Phytophthora capsici on Vegetable Crops: Research Progress and Management Challenges

Synthesis of paeonol hydrazone derivatives and their anti-oomycete, anti-fungal, and nematicidal activities

BACKGROUND

The natural product paeonol is a rich and sustainable natural bioresource, and its derivatives have various unique biological efficacy. As is well known, Schiff bases are a class of organic compounds with a wide range of biological activities, including anti-fungal, insecticidal, anti-viral, and nematicidal.

RESULTS

To discover biorational natural product-based pesticides, nine intermediates (2-10), 12 sulfonylhydrazones (11a-11c, 12a-12c, 13a-13c, and 14a-14c) and 20 benzylidene hydrazones (18a-18r, 19a, and 20a) were synthesized by structural modification of paeonol, and their structures were characterized by proton nuclear magnetic resonance (1H NMR), carbon-13 (13C) NMR, and high-resolution mass spectrometry (HRMS). The stereochemical configurations of compounds 14a, 18d, and 18r were unambiguously confirmed by single-crystal X-ray diffraction. Furthermore, bioactivities of these compounds as anti-oomycete, anti-fungal, and nematicidal agents against three serious agricultural pests, Phytophthora capsici, Fusarium graminearum, and Heterodera glycines were evaluated. Among all tested compounds: (i) compound 7 exhibited promising anti-oomycete against Phytophthora capsici, with a half maximal effective concentration (EC50) value of 15.81 mg L-1; (ii) compounds 2, 7, 10, and 19a displayed promising anti-fungal against F. graminearum, with EC50 values of 12.22, 14.72, 23.39, and 33.10 mg L-1, respectively; (iii) ten compounds (12a-12c, 14c, 18g-18j, 18m, and 19a) showed significant nematicidal activity against H. glycines, with median lethal concentration (LC50) values all less than 30.00 mg L-1. Especially for compound 18g, its LC50 value is the smallest, at 12.65 mg L-1.

CONCLUSION

The research results indicate that introducing nitro groups at the C5 position of paeonol, or introducing halogens at both C5 and C3 positions, can significantly enhance its biological activity against Phytophthora capsici, F. graminearum, and H. glycines. © 2024 Society of Chemical Industry.

Integrating multilocus phylogeny and morphological analysis reveals the prevalence of Phytophthora meadii (McRae) associated with abnormal leaf fall disease of Hevea brasiliensis in India

The Oomycetes fungus Phytophthora spp. which causes Abnormal leaf fall (ALF) disease poses a significant threat as one of the most devastating diseases affecting rubber trees in India. A total of 30 Phytophthora isolates were obtained from ALF-affected samples collected during the Southwest monsoon season of Kerala. The colony morphology of Phytophthora isolates revealed eight different types of growth patterns, with stellate, stellate striated, and petaloid patterns growing rapidly, whereas chrysanthemum pattern grew slowly. Sporangia were papillate to non-papillate in various shapes, and sporangiophores exhibited simple, simple sympodial, or irregularly branching patterns. Highly virulent isolates exhibited petaloid morphology and rapid growth rates. Regardless of their virulence, all isolates showed susceptibility to the fungicide metalaxyl. Under in vitro conditions, the highly virulent isolate (R17) from rubber caused severe infections in chili, brinjal, and tomato with brown water-soaked lesions. Sequence analysis and multi-locus phylogeny of Internal transcribed spacer (ITS), cCytochrome c oxidase 1 (COX 1), Heat shock protein 90 (HSP 90), and Ribosomal protein L10 (RPL 10) confirmed the pathogen as Phytophthora meadii. A comprehensive understanding of both morphological and molecular traits of P. meadii is crucial for precise identification and future genetic variability studies.

Biological activity and systemic translocation of the new tetrazolyloxime fungicide picarbutrazox against plant-pathogenic oomycetes

BACKGROUND

Picarbutrazox is a new tetrazolyloxime fungicide discovered in 2014 by Nippon Soda. It is mostly used to protect against Phytophthora spp. and Pythium spp. However, little is known of its inhibition spectrum, protective and curative activity, and systemic translocation in plants.

RESULTS

While picarbutrazox did not show obvious antifungal activity, it exhibited significant activity against oomycetes, including Phytophthora spp., Pythium spp. and Phytopythium spp.. The effective concentration for 50% growth inhibition (EC50) values of picarbutrazox against 16 oomycetes ranged from 3.1 × 10-4 and 7.27 × 10-3 μg mL-1. Furthermore, picarbutrazox could markedly inhibited the mycelial development, sporangia production, zoospore release, and cyst germination of Phytophthora capsici, with EC50 values of 1.34 × 10-3, 1.11 × 10-3, 4.85 × 10-3, and 5.88 × 10-2 μg mL-1, respectively. Additionally, under greenhouse conditions, the protective and curative activities of picarbutrazox at 200 mg L-1 (100%, 41.03%) against the P. capsici infection in peppers were higher than those of the reference fungicide dimethomorph at 200 mg L-1 (77.52%, 36.15%). High-performance liquid chromatography analysis confirmed that picarbutrazox showed excellent systemic translocation in pepper plants.

CONCLUSION

The results showed that picarbutrazox markedly inhibited the important plant oomycete pathogens including Phytophthora spp., Pythium spp. and Phytopythium spp.. It also displayed excellent protective, curative and systemic translocation activity. Picarbutrazox thus has significant potential for preventing and controlling diseases caused by oomycetes. © 2024 Society of Chemical Industry.

Dual actions of chloroinconazide on pepper blight in Capsicum annuum: disruption of Phytophthora capsici mycelium and activation of CaCNGC9-mediated SA signaling

BACKGROUND

Pepper blight, caused by Phytophthora capsici, is a devastating disease that seriously threatens pepper production worldwide. With the emergence of resistance in P. capsici against conventional fungicides, there is an urgent need to explore novel alternatives for pepper blight management. This study aims to assess the inhibitory effect of chloroinconazide (CHI), a compound synthesized from tryptophan, against pepper blight, and to explore its potential mechanisms of action.

RESULTS

The results demonstrated that CHI effectively targeted P. capsici, disrupting its growth and mycelial structure, which resulted in the release of dissolved intracellular substances. Additionally, CHI significantly inhibited the sporangium formation, zoospores release, and zoospores germination, thereby reducing the re-infection of P. capsici. In contrast, the commercial pesticide methylaxyl only inhibited mycelial growth and had limited effect on re-infection, while azoxystrobin inhibited re-infection but had a weak inhibitory effect on mycelial growth. Furthermore, CHI activated the salicylic acid (SA) signaling pathway-mediated immune response to inhibit P. capsici infection in pepper, with this activation being contingent upon cyclic nucleotide-gated ion channel CaCNGC9.

CONCLUSION

CHI exhibited potent dual inhibitory effects on P. capsici by disrupting mycelial structure and activating the CaCNGC9-mediated SA signaling pathway. These dual mechanisms of action suggested that CHI could serve as a promising alternative chemical fungicide for the effective management of pepper blight, offering a new approach to control this devastating disease. Our findings highlighted the potential of CHI as a sustainable and efficient solution to combat the increasing resistance of P. capsici to conventional fungicides, ensuring better crop protection and yield. © 2024 Society of Chemical Industry.

Evaluation of a Formulation of Bacillus subtilis for Control of Phytophthora Blight of Bell Pepper

Phytophthora blight, caused by Phytophthora capsici, is one of the most economically significant diseases of bell pepper in the United States. Over the past several decades, isolates of P. capsici exhibiting resistance to mefenoxam and other fungicides have been reported. Fungicide resistance coupled with an increased market for organically grown crops has led to interest in biological control as a disease management option. In this work, an isolate of Bacillus subtilis (AFS032321) was evaluated for control of Phytophthora blight of bell pepper in the greenhouse and field. A 28% active ingredient wettable powder formulation of the strain was applied as a soil drench at transplanting prior to inoculation. Treatment with this formulation of B. subtilis significantly reduced the area under the disease progress curve (AUDPC) by up to 52% compared to untreated control plants in greenhouse tests. Comparisons between applying the biocontrol weekly after seeding for 5 weeks versus a single application at transplanting (5 weeks) indicated no significant benefits of additional applications. The formulation of B. subtilis reduced disease caused by a mefenoxam-resistant isolate of P. capsici, while mefenoxam failed. The biocontrol efficacy of formulated strains was not affected in different soil types or potting media. However, disease was more severe in sandy soils. In field experiments that were conducted with a mefenoxam-sensitive isolate, disease incidence and severity of Phytophthora blight were significantly reduced at all rates of B. subtilis in 2019 except the 16.8 kg ha-1 rate. In both years, mefenoxam was more effective than B. subtilis in controlling disease in the field. B. subtilis did not affect the spatial dynamics of pathogen spread within rows. While the precise mechanism(s) of action is unclear, in vitro dual-culture tests suggest direct antagonism, as B. subtilis significantly inhibited colony growth of P. capsici. AgBiome has recently released a new formulation of the AFS032321 strain named Theia, with higher active ingredients for commercial applications and biocontrol of P. capsici.

Antimicrobial mechanisms of g-C3 N4 @ZnO against oomycetes Phytophthora capsici: from its metabolism, membrane structures and growth

BACKGROUND

Phytophthora capsici, a refractory and model oomycete plant pathogen, especially threatens multiple vegetable crops. A limited number of chemical pesticides play a vital role in controlling oomycete plant diseases. However, this approach often leads to excessive use of chemical agent, exacerbates environmental issues and more and more drug-resistant strains of oomycete. Therefore, it is imperative to devise innovative solutions that can effectively address the infection of oomycete while maintaining high levels of environmental sustainability and low toxicity.

RESULTS

In this study, g-C3 N4 @ZnO heterostructure was synthesized and characterized. The g-C3 N4 @ZnO showed higher toxicity on Phytophthora capsici than graphitic carbon nitride (g-C3 N4 ) nanosheets and zinc oxide (ZnO) nanoparticles in vitro and in vivo. Except the hyphal growth of Phytophthora capsici, their germination rate of spores, sporangium formation and number of spores were all suppressed by g-C3 N4 @ZnO heterostructure. Furthermore, we found that this g-C3 N4 @ZnO heterostructure has higher photocatalytic activity under visible light, which potentially enhanced the reactive oxygen species (ROS) mediated stress on Phytophthora capsici. Ultrastructural morphology, global changes of gene expression and weighted gene co-expression network analysis all supported that the anti-oomycete activity of g-C3 N4 @ZnO was manifested in the destruction of membrane system and inhibition of multiple metabolisms of Phytophthora capsici under visible irradiation, which also could be attributed to the ROS and zinc ion (Zn2+ ) mediated stress.

CONCLUSION

This works offers a novel oomycete disease management strategy by using g-C3 N4 @ZnO, which were attributed to the ROS stress, destruction of membrane system and inhibition of multiple metabolisms. © 2023 Society of Chemical Industry.

Semisynthesis, anti-oomycete and anti-fungal activities of ursolic acid ester derivatives

Using ursolic acid (UA) as the lead compound, thirteen UA ester derivatives (3 and 7a-l) were synthesized by modifying their C-3 and C-28 positions, respectively, and their structures were well characterized by 1H NMR, 13C NMR, HRMS and melting points. Furthermore, we evaluated the anti-oomycete and anti-fungal activities of these compounds against Phytophthora capsici and Fusarium graminearum in vitro. The results showed that compound 7h exhibited prominent anti-oomycete and anti-fungal activities, and the median effective concentration (EC50) values of 7h against P. capsici and F. graminearum were 70.49 and 113.21 mg/L, respectively. This study suggested that the anti-oomycete and anti-fungal activities of esters synthesized by introducing acyloxy group at C-3 position of UA was more conspicuous than that of esters synthesized by introducing benzyloxy group at C-28 position. This result will pave the way for further modification of UA to develop potential new fungicides.

Unveiling molecular mechanisms of pepper resistance to Phytophthora capsici through grafting using iTRAQ-based proteomic analysis

Phytophthora blight severely threatens global pepper production. Grafting bolsters plant disease resistance, but the underlying molecular mechanisms remain unclear. In this study, we used P. capsici-resistant strain 'ZCM334' and susceptible strain 'Early Calwonder' for grafting. Compared to self-rooted 'Early Calwonder' plants, 'ZCM334' grafts exhibited delayed disease onset, elevated resistance, and reduced leaf cell damage, showcasing the potential of grafting in enhancing pepper resistance to P. capsici. Proteomic analysis via the iTRAQ technology unveiled 478 and 349 differentially expressed proteins (DEPs) in the leaves and roots, respectively, between the grafts and self-rooted plants. These DEPs were linked to metabolism and cellular processes, stimulus responses, and catalytic activity and were significantly enriched in the biosynthesis of secondary metabolites, carbon fixation in photosynthetic organizations, and pyruvate metabolism pathways. Twelve DEPs exhibiting consistent expression trends in both leaves and roots, including seven related to P. capsici resistance, were screened. qRT-PCR analysis confirmed a significant correlation between the protein and transcript levels of DEPs after P. capsici inoculation. This study highlights the molecular mechanisms whereby grafting enhances pepper resistance to Phytophthora blight. Identification of key genes provides a foundation for studying the regulatory network governing the resistance of pepper to P. capsici.

Characterizing the Dynamics of Virulence and Fungicide Resistance of Phytophthora capsici in Michigan Vegetable Fields Reveals Loci Associated with Virulence

The oomycete Phytophthora capsici is a destructive pathogen infecting more than 50 plant species and is one of the most serious threats to cucurbit production. Phytophthora blight caused by Phytophthora capsici can affect all plant growth stages, and fungicides and cultural controls are used to limit losses. Dissecting pathogen virulence and fungicide resistance can provide insights into pathogenic mechanisms and inform effective management practices to control P. capsici. In this study, we assessed virulence, mefenoxam sensitivity, and genetic diversity of nine P. capsici populations collected from Cucurbitaceae, Solanaceae, and Fabaceae host families in Michigan from 2002 to 2016. We developed 992 simple sequence repeats (SSRs) in the P. capsici genome and identified 60 SSRs located within or close to RXLR-class (Arginine-any amino acid-Leucine-Arginine) effectors and 29 SSRs within or close to effector CRN (CRinkling and Necrosis) family protein, which represent 62 RXLR and 34 putative CRNs. Population structure analysis shows that mefenoxam resistance was not associated with the year of collection, host type, or location, but there were significant differences in virulence among the populations. Using the general linear model and mixed linear model-based association analyses with all effector-related SSR markers, we identified four SSR markers significantly associated with at least one of the virulence-related parameters. Of these, one (Pce_SC18) was in a predicted CRN effector and had high identity with the putative PhCRN37 effector in the pathogen Plasmopara halstedii, which can be further verified for virulence identification in P. capsici.

Identification of QTL associated with resistance to Phytophthora fruit rot in cucumber (Cucumis sativus L.)

Phytophthora fruit rot (PFR) caused by the soilborne oomycete pathogen, Phytophthora capsici, can cause severe yield loss in cucumber. With no resistant variety available, genetic resources are needed to develop resistant varieties. The goal of this work was to identify quantitative trait loci (QTL) associated with resistance to PFR using multiple genomic approaches and populations. Two types of resistances have been identified: age-related resistance (ARR) and young fruit resistance. ARR occurs at 12-16 days post pollination (dpp), coinciding with the end of exponential fruit growth. A major QTL for ARR was discovered on chromosome 3 and a candidate gene identified based on comparative transcriptomic analysis. Young fruit resistance, which is observed during the state of rapid fruit growth prior to commercial harvest, is a quantitative trait for which multiple QTL were identified. The largest effect QTL, qPFR5.1, located on chromosome 5 was fine mapped to a 1-Mb region. Genome-wide association studies (GWAS) and extreme-phenotype genome-wide association study (XP-GWAS) for young fruit resistance were also performed on a cucumber core collection representing > 96% of the genetic diversity of the USDA cucumber germplasm. Several SNPs overlapped with the QTL identified from QTL-seq analysis on biparental populations. In addition, novel SNPs associated with the resistance were identified from the germplasm. The resistant alleles were found mostly in accessions from India and South Asia, the center of diversity for cucumber. The results from this work can be applied to future disease resistance studies and marker-assisted selection in breeding programs.

Synthesis of novel 18β-glycyrrhetinic acid sulfonate derivatives displaying significant anti-oomycete activity against Phytophthora capsici

Using 18β-glycyrrhetinic acid (GA) as the lead compound, fourteen GA sulphonate derivatives (3a-n) were prepared by modifying its C-3 OH group, and their structures were well confirmed by 1H NMR, 13C NMR, HRMS and melting points. Moreover, we screened the anti-oomycete activity of these compounds against Phytophthora capsici by using the mycelial growth rate method. Among the fourteen GA sulphonate derivatives evaluated, four compounds 3f, 3j, 3k and 3l exhibited more potent anti-oomycete activity than that of the positive control zoxamide (EC50 = 25.17 mg/L), and had the median effective concentration (EC50) values of 23.04, 16.16, 22.55, and 13.93 mg/L, respectively. Especially compound 3l showed the best anti-oomycete activity against P. capsici with EC50 value of 13.93 mg/L. Overall, the introduction of sulfonyloxy groups at the C-3 position of GA has a significant impact on its anti-oomycete activity, and the corresponding derivative activity varies significantly with different substituents R.

Transcriptome and metabolome analyses revealed the response mechanism of pepper roots to Phytophthora capsici infection

BACKGROUND

Phytophthora root rot caused by the oomycete Phytophthora capsici is the most devastating disease in pepper production worldwide, and current management strategies have not been effective in preventing this disease. Therefore, the use of resistant varieties was regarded as an important part of disease management of P. capsici. However, our knowledge of the molecular mechanisms underlying the defense response of pepper roots to P. capsici infection is limited.

METHODS

A comprehensive transcriptome and metabolome approaches were used to dissect the molecular response of pepper to P. capsici infection in the resistant genotype A204 and the susceptible genotype A198 at 0, 24 and 48 hours post-inoculation (hpi).

RESULTS

More genes and metabolites were induced at 24 hpi in A204 than A198, suggesting the prompt activation of defense responses in the resistant genotype, which can attribute two proteases, subtilisin-like protease and xylem cysteine proteinase 1, involved in pathogen recognition and signal transduction in A204. Further analysis indicated that the resistant genotype responded to P. capsici with fine regulation by the Ca2+- and salicylic acid-mediated signaling pathways, and then activation of downstream defense responses, including cell wall reinforcement and defense-related genes expression and metabolites accumulation. Among them, differentially expressed genes and differentially accumulated metabolites involved in the flavonoid biosynthesis pathways were uniquely activated in the resistant genotype A204 at 24 hpi, indicating a significant role of the flavonoid biosynthesis pathways in pepper resistance to P. capsici.

CONCLUSION

The candidate transcripts may provide genetic resources that may be useful in the improvement of Phytophthora root rot-resistant characters of pepper. In addition, the model proposed in this study provides new insight into the defense response against P. capsici in pepper, and enhance our current understanding of the interaction of pepper-P. capsici.

Additive effect of the Streptomyces albus XJC2-1 and dimethomorph control pepper blight (Capsicum annuum L.)

BACKGROUND

Pepper blight, caused by Phytophthora capsici, is a destructive soilborne disease, which poses a serious threat to pepper, Capsicum annuum L., production. Chemical fungicides, which mainly are used to control pepper blight, have a negative effect on the environment, rendering biological control as a promising alternative to maintain the balance between ecology and pest management. The purpose of this study was to screen the biocontrol bacteria, reduce the dosage of fungicides and increase the stability of biocontrol bacteria, and to find the mixing ratio of biocontrol bacteria and fungicides giving the best control effect.

RESULTS

We isolated actinomycetes strains from the soil surrounding the roots of healthy pepper plants amongst field-grown plants infected with P. capsici. Of these, Streptomyces albus XJC2-1 showed a strong inhibition effect on the growth of P. capsici, with an inhibition rate of ≤85%. XJC2-1 effectively inhibited the formation of sporangium and release of zoospores of P. capsici as well as directly destroyed its hyphae, to achieve the inhibitory effect. Transcriptomic profiling of pepper leaves, postirrigation of plants with the XJC2-1 fermentation broth, revealed upregulation of genes related to the photosynthesis pathway in pepper. Furthermore, XJC2-1 treatment improved the net photosynthetic rate and intercellular CO2 concentration, thereby improving the pepper plant's resistance to pathogens. The combination of XJC2-1 with the fungicide dimethomorph (8 μg mL-1 ) displayed strong synergism in inhibition of P. capsici infection, with a control efficiency as high as 75.16%, thus providing a basis for its application in the field.

CONCLUSION

Our study demonstrated that S. albus XJC2-1 inhibited Phytophthora pathogens from infecting pepper plants and enhanced plant host resistance. The combination of XJC2-1 and dimethomorph displayed a more stable and stronger control effect on pepper blight, showing potential for the future application of XJC2-1 in the field of biological control. © 2023 Society of Chemical Industry.

Phytophthora capsici: Recent Progress on Fundamental Biology and Disease Management 100 Years After Its Description

Phytophthora capsici is a destructive oomycete pathogen of vegetable, ornamental, and tropical crops. First described by L.H. Leonian in 1922 as a pathogen of pepper in New Mexico, USA, P. capsici is now widespread in temperate and tropical countries alike. Phytophthora capsici is notorious for its capability to evade disease management strategies. High genetic diversity allows P. capsici populations to overcome fungicides and host resistance, the formation of oospores results in long-term persistence in soils, zoospore differentiation in the presence of water increases epidemic potential, and a broad host range maximizes economic losses and limits the effectiveness of crop rotation. The severity of disease caused by P. capsici and management challenges have led to numerous research efforts in the past 100 years. Here, we discuss recent findings regarding the biology, genetic diversity, disease management, fungicide resistance, host resistance, genomics, and effector biology of P. capsici.

Plant secondary metabolite citral interferes with Phytophthora capsici virulence by manipulating the expression of effector genes

Phytophthora capsici is a notorious pathogen that infects various economically important plants and causes serious threats to agriculture worldwide. Plants deploy a variety of plant secondary metabolites to fend off pathogen attacks, but the molecular mechanisms are largely unknown. In this study, we screened 11 plant secondary metabolites to evaluate their biofumigation effects against P. capsici, and found that citral, carvacrol, and trans-2-decenal exhibited strong antimicrobial effects. Intriguingly, a low concentration of citral was effective in restricting P. capsici infection in Nicotiana benthamiana, but it was unable to inhibit the mycelial growth. A high concentration of citral affected the mycelial growth and morphology, zoospore germination, and cell membrane permeability of P. capsici. Further investigations showed that citral did not induce expression of tested plant immunity-related genes and reactive oxygen species (ROS) production, suggesting that a low concentration of citral could not trigger plant immunity. Moreover, RNA-Seq analysis showed that citral treatment regulated the expression of some P. capsici effector genes such as RxLR genes and P. cactorum-fragaria (PCF)/small cysteine-rich (SCR)74-like genes during the infection process, which was also verified by reverse transcription-quantitative PCR assay. Five candidate effector genes suppressed by citral significantly facilitated P. capsici infection in N. benthamiana or inhibited ROS triggered by flg22, suggesting that they were virulence factors of P. capsici. Together, our results revealed that plant-derived citral exhibited excellent inhibitory efficacy against P. capsici by suppressing vegetative growth and manipulating expression of effector genes, which provides a promising application of citral for controlling Phytophthora blight.

Development and Application of a Cleaved Amplified Polymorphic Sequence Marker (Phyto) Linked to the Pc5.1 Locus Conferring Resistance to Phytophthora capsici in Pepper (Capsicum annuum L.)

Phytophthora capsici causes destructive disease in several crop species, including pepper (Capsicum annuum L.). Resistance in this species is physiologically and genetically complex due to many P. capsici virulence phenotypes and different QTLs and R genes among the identified resistance sources. Several primer pairs were designed to follow an SNP (G/A) within the CA_011264 locus linked to the Pc5.1 locus. All primer pairs were designed on DNA sequences derived from CaDMR1, a homoserine kinase (HSK), which is a gene candidate responsible for the major QTL on chromosome P5 for resistance to P. capsici. A panel of 69 pepper genotypes from the Southern Seed germplasm collection was used to screen the primer pairs designed. Of these, two primers (Phyto_for_2 and Phyto_rev_2) surrounding the SNP proved successful in discriminating susceptible and resistant genotypes when combined with a restriction enzyme (BtgI). This new marker (called Phyto) worked as expected in all genotypes tested, proving to be an excellent candidate for marker-assisted selection in breeding programs aimed at introgressing the resistant locus into pure lines.

Synthesis, Anti-Oomycete and Anti-fungal Activities of Novel Cinchona Alkaloid Derivatives Containing Sulfonate Moiety

Using cinchona alkaloid as the lead compound, twenty-four cinchona alkaloid sulfonate derivatives (1 a-l, 2 a-c, 3 a-c, 4 a-c, and 5 a-c) were designed and prepared by modifying their C9 position, and structurally confirmed by 1 H-NMR, 13 C-NMR, HR-MS and melting points. Moreover, the stereochemical configurations of compounds 1 f and 1 l were unambiguously confirmed by single-crystal X-ray diffraction. Furthermore, we determined the anti-oomycete and anti-fungal activities of these target compounds against Phytophthora capsici and Fusarium graminearum in vitro. The results showed that two compounds 4 b and 4 c exhibited prominent anti-oomycete activity, and the median effective concentration (EC50 ) values of 4 b and 4 c against P. capsici were 22.55 and 16.32 mg/L, respectively. This study suggested that when the C9 position of cinchona alkaloid sulfonate derivatives is in the S configuration and the 6'-position methoxy group is not present, the anti-oomycete activity is superior. In addition, five compounds 1 e, 1 f, 1 k, 3 c and 4 c displayed significant anti-fungal activity, with EC50 values of 43.64, 45.07, 80.18, 48.58 and 41.88 mg/L against F. graminearum, respectively. This result indicates that only when a specific substituent is introduced into the structural framework of the target compound, the corresponding compound exhibits significant inhibitory activity against fungi.

Phytophthora capsici, 100 Years Later: Research Mile Markers from 1922 to 2022

In 1922, Phytophthora capsici was described by Leon Hatching Leonian as a new pathogen infecting pepper (Capsicum annuum), with disease symptoms of root rot, stem and fruit blight, seed rot, and plant wilting and death. Extensive research has been conducted on P. capsici over the last 100 years. This review succinctly describes the salient mile markers of research on P. capsici with current perspectives on the pathogen's distribution, economic importance, epidemiology, genetics and genomics, fungicide resistance, host susceptibility, pathogenicity mechanisms, and management.

Evaluation of Inhibitory Effect and Mechanism of Euphorbia Factor L3 against Phytophthora capsici

Phytophthora capsici is a highly destructive phytopathogenic oomycete with a broad host range and is responsible for tremendous losses. Euphorbia factor L3 (EFL3) is a natural plant-derived compound that has been widely studied in medicine and cosmetic applications. In this study, the sensitivity of 105 P. capsici isolates to EFL3 was determined, and the biological activity and physiological effects of EFL3 against P. capsici were investigated. The median effective concentration (EC50) values for EFL3 inhibition mycelial growth and spore germination ranged from 0.66 to 8.94 μg/mL (mean, 2.96 ± 0.91 μg/mL) and 1.63 to 13.16 μg/mL (mean, 5.30 ± 1.64 μg/mL), respectively. EFL3 treatment resulted in cell wall and cell membrane damage of P. capsici, which was revealed by morphological and ultrastructural observations, propidium iodide (PI) and calcofluor white (CFW) staining, and measurements of relative conductivity as well as malondialdehyde (MDA) and glycerol contents. In addition, the contents of phospholipid and cellulose, which are the major components of cell membrane and cell wall, were significantly reduced following EFL3 treatment. Furthermore, EFL3 provided protective as well as curative efficacies against P. capsici on detached tomato leaves and pepper seedlings in vivo. These data show that EFL3 exhibits strong inhibitory activity against P. capsici, thereby suggesting that it could be an effective alternative for controlling P. capsici-induced diseases.

Biocontrol of Diseases Caused by Phytophthora capsici and P. parasitica in Pepper Plants

The main objective of this study was to evaluate the ability of Trichoderma aggressivum f. europaeum, T. longibrachiatum, Paecilomyces variotii, and T. saturnisporum as biological control agents (BCAs) against diseases caused by P. capsici and P. parasitica in pepper. For this purpose, their antagonistic activities were evaluated both in vitro and in vivo. We analysed the expression patterns of five defence related genes, CaBGLU, CaRGA1, CaBPR1, CaPTI1, and CaSAR8.2, in leaves. All BCAs showed a high in vitro antagonistic activity, significantly reducing the mycelial growth of P. capsici and P. parasitica. The treatments with T. aggressivum f. europaeum, T. longibrachiatum, and P. variotii substantially reduced the severity of the disease caused by P. capsici by 54, 76, and 70%, respectively, and of the disease caused by P. parasitica by 66, 55, and 64%, respectively. T. saturnisporum had the lowest values of disease reduction. Reinoculation with the four BCAs increased the control of both plant pathogens. Markedly different expression patterns were observed in the genes CaBGLU, CaRGA1, and CaSAR8.2. Based on the results, all four BCAs under study could be used as a biological alternative to chemicals for the control of P. capsici and P. parasitica in pepper with a high success rate.

Antifungal alkaloids from the branch-leaves of Clausena lansium Lour. Skeels (Rutaceae)

BACKGROUND

The rational utilization of botanical secondary metabolites is one of the strategies to reduce the application of chemical fungicides. The extensive biological activities of Clausena lansium indicate that it has the potential to develop botanical fungicides.

RESULTS

A systematic investigation on the antifungal alkaloids from C. lansium branch-leaves following bioassay-guided isolation was implemented. Sixteen alkaloids, including two new and nine known carbazole alkaloids, one known quinoline alkaloid and four known amides, were isolated. Compounds 4, 7, 12 and 14 showed strong antifungal activity on Phytophthora capsiciwith EC₅₀ values ranging from 50.67 to 70.82 μg mL^-1 . Compounds 1, 3, 8, 10, 11, 12 and 16 displayed different degrees of antifungal activity against Botryosphaeria dothidea with EC₅₀ values ranging from 54.18 to 129.83 μg mL^-1 . It was reported for the first time that these alkaloids had antifungal effects on P. capsici or B. dothidea, and their structure-activity relationships were further discussed systematically. Additionally, among all alkaloids, dictamine (12) had the strongest antifungal activities against P. capsici (EC₅₀  = 50.67 μg mL^-1 ) and B. dothidea (EC₅₀  = 54.18 μg mL^-1 ), and its physiological effects on P. capsici and B. dothidea also were further evaluated.

CONCLUSION

Capsicum lansium is a potential source of antifungal alkaloids, and C. lansium alkaloids had the potential as lead compounds of botanical fungicides in the development of new fungicides with novel action mechanism. © 2023 Society of Chemical Industry.

Multimodal pathogen transmission as a limiting factor in host distribution

Theoretical models suggest that infectious diseases could play a substantial role in determining the spatial extent of host species, but few studies have collected the empirical data required to test this hypothesis. Pathogens that sterilize their hosts or spread through frequency-dependent transmission could have especially strong effects on the limits of species' distributions because diseased hosts that are sterilized but not killed may continue to produce infectious stages and frequency-dependent transmission mechanisms are effective even at very low population densities. We collected spatial pathogen prevalence data and population abundance data for alpine carnations infected by the sterilizing pathogen Microbotryum dianthorum, a parasite that is spread through both frequency-dependent (vector-borne) and density-dependent (aerial spore transmission) mechanisms. Our 13-year study reveals rapid declines in population abundance without a compensatory decrease in pathogen prevalence. We apply a stochastic, spatial model of parasite spread that accommodates spatial habitat heterogeneity to investigate how the population dynamics depend on multimodal (frequency-dependent and density-dependent) transmission. We found that the observed rate of population decline could plausibly be explained by multimodal transmission, but is unlikely to be explained by either frequency-dependent or density-dependent mechanisms alone. Multimodal pathogen transmission rates high enough to explain the observed decline predicted that eventual local extinction of the host species is highly likely. Our results add to a growing body of literature showing how multimodal transmission can constrain species distributions in nature.

Anti-Oomycete Activity and Pepper Root Colonization of Pseudomonas plecoglossicida YJR13 and Pseudomonas putida YJR92 against Phytophthora capsici

Previously, Pseudomonas plecoglossicida YJR13 and Pseudomonas putida YJR92 from a sequential screening procedure were proven to effectively control Phytophthora blight caused by Phytophthora capsici. In this study, we further investigated the anti-oomycete activities of these strains against mycelial growth, zoospore germination, and germ tube elongation of P. capsici. We also investigated root colonization ability of the bacterial strains in square dishes, including cell motility (swimming and swarming motilities) and biofilm formation. Both strains significantly inhibited mycelial growth in liquid and solid V8 juice media and M9 minimal media, zoospore germination, and germ tube elongation compared with Bacillus vallismortis EXTN-1 (positive biocontrol strain), Sphingomonas aquatilis KU408 (negative biocontrol strain), and MgSO4 solution (untreated control). In diluted (nutrient-deficient) V8 juice broth, the tested strain populations were maintained at >108 cells/ml, simultaneously providing mycelial inhibitory activity. Additionally, these strains colonized pepper roots at a 106 cells/ml concentration for 7 days. The root colonization of the strains was supported by strong swimming and swarming activities, biofilm formation, and chemotactic activity towards exudate components (amino acids, organic acids, and sugars) of pepper roots. Collectively, these results suggest that strains YJR13 and YJR92 can effectively suppress Phytophthora blight of pepper through direct anti-oomycete activities against mycelial growth, zoospore germination and germ tube elongation. Bacterial colonization of pepper roots may be mediated by cell motility and biofilm formation together with chemotaxis to root exudates.

Major Soilborne Pathogens of Field Processing Tomatoes and Management Strategies

Globally, tomato is the second most cultivated vegetable crop next to potato, preferentially grown in temperate climates. Processing tomatoes are generally produced in field conditions, in which soilborne pathogens have serious impacts on tomato yield and quality by causing diseases of the tomato root system. Major processing tomato-producing countries have documented soilborne diseases caused by a variety of pathogens including bacteria, fungi, nematodes, and oomycetes, which are of economic importance and may threaten food security. Recent field surveys in the Australian processing tomato industry showed that plant growth and yield were significantly affected by soilborne pathogens, especially Fusarium oxysporum and Pythium species. Globally, different management methods have been used to control diseases such as the use of resistant tomato cultivars, the application of fungicides, and biological control. Among these methods, biocontrol has received increasing attention due to its high efficiency, target-specificity, sustainability and public acceptance. The application of biocontrol is a mix of different strategies, such as applying antagonistic microorganisms to the field, and using the beneficial metabolites synthesized by these microorganisms. This review provides a broad review of the major soilborne fungal/oomycete pathogens of the field processing tomato industry affecting major global producers, the traditional and biological management practices for the control of the pathogens, and the various strategies of the biological control for tomato soilborne diseases. The advantages and disadvantages of the management strategies are discussed, and highlighted is the importance of biological control in managing the diseases in field processing tomatoes under the pressure of global climate change.

Polymorphic Microsatellite Development, Genetic Diversity, Population Differentiation and Sexual State of Phytophthora capsici on Commercial Peppers in Three Provinces of Southwest China

Pepper blight, caused by the oomycete pathogen Phytophthora capsici (P. capsici), is one of the most destructive soilborne diseases worldwide. Between 2019 and 2020, 235 single spore isolates of P. capsici were collected from 36 commercial pepper planting areas in Sichuan, Chongqing, and Guizhou provinces in China. A novel full set of 323 high-quality polymorphic microsatellites was obtained by resequencing 10 isolates. In total, 163 isolates with two alleles per microsatellite locus were used for population analysis and resulted in 156 genotypes on 10 microsatellite loci. The genetic diversity, population differentiation, principal component, genetic structure, and genetic relationships analyses showed an extensive variety of the P. capsici in Sichuan and Guizhou with clonal lineages, two shared genotypes, and no geographic differentiation. The population from Chongqing was differentiated from that of Sichuan and Guizhou and had the highest genetic diversity. There was no significant distinction between the populations of the two sampling years, but there was a small differentiation between the populations from bell peppers and hot peppers. The isolates from Southwest China were largely distant from the two reference isolates from the USA. The analysis of molecular variance showed that the major variance of the populations was within populations. The linkage equilibrium test, mating type composition, and oospore detection indicated that only P. capsici from the Jiulongpo district of Chongqing had appeared in sexual recombination. Overall, this study revealed that the high and complex genetic diversity population of P. capsici in Sichuan, Chongqing, and Guizhou with uneven geographic variation and limited sexual reproductive behavior in Chongqing, potentially driven by differences in the geographical environment, reproductive patterns, different cultivars, and artificial long-distance transfers. IMPORTANCE Phytophthora capsici, a notorious soilborne and rapidly evolving pathogen with a wide range of hosts, is a huge threat to pepper production worldwide. However, the detailed genetic structure and dynamics of P. capsici in most Chinese provinces are still unclear, even though China is the world's largest producer and consumer of peppers. Here, a novel full set of high-quality polymorphic microsatellites, obtained by genome resequencing data of 10 isolates from Southwest China, was provided for future population analyses. In this study, we further investigated and established the genetic structure, sexual recombination, geographic subdivisions, interannual stability, differentiation in different types of host peppers, and member relationships of P. capsici from three provinces in Southwest China. These results reveal the genetic structure and dynamics of P. capsici in three provinces of Southwest China and help us to execute more effective management strategies in the future.

A Phytophthora capsici RXLR effector manipulates plant immunity by targeting RAB proteins and disturbing the protein trafficking pathway

The oomycete pathogen Phytophthora capsici encodes hundreds of RXLR effectors that enter the plant cells and suppress host immunity. Only a few of these genes are conserved across different strains and species. Such core effectors might target hub genes and immune pathways in hosts. Here, we describe the functional characterization of the core P. capsici RXLR effector RXLR242. The expression of RXLR242 was up-regulated during infection, and its ectopic expression in Nicotiana benthamiana, an experimental plant host, further promoted Phytophthora infection. RXLR242 physically interacted with a group of RAB proteins that belong to the small GTPase family and play a role in regulating transport pathways in the intracellular membrane trafficking system. In addition, RXLR242 impeded the secretion of PATHOGENESIS-RELATED 1 (PR1) protein to the apoplast. This phenomenon resulted from the competitive binding of RXLR242 to RABE1-7. We also found that RXLR242 interfered with the association between RABA4-3 and its binding protein, thereby disrupting the trafficking of the membrane receptor FLAGELLIN-SENSING 2. Thus, RXLR242 manipulates plant immunity by targeting RAB proteins and disrupting protein trafficking in the host plants.

Biological Control of Oomycete Soilborne Diseases Caused by Phytophthora capsici, Phytophthora infestans, and Phytophthora nicotianae in Solanaceous Crops

Oomycete pathogens that belong to the genus Phytophthora cause devastating diseases in solanaceous crops such as pepper, potato, and tobacco, resulting in crop production losses worldwide. Although the application of fungicides efficiently controls these diseases, it has been shown to trigger negative side effects such as environmental pollution, phytotoxicity, and fungicide resistance in plant pathogens. Therefore, biological control of Phytophthora-induced diseases was proposed as an environmentally sound alternative to conventional chemical control. In this review, progress on biological control of the soilborne oomycete plant pathogens, Phytophthora capsici, Phytophthora infestans, and Phytophthora nicotianae, infecting pepper, potato, and tobacco is described. Bacterial (e.g., Acinetobacter, Bacillus, Chryseobacterium, Paenibacillus, Pseudomonas, and Streptomyces) and fungal (e.g., Trichoderma and arbuscular mycorrhizal fungi) agents, and yeasts (e.g., Aureobasidium, Curvibasidium, and Metschnikowia) have been reported as successful biocontrol agents of Phytophthora pathogens. These microorganisms antagonize Phytophthora spp. via antimicrobial compounds with inhibitory activities against mycelial growth, sporulation, and zoospore germination. They also trigger plant immunity-inducing systemic resistance via several pathways, resulting in enhanced defense responses in their hosts. Along with plant protection, some of the microorganisms promote plant growth, thereby enhancing their beneficial relations with host plants. Although the beneficial effects of the biocontrol microorganisms are acceptable, single applications of antagonistic microorganisms tend to lack consistent efficacy compared with chemical analogues. Therefore, strategies to improve the biocontrol performance of these prominent antagonists are also discussed in this review.

Synthesis of hinokitiol sulfonate derivatives and their anti-oomycete and nematicidal activities

In order to explore novel natural product-based anti-oomycete and nematicidal agents, sixteen unreported 2-sulfonyloxyhinokitiol derivatives were prepared using the principle of active splicing, and structurally confirmed by proton nuclear magnetic resonance (1H NMR), carbon-13 nuclear magnetic resonance (13C NMR), high-resolution mass spectrometry (HRMS), and melting point. Moreover, we evaluated the title compounds as anti-oomycete and nematicidal agents against two serious agricultural pests of Phytophthora capsici and Meloidogyne incongnita. Among the sixteen hinokitiol esters tested: (1) Compounds 3a and 3m exhibited the most potent anti-oomycete activity compared to zoxamide against P. capsici, and the median effective concentration (EC50) values of 3a, 3m, and zoxamide were 18.64, 21.11, and 23.15 mg/L, respectively; Further studies showed that the existence of seven membered ring and carbonyl group was the necessary condition for the high anti-oomycete activity of hinokitiol. (2) Compounds 3n and 3p exhibited more promising nematicidal activity than hinokitiol, and the median lethal concentration (LC50) values of 3n, 3p and 1 against M. incongnita were 0.2111, 0.2079, and 0.3933 mg/L, respectively. This result will pave the way for further modification of hinokitiol to develop potential new fungicides and nematicides.

Synthesis of paeonol ester derivatives and their insecticidal, nematicidal, and anti-oomycete activities

BACKGROUND

Paeonol is extracted and isolated as a rich and sustainable natural bioresource from the root bark of Paeonia suffruticosa, the derivatives of which exhibit numerous biological activities. It is well known that ester compounds play a very important role in pest control, such as organophosphorus, carbamate and pyrethroid pesticides.

RESULTS

To discover biorational natural product-based pesticides, three series of (60) paeonol ester derivatives (7a-t, 8g,p, 9g,p, 10g-j,n-u, 11g,u, 12g,u, 13a-p, 14b,c, and 15b,c) were prepared by structural modification of paeonol, and their structures were well characterized by proton nuclear magnetic resonance (¹ H-NMR), carbon-13 nuclear magnetic resonance (¹³ C-NMR), high-resolution mass spectrometry (HRMS), and melting point. Furthermore, we assessed the compounds as insecticidal, nematicidal, and anti-oomycete agents against three serious agricultural pests, Mythimna separata, Heterodera glycines, and Phytophthora capsici. Among all tested compounds: (i) compound 8p showed more significant insecticidal activity than toosendanin, and the final mortality rates of 8p and toosendanin against M. separata (1 mg mL^-1 ) were 70.4%, and 51.9%, respectively; (ii) compound 7a exhibited more promising nematicidal activity than paeonol, and the median lethal concentration (LC₅₀ ) values of 7a and 1 against H. glycines were 15.47 and 50.80 mg L^-1 , respectively; (iii) compounds 7n and 13m displayed more significant anti-oomycete activity compared to zoxamide against Phytophthora capsici, and the median effective concentration (EC₅₀ ) values of 7n, 13m, and zoxamide were 23.72, 24.51, and 26.87 mg L^-1 , respectively; and the protective effect of the compounds against Phytophthora capsici in vivo further confirmed the effectiveness of the agents.

CONCLUSION

This study suggested that the introduction of a nitro at the C5 or C3 position of paeonol could improve its bioactivity against M. separata, H. glycines, and Phytophthora capsici. © 2022 Society of Chemical Industry.

Discovery of Tropolone Stipitaldehyde as a Potential Agent for Controlling Phytophthora Blight and Its Action Mechanism Research

The fermentation of endophytic Nigrospora chinensis GGY-3 resulted in the isolation of tropolone stipitaldehyde (1), which exhibited broad-spectrum inhibition activity against fungi and bacteria, especially against Phytophthora capsici, with an EC₅₀ value of 0.83 μg/mL and Xanthomonas oryzae pv. oryzicola, with a minimum inhibitory concentration value of 4.0 μg/mL. The in vitro and in vivo assays demonstrated that 1 had a significant protective effect on P. capsici. Furthermore, 1 inhibited the spore germination of P. capsici and damaged the plasma membrane structure. As observed by SEM and TEM, after exposure to 1, mycelia exhibited swelling, shrunken, branch-increasing phenomena, cell wall and membrane damage, and disordered content. Transcriptome analysis revealed that 1 might affect starch and sucrose metabolism and fatty acid biosynthesis by suppressing the expression of genes relevant to cell wall synthetases and cell membrane-associated genes. These findings indicate that 1 may be a potential agrochemical fungicide for controlling phytophthora blight.

Phytophthora capsici Populations Are Structured by Host, Geography, and Fluopicolide Sensitivity

Phytophthora capsici epidemics are propelled by warm temperatures and wet conditions. With temperatures and inland flooding in many locations worldwide expected to rise as a result of global climate change, understanding of population structure can help to inform management of P. capsici in the field and prevent devastating epidemics. Thus, we investigated the effect of host crop, geographical origin, fungicide sensitivity, and mating type on shaping the population structure of P. capsici in the eastern United States. Our fungicide in vitro assays identified the emergence of insensitive isolates for fluopicolide and mefenoxam. A set of 12 microsatellite markers proved informative to assign 157 P. capsici isolates to five distinct genetic clusters. Implementation of Bayesian structure, population differentiation, genetic diversity statistics, and index of association analysis, allowed us to identify population structure by host with some correspondence with genetic clusters for cucumber and squash isolates. We found weak population structure by state for geographically close isolates. In this study, we discovered that North Carolina populations stratify by fluopicolide sensitivity with insensitive isolates experiencing nonrandom mating. Our findings highlight the need for careful monitoring of local field populations, improved selection of relevant isolates for breeding efforts, and hypervigilant surveillance of resistance to different fungicides.

Synthesis and Anti-Oomycete Preliminary Mechanism of Sulfonate Derivatives of Ethyl Maltol

To discover novel molecules with unique mechanism against plant pathogenic oomycetes, sixteen new sulfonate derivatives of ethyl maltol (3a-p) were synthesized by structural modification of 2-ethyl-3-hydroxy-4H-pyran-4-one, and their anti-oomycete activity against a serious agricultural disease, Phytophthora capsici Leonian was determined in this study. Among all tested compounds, derivatives 3e, 3m and 3p exhibited the most potent anti-oomycete activity against P. capsici with EC₅₀ values of 19.40, 21.04 and 31.10 mg/L, respectively; especially 3e and 3m showed the best promising and pronounced anti-oomycete activity than zoxamide (EC₅₀ =26.87 mg/L). The results further proved that 4-tert-butylphenylsulfonyl group, 3-nitro-4-chlorophenylsulfonyl group and 8-quinolinesulfonyl group introduced at the hydroxy position of ethyl maltol or maltol were necessary for obtaining the most potent compounds. Further mechanism studies of P. capsici treated with 3e demonstrated that this compound can affect the growth of mycelia by disrupting the integrity of the membrane, and the higher the concentration of the compound is, the greater the degree of membrane integrity damage. These important results will pave the way for further modification of ethyl maltol to develop potential new fungicides.

iTRAQ proteomic analysis of the inhibitory effect of 1,6-O,O-diacetylbritannilactone on the plant pathogenic oomycete Phytophthora capsici

Phytophthora capsici is a highly destructive oomycete of vegetables; its management is challenging due to its broad host range, rapid dispersion, resilient spores and severe fungicide resistance. Identifying an effective alternative fungicide is important for the control of P. capsici. 1,6-O,O-diacetylbritannilactone (ABLOO), one of the secondary metabolites of Inula Britannica, showed a favorable inhibitory activity against P. capsici at different developmental stages, with a sensitivity order as follows: sporangia formation (30.45 mg/L) > zoospore discharge (77.69 mg/L) > mycelial growth (93.18 mg/L) > cystospore germination (591.48 mg/L). To investigate the mode of action of ABLOO in P. capsici, iTRAQ-based quantitative proteomic analysis was performed by comparing the expression levels of proteins in the control and ABLOO-treated (400 mg/L, inhibition rate of 80%) mycelial groups. A total of 65 downregulated and 75 upregulated proteins were identified in the proteomic analysis. Functional enrichment analyses showed that proteins with transmembrane transport activity were significantly inhibited, while proteins involved in energy production were significantly increased, including proteins involved in ubiquinone and other terpenoid-quinone biosynthesis, oxidative phosphorylation, and glycolysis/gluconeogenesis. The morphological results indicated that ABLOO treatment could decrease the thickness of the cell walls of P. capsici mycelia. Correspondingly, biochemical results showed that ABLOO treatment reduced the β-1,3-glucan contents (the key component of the cell wall of P. capsici) and increased the cell membrane permeability of P. capsici. ABLOO may exhibit antioomycete activity by destroying the cell membrane of P. capsici. This study provides new evidence regarding the inhibitory mechanisms of ABLOO against P. capsici.

Uncoupler SYP-14288 inducing multidrug resistance of Phytophthora capsici through overexpression of cytochrome P450 monooxygenases and P-glycoprotein

BACKGROUND

Fungicide resistance has become a serious problem for different mode of action groups except for uncouplers, which makes their resistance mechanism a hot topic, which until now, has not been well clarified. SYP-14288, a newly developed diarylamine fungicide modeled on fluazinam, has shown good toxicity to both oomycete and fungus by the action of uncoupling. In this research, the resistance of Phytophthora capsici to SYP-14288 was studied to clarify the resistance mechanism of uncouplers.

RESULTS

The toxicity tests of resistant strains against SYP-14288 showed multidrug resistance. The high-performance liquid chromatography (HPLC) results showed that resistant strains could efflux the fungicide, and this ability could be inhibited by the efflux pump inhibitor amitriptyline. The target protein of amitriptyline is P-glycoprotein (P-gp), which was overexpressed in resistant strains. Three products of nitrate reduction of SYP-14288 were detected and determined by HPLC-Q-TOF. Eight cytochrome P450 monooxygenase (P450) proteins were differentially involved in the reduction reaction.

CONCLUSION

Both fungicide efflux and detoxification metabolism were involved in the resistance mechanisms of P. capsici to SYP-14288. © 2022 Society of Chemical Industry.

Genetic diversity and marker trait association for phytophthora resistance in chilli

BACKGROUND

Chilli is an important commercial crop with positive returns tendency. Phytophthora root rot causes drastic damage to chilli plant. Dearth of detecting marker trait associations is a major hinderance in practicing marker assisted selection in chilli breeding.

METHODS AND RESULTS

Herein, 110 chilli accessions were assessed for 15 agronomic traits under control and disease infected conditions for two crop seasons (2018-2019). The SSR genotyping revealed high values of major allele frequency (MAF = 0.70), genetic diversity (GD = 0.39) and Polymorphic Information Content (PIC = 0.31). Principal coordinate analysis and population structure analysis showed distribution of diverse genotypes in all groups by dividing 110 genotypes in three populations and nine sub-populations. The UPGMA based Archaeopteryx tree was in concordance with population structure analysis. Linkage disequilibrium analysis evaluated that LD decays within 3-10 bp. Marker trait association (MTA) revealed the associations of 35 SSRs with 14 morphological traits. The significant MTA for marker CAeMS073 with relative leaf damage (RLD, 0.183 R²) under control and treated conditions was consistently observed in both models. The markers, CAMS173 and CAMS194 were found to be strongly associated with RLD and Disease Index (DI), respectively. The absence of MTA was detected for height of first branch.

CONCLUSION

The MTAs reported in this study can facilitate marker assisted breeding for developing chilli germplasm resistant against Phytophthora capsici.

PcCesA1 is involved in the polar growth, cellulose synthesis, and glycosidic linkage crosslinking in the cell wall of Phytophthora capsici

Phytophthora capsici is a destructive plant pathogen that infects a wide range of hosts worldwide. The P. capsici cell wall, rich in cellulose, is vital for hyphal growth and host interactions. However, the enzymes involved in its synthesis remain largely unelucidated. In the current study, we functionally characterized the cellulose synthase gene PcCesA1, which is highly conserved in Phytophthora. By using CRISPR/Cas9-mediated gene replacement and in situ complementation system, it was found PcCesA1 is essential for the mycelial growth, cystospore germination, and pathogenicity of P. capsici. The normal deposition of newly synthesized cell wall components and the polar growth point formation were disrupted in PcCesA1 knockout mutants, suggesting that PcCesA1 plays an important role in the polar growth of P. capsici. Compared with the wild-type strains, PcCesA1 knockout mutants displayed a thicker inner layer cell wall and were more sensitive to carboxylic acid amide fungicides (CAAs). The contents of the cell wall polysaccharides 1,4-Glc, 1,4,6-Glc, and 1,3,4-Glc were reduced in PcCesA1 knockout mutants, suggesting that PcCesA1 affected cellulose content and glycosidic linkage crosslinking in the cell wall. Our findings demonstrate that PcCesA1 is required for cell wall biogenesis. Therefore, PcCesA1 may be a potential target for Phytophthora disease control.

Comparative Study of Phosphorous-Acid-Containing Products for Managing Phytophthora Blight of Bell Pepper

Phytophthora blight of pepper caused by Phytophthora capsici is a major constraint to bell pepper (Capsicum annuum) production. The long-term effectiveness of chemicals currently in use against P. capsici is uncertain due to the development of fungicide resistance by this pathogen. Hence, the efficacy of alternative chemicals such as phosphorous-acid-containing products was evaluated in this study. In in vitro tests, ProPhyt, K-Phite, Lexx-A-Phos, Agri-Fos, and Nutri-Phite were less effective in inhibiting mycelial growth (EC50 = 50.5 to 324.4 µg mL−1) and sporangium formation (EC50 = 6.1 to 225.7 µg mL−1) of two P. capsici isolates, but more effective against zoospore germination compared with mefenoxam. Among phosphorous-acid-containing products tested, Nutri-Phite was most effective in inhibiting mycelial growth of both P. capsici isolates. In greenhouse studies, Nutri-Phite was effective against Phytophthora blight used as drench. The use of Nutri-Phite, Agri-Fos, ProPhyt, and K-Phite could induce systemic resistance against foliar blight when applied to the root and potting mix. The results indicated that some phosphorous-acid-containing products have the potential to lower disease occurrence and delay Phytophthora blight of bell pepper without phytotoxic effects. The utility of the systemic protection induced by these products is promising in Phytophthora blight management.

Genome-Wide Association Study of Resistance to Phytophthora capsici in the Pepper (Capsicum spp.) Collection

One of the most serious pepper diseases is Phytophthora blight, which is caused by Phytophthora capsici. It is crucial to assess the resistance of pepper genetic resources to Phytophthora blight, understand the genetic resistances, and develop markers for selecting resistant pepper materials in breeding programs. In this study, the resistance of 342 pepper accessions to P. capsici was evaluated. The disease severity score method was used to evaluate the phenotypic responses of pepper accessions inoculated with the KCP7 isolate. A genome-wide association study (GWAS) was performed to identify single nucleotide polymorphisms (SNPs) linked to P. capsici (isolate KCP7) resistance. The pepper population was genotyped using the genotype-by-sequencing (GBS) method, and 45,481 SNPs were obtained. A GWAS analysis was performed using resistance evaluation data and SNP markers. Significantly associated SNPs for P. capsici resistance at 4 weeks after inoculation of the GWAS pepper population were selected. These SNPs for Phytophthora blight resistance were found on all chromosomes except Chr.05, Chr.09, and Chr.11. One of the SNPs found on Chr.02 was converted into a high-resolution melting (HRM) marker, and another marker (QTL5-1) from the previous study was applied to pepper accessions and breeding lines for validation and comparison. This SNP marker was selected because the resistance phenotype and the HRM marker genotype matched well. The selected SNP was named Chr02-1126 and was located at 112 Mb on Chr.02. The Chr02-1126 marker predicted P. capsici resistance with 78.5% accuracy, while the QTL5-1 marker predicted resistance with 80.2% accuracy. Along with the marker for major quantitative traits loci (QTLs) on Chr.05, this Chr02-1126 marker could be used to accurately predict Phytophthora blight resistance in pepper genetic resources. Therefore, this study will assist in the selection of resistant pepper plants in order to breed new phytophthora blight-resistant varieties.

Mecanismos de defensa en plantas de Capsicum contra Phytophthora capsici

Phytophthora capsici es un oomiceto causante de la pudrición de raíz, tallo, frutos y tizón foliar en varias especies vegetales de importancia agrícola, principalmente en Solanáceas del género Capsicum como ají y pimiento. Este fitopatógeno cosmopolita posee mecanismos de ataque que favorecen la rápida infección, colonización y reproducción en huéspedes susceptibles. Contrariamente, estos procesos son retrasados o evitados fuertemente por genotipos resistentes, debido principalmente a sus mecanismos de defensa. En esas interacciones incompatibles, las plantas resistentes de Capsicum reconocen el oomiceto y rápidamente expresan múltiples genes que posteriormente señalizan moléculas, que permiten la acumulación de compuestos fenólicos, fitoalexinas y especies reactivas de oxígeno, la actividad de diferentes enzimas, que pueden permitir incluso la formación de barreras físicas. Esta revisión aborda, expone y discute los avances y el progreso de las investigaciones a lo largo de los ultimos veinte años, referente a los mecanismos de defensa estructurales, bioquimicos y moleculares que utilizan las plantas resistentes de Capsicum para defenderse de P. capsici. Palabras claves. ají, pimiento, pudrición de raíz y corona, tizón foliar, resistencia vegetal

Phytophthora capsici sterol reductase PcDHCR7 has a role in mycelium development and pathogenicity

The de novo biosynthesis of sterols is critical for the majority of eukaryotes; however, some organisms lack this pathway, including most oomycetes. Phytophthora spp. are sterol auxotrophic but, remarkably, have retained a few genes encoding enzymes in the sterol biosynthesis pathway. Here, we show that PcDHCR7, a gene in Phytophthora capsici predicted to encode Δ7-sterol reductase, displays multiple functions. When expressed in Saccharomyces cerevisiae, PcDHCR7 showed the Δ7-sterol reductase activity. Knocking out PcDHCR7 in P. capsici resulted in loss of the capacity to transform ergosterol into brassicasterol, which means PcDHCR7 has the Δ7-sterol reductase activity in P. capsici itself. This enables P. capsici to transform sterols recruited from the environment for better use. The biological characteristics of ΔPcDHCR7 transformants were compared with those of the wild-type strain and a PcDHCR7 complemented transformant, and the results showed that PcDHCR7 plays a key role in mycelium development and pathogenicity of zoospores. Further analysis of the transcriptome indicated that the expression of many genes changed in the ΔPcDHCR7 transformant, which involve in different biological processes. It is possible that P. capsici compensates for the defects caused by the loss of PcDHCR7 by remodelling its transcriptome.

Synthesis and Anti-Oomycete Activity of Sulfonate Derivatives of Fenjuntong

In order to discover highly active fungicides, sixteen novel sulfonate derivatives of Fenjuntong were synthesized by structural modification of 2'-hydroxybutyrophenone, and their anti-oomycete activity against Phytophthora capsici Leonian was determined in this study. Among all tested compounds, compound 3b displayed more significant anti-oomycete activity than the precursor Fenjuntong against P. capsici, and the EC₅₀ values of 3b and Fenjuntong were 84.50 and 517.25 mg/L, respectively. By comparing the anti-oomycete activity of compounds 3a-p, I-a-p, and II-a-p, the following conclusions were drawn: (1) Hydroxy group is well tolerated, and sulfonylation of hydroxy group enhances its anti-oomycete activity. (2) The proper length of the ketone carbonyl chain is very important for their anti-oomycete activity. (3) The presence of a site methoxy group in the structural skeleton is closely related to the anti-oomycete activity. These important results will pave the way for further modification of Fenjuntong to develop potential new fungicides.

Population structure of Phytophthora capsici in the state of Tennessee

The plant pathogen Phytophthora capsici can be found all throughout the USA, and the population genetics of this organism have been studied within many of these states. Until now, no work has been done in the state of Tennessee to investigate the population structure and genetics of P. capsici found there. The population structure of P. capsici was explored using 296 isolates collected from five counties in Tennessee in 2004, 2007, 2018, and 2019. Samples were genotyped using 39 single nucleotide polymorphism (SNP) genetic markers. Multiple analyses indicate that the population structure of P. capsici in Tennessee exists in isolated clusters structured by geography. Geographically separate populations were genetically distinct, suggesting there is limited or no outcrossing among populations, but there is significant sexual reproduction occurring within populations. These findings corroborate previous studies of P. capsici throughout the midwestern and northeastern USA, where populations are generally sexually reproducing and structured by geography. This study provides the first characterization of P. capsici population structure in Tennessee.

Broad Resistance to Post-Harvest Fruit Rot in USVL Watermelon Germplasm Lines to Isolates of Phytophthora capsici Across the United States

Watermelon is an important cucurbit vegetable crop grown in most of the United States. Phytophthora fruit rot of watermelon caused by Phytophthora capsici has been a major factor, limiting production for the past 15 years in the southeastern United States. The U.S. Department of Agriculture, Agricultural Research Service released five Phytophthora fruit rot-resistant germplasm lines for use in breeding programs. These lines were developed by phenotyping using a local isolate of P. capsici from South Carolina. The present study was undertaken to determine if these resistant lines had broad resistance to diverse P. capsici isolates collected from different states and crops. Five resistant germplasm lines (USVL020-PFR, USVL203-PFR, USVL782-PFR, USVL489-PFR, and USVL531-MDR) and two susceptible cultivars, Sugar Baby and Mickey Lee, used as checks were grown in a field in 2014 and 2015 to produce fruit for evaluation. Mature fruit were harvested and placed in a walk-in growth chamber and inoculated with 20 different P. capsici isolates. The chamber was maintained at 26 ± 2°C and high relative humidity (>95%) using a humidifier. All five resistant germplasm lines were significantly more resistant than the two susceptible checks to all 20 P. capsici isolates. Among the five resistant germplasm lines, USVL020-PFR, USVL782-PFR, and USVL531-MDR had broad resistance. Some P. capsici isolates induced minor lesions and rot on USVL489-PFR compared with the other resistant lines. Variation in virulence and genetic diversity among the 20 P. capsici isolates was also observed. The five watermelon germplasm lines will be useful for developing commercial watermelon cultivars with broad resistance to P. capsici.

A G-type lectin receptor-like kinase regulates the perception of oomycete apoplastic expansin-like proteins

Phytophthora capsici is one of the most harmful pathogens in agriculture, which threatens the safe production of multiple crops and causes serious economic losses worldwide. Here, we identified a P. capsici expansin-like protein, PcEXLX1, by liquid chromatography-tandem mass spectrometry from Nicotiana benthamiana apoplastic fluid infected with P. capsici. Clustered regularly interspaced short palindromic repeats/crispr associated protein 9 (CRISPR/Cas9)-mediated PcEXLX1 knockout mutants exhibited significantly enhanced virulence, while the overexpression of PcEXLX1 impaired the virulence. Prokaryotically expressed PcEXLX1 activated multiple plant immune responses, which were BRI1-associated kinase 1 (BAK1)- and suppressor of BIR1-1 (SOBIR1)-dependent. Furthermore, overexpression of PcEXLX1 homologs in N. benthamiana could also increase plant resistance to P. capsici. A G-type lectin receptor-like kinase from N. benthamiana, expansin-regulating kinase 1 (ERK1), was shown to regulate the perception of PcEXLX1 and positively mediate the plant resistance to P. capsici. These results reveal that the expansin-like protein, PcEXLX1, is a novel apoplastic effector with plant immunity-inducing activity of oomycetes, perception of which is regulated by the receptor-like kinase, ERK1.

Multiple mutations in the predicted transmembrane domains of the cellulose synthase 3 (CesA3) of Phytophthora capsici can confer semi-dominant resistance to carboxylic acid amide fungicides

The current study established a clearer understanding of the molecular basis for resistance to carboxylic acid amide (CAA) fungicides. Although four cellulose synthase (CesA) genes were investigated, only F1073L, G1105A, V1109L in CesA3 were found to link to CAA-resistance in Phytophthora capsici. Back-transformation experiments confirmed the role of the three mutations in CAA-resistance. Inheritance studies also confirmed the link and indicated the resistance was semi-dominant with the heterozygous F₁ and F₂ progeny exhibiting intermediate resistance levels compared to the homozygous parents, which was validated by the pyrosequencing results. The semi-dominant nature of CAA-resistance implies that it could be easy for resistance to spread once resistance emerged, being facilitated by both sexual and asexual reproduction. Bioinformatic analysis indicated all mutations occurred in either the first or second of the predicted transmembrane domains at C-terminus of CesA3. Resistant isolates bearing different combinations of mutations were found to exhibit different resistance levels to different CAAs, indicating that each mutation could make different contributions to resistance phenotype depending on structural differences in different CAAs. The current results highlight the complex combinations of mutations and resistance phenotype, and further reinforces the research necessity to completely characterize CAA-resistance to develop appropriate strategies to manage resistance development.

Plant-Growth-Promoting Rhizobacteria Emerging as an Effective Bioinoculant to Improve the Growth, Production, and Stress Tolerance of Vegetable Crops

Vegetable cultivation is a promising economic activity, and vegetable consumption is important for human health due to the high nutritional content of vegetables. Vegetables are rich in vitamins, minerals, dietary fiber, and several phytochemical compounds. However, the production of vegetables is insufficient to meet the demand of the ever-increasing population. Plant-growth-promoting rhizobacteria (PGPR) facilitate the growth and production of vegetable crops by acquiring nutrients, producing phytohormones, and protecting them from various detrimental effects. In this review, we highlight well-developed and cutting-edge findings focusing on the role of a PGPR-based bioinoculant formulation in enhancing vegetable crop production. We also discuss the role of PGPR in promoting vegetable crop growth and resisting the adverse effects arising from various abiotic (drought, salinity, heat, heavy metals) and biotic (fungi, bacteria, nematodes, and insect pests) stresses.

The Plant-Beneficial Rhizobacterium Bacillus velezensis FZB42 Controls the Soybean Pathogen Phytophthora sojae Due to Bacilysin Production

Soybean root rot caused by the oomycete Phytophthora sojae is a serious soilborne disease threatening soybean production in China. Bacillus velezensis FZB42 is a model strain for Gram-positive plant growth-promoting rhizobacteria and is able to produce multiple antibiotics. In this study, we demonstrated that B. velezensis FZB42 can efficiently antagonize P. sojae. The underlying mechanism for the inhibition was then investigated. The FZB42 mutants deficient in the synthesis of lipopeptides (bacillomycin D and fengycin), known to have antifungal activities, and polyketides (bacillaene, difficidin, and macrolactin), known to have antibacterial activities, were not impaired in their antagonism toward P. sojae; in contrast, mutants deficient in bacilysin biosynthesis completely lost their antagonistic activities toward P. sojae, indicating that bacilysin was responsible for the activity. Isolated pure bacilysin confirmed this inference. Bacilysin was previously shown to be antagonistic mainly toward prokaryotic bacteria rather than eukaryotes. Here, we found that bacilysin could severely damage the hyphal structures of P. sojae and lead to the loss of its intracellular contents. A device was invented allowing interactions between P. sojae and B. velezensis FZB42 on nutrient agar. In this manner, the effect of FZB42 on P. sojae was studied by transcriptomics. FZB42 significantly inhibited the expression of P. sojae genes related to growth, macromolecule biosynthesis, pathogenicity, and ribosomes. Among them, the genes for pectate lyase were the most significantly downregulated. Additionally, we showed that bacilysin effectively prevents soybean sprouts from being infected by P. sojae and could antagonize diverse Phytophthora species, such as Phytophthora palmivora, P. melonis, P. capsici, P. litchi, and, most importantly, P. infestans. IMPORTANCEPhytophthora spp. are widespread eukaryotic phytopathogens and often extremely harmful. Phytophthora can infect many types of plants important to agriculture and forestry and thus cause large economic losses. Perhaps due to inappropriate recognition of Phytophthora as a common pathogen in history, research on the biological control of Phytophthora is limited. This study shows that B. velezensis FZB42 can antagonize various Phytophthora species and prevent the infection of soybean seedlings by P. sojae. The antibiotic produced by FZB42, bacilysin, which was already known to have antibacterial effectiveness, is responsible for the inhibitory action against Phytophthora. We further showed that some Phytophthora genes and pathways may be targeted in future biocontrol studies. Therefore, our data provide a basis for the development of new tools for the prevention and control of root and stem rot in soybean and other plant diseases caused by Phytophthora.

The use of phosphonates in agriculture. Chemical, biological properties and legislative issues

The Phosphorus (III) derivatives, named Phosphonates, include congeners with properties as fungicides that are effective in controlling Oomycetes. Examples are organic compounds like Fosetyl-Al [Aluminium tris-(ethylphosphonate)] and salts formed with the anion of phosphonic acid [(OH)₂HPO] and Potassium, Sodium and Ammonium cations. According to IUPAC, the correct nomenclature for these compounds is "phosphonates", but in common language and scientific literature they are often named "phosphites", creating ambiguity. The European legislation restricts the use of phosphonates, with the ban for application in organic agriculture. However, phosphonate residues were detected in some organic products due to their addition to fertilizers allowed in organic agriculture. The legitimacy of this addition is controversial, as it is not evident if phosphonates have also a nutritional role in addition to their fungicidal properties. The new European Directive EU 1009/2019 resolves the problem by banning the phosphonates addition to fertilizers and placing a limit of 0.5% by mass for unintentional addition. However, an official method is not available for phosphonates determination in fertilizers and approval by the European Committee for Standardization (CEN) is necessary in a short time. This review presents an overview about the chemical, biological, analytical and legislative aspects of phosphonates and aims at providing: clarity on the correct nomenclature to avoid misunderstandings; the evaluation of phosphonates properties with the absence of a nutritional role, justifying the ban on adding to fertilizers; a summary of analytical techniques that could be considered by CEN to complete the analytical background for the agricultural use of phosphonates.

A Novel QTL for Resistance to Phytophthora Crown Rot in Squash

Phytophthora capsici Leonian causes significant yield losses in commercial squash (Cucurbita pepo) production worldwide. The deployment of resistant cultivars can complement integrated management practices for P. capsici, but resistant cultivars are currently unavailable for growers. Moderate resistance to Phytophthora crown rot in a selection of accession PI 181761 (C. pepo) (designated line #181761-36P) is controlled by three dominant genes (R4, R5 and R6). Introgression of these loci into elite germplasm through marker-assisted selection (MAS) can accelerate the release of new C. pepo cultivars resistant to crown rot, but these tools are currently unavailable. Here we describe the identification of a quantitative trait locus (QTL), molecular markers and candidate genes associated with crown rot resistance in #181761-36P. Five hundred and twenty-three SNP markers were genotyped in an F₂ (n = 83) population derived from a cross between #181761-36P (R) and Table Queen (S) using targeted genotyping by sequencing. A linkage map (2068.96 cM) consisting of twenty-one linkage groups and an average density of 8.1 markers/cM was developed for the F₂ population. The F₂:₃ families were phenotyped in the greenhouse with a virulent strain of P. capsica, using the spore-spray method. A single QTL (QtlPC-C13) was consistently detected on LG 13 (chromosome 13) across three experiments and explained 17.92-21.47% of phenotypic variation observed in the population. Nine candidate disease resistance gene homologs were found within the confidence interval of QtlPC-C13. Single nucleotide polymorphism (SNP) markers within these genes were converted into Kompetitive Allele Specific PCR (KASP) assays and tested for association with resistance in the F₂ population. One SNP marker (C002686) was significantly associated with resistance to crown rot in the F₂ population (p < 0.05). This marker is a potential target for MAS for crown rot resistance in C. pepo.

Sensitivity of Phytophthora capsici from Tennessee to Mefenoxam, Fluopicolide, Oxathiapiprolin, Dimethomorph, Mandipropamid, and Cyazofamid

Phytophthora blight is a destructive disease caused by the oomycete Phytophthora capsici, which affects vegetable production throughout the state of Tennessee and worldwide. Fungicides are a primary control method used in managing Phytophthora blight, but in some cases the efficacy of these products has been reduced or lost in the field. In 2018 and 2019, the efficacy of six fungicides was tested in vitro on 184 P. capsici isolates collected in Tennessee using radial growth assays. The fungicides included in the study were mefenoxam, fluopicolide, oxathiapiprolin, dimethomorph, mandipropamid, and cyazofamid. Seven isolates were resistant to mefenoxam, 86 were resistant to fluopicolide, one was resistant to oxathiapiprolin, and 13 were resistant to cyazofamid. None were resistant to dimethomorph or mandipropamid. Of the 86 isolates resistant to fluopicolide, five were also resistant to mefenoxam. Resistance to fluopicolide and cyazofamid was widespread in Tennessee, and it was more localized for mefenoxam and oxathiapiprolin. The results of this study show that fungicide resistance is widespread in P. capsici in Tennessee, and the implications for Phytophthora blight management are discussed.