Constraining Evolution of Alternaria alternata Resistance to a Demethylation Inhibitor (DMI) Fungicide Difenoconazole

Biocontrol Potential of Streptomyces odonnellii SZF-179 toward Alternaria alternata to Control Pear Black Spot Disease

Pear black spot disease, caused by Alternaria alternata, is a devastating disease in pears and leads to enormous economic losses worldwide. In this investigation, we isolated a Streptomyces odonnellii SZF-179 from the rhizosphere soil of pear plants in China. Indoor confrontation experiments results showed that both SZF-179 and its aseptic filtrate had excellent inhibitory effects against A. alternata. Afterwards, the main antifungal compound of SZF-179 was identified as polyene, with thermal and pH stability in the environment. A microscopic examination of A. alternata mycelium showed severe morphological abnormalities caused by SZF-179. Protective studies showed that SZF-179 fermentation broth could significantly reduce the diameter of the necrotic lesions on pear leaves by 42.25%. Furthermore, the potential of fermentation broth as a foliar treatment to control black leaf spot was also evaluated. Disease indexes of 'Hosui' and 'Wonwhang' pear plants treated with SZF-179 fermentation broth were lower than that of control plants. Overall, SZF-179 is expected to be developed into a safe and broad-spectrum biocontrol agent. No studies to date have evaluated the utility of S. odonnellii for the control of pear black spot disease; our study fills this research gap. Collectively, our findings provide new insights that will aid the control of pear black spot disease, as well as future studies of S. odonnellii strains.

Three point mutations in AaCYP51 combined with induced overexpression of AaCYP51 conferred low-level resistance to mefentrifluconazole in Alternaria alternata

Tomato early blight is a significant disease that causes substantial losses to tomato yield and quality. Mefentrifluconazole, an isopropanol-azole subgroup of triazole fungicides, has been registered in China for controlling various plant diseases, including tomato early blight, grape anthracnose, and apple brown spot. However, limited information is available on the mefentrifluconazole resistance risk and mechanism in plant pathogens. The sensitivity to mefentrifluconazole of 122 isolates of Alternaria alternata, one of the causal agents of tomato early blight, collected from different provinces in China, was evaluated. The results showed a unimodal curve for the sensitivity frequency, with an average EC50 of 0.306 μg/mL. Through fungicide adaption, six resistant mutants (N4, N5, T4, T5, NG1, and NG10) were obtained from three parental isolates, with a mutation frequency of 3.28 × 10-4 and resistance factors ranging between 19 and 147. The survival fitness of the resistant mutants, except for NG1, was significantly lower than that of their parental isolates. Positive cross-resistance was observed between mefentrifluconazole and difenoconazole or fenbuconazole, whereas no cross-resistance was found with three non-DMI fungicides. Furthermore, three distinct point mutations were detected in the AaCYP51 protein of the resistant mutants: I300S in T4 and T5; A303T in N4, NG1, and NG10; and A303V in N5. Compared to the parental isolates, the AaCYP51 gene was overexpressed in all six resistant mutants when treated with mefentrifluconazole. In summary, the resistance risk of A. alternata to mefentrifluconazole was low, and point mutations and overexpression of the AaCYP51 gene were identified as contributing factors to mefentrifluconazole resistance in A. alternata.

Mechanisms of Alternaria pathogenesis in animals and plants

Alternaria species are cosmopolitan fungi darkly pigmented by melanin that infect numerous plant species causing economically important agricultural spoilage of various food crops. Alternaria spp. also infect animals, being described as entomopathogenic fungi but also infecting warm-blooded animals, including humans. Their clinical importance in human health, as infection agents, lay in the growing number of immunocompromised patients. Moreover, Alternaria spp. are considered some of the most abundant and potent sources of airborne sensitizer allergens causing allergic respiratory diseases, as severe asthma. Among the numerous strategies deployed by Alternaria spp. to attack their hosts, the production of toxins, carrying critical concerns to public health as food contaminant, and the production of hydrolytic enzymes such as proteases, can be highlighted. Alternaria proteases also trigger allergic symptoms in individuals with fungal sensitization, acting as allergens and facilitating antigen access to the host subepithelium. Here, we review the current knowledge about the mechanisms of Alternaria pathogenesis in plants and animals, the strategies used by Alternaria to cope with the host defenses, and the involvement Alternaria allergens and mechanisms of sensitization.

Helichrysum stoechas (L.) Moench Inflorescence Extract for Tomato Disease Management

Helichrysum stoechas is a singular halophyte that has been shown to have anti-inflammatory, antioxidant, and allelopathic properties. In the work presented herein, we have characterized its inflorescences hydromethanolic extract and assessed its antifungal activity for the pre- and postharvest management of tomato crop diseases. Gas chromatography-mass spectrometry characterization of the extract showed that 4-ethenyl-1,3-benzenediol, 2,3-dihydro-benzofuran, quinic acid, 3,5-dihydroxy-6,7,8-trimethoxy-2-phenyl-4H-1-benzopyran-4-one, 1,6-anhydro-β-D-glucopyranose, catechol, scopoletin, and maltol were the main constituents. The co-occurrence of pyranones, benzenediols, and quinic acids as phytoconstituents of H. stoechas extract resulted in promising in vitro minimum inhibitory concentrations of 500, 375, 500, 187.5, 187.5, and 375 μg·mL-1 against mycelia of Alternaria alternata, Colletotrichum coccodes, Fusarium oxysporum f. sp. lycopersici, Rhizoctonia solani, Sclerotinia sclerotiorum, and Verticillium dahliae, respectively. Further, to assess the potential of H. stoechas inflorescence extract for postharvest tomato crop protection, ex situ tests were conducted against C. coccodes, obtaining high protection at a dose of 750 μg·mL-1. Taking into consideration that the demonstrated activity is among the highest reported to date for plant extracts and comparable to that of the synthetic fungicides tested as positive controls, H. stoechas inflorescence extract may be put forward as a promising biorational and may deserve further testing in field-scale studies.

Biogenic synthesis, characterization, and evaluation of synthesized nanoparticles against the pathogenic fungus Alternaria solani

In the present study, Trichoderma harzianum culture filtrate (CF) was used as a reducing and capping agent to synthesize silver nanoparticles (Ag NPs) in a quick, simple, cost-effective, and eco-friendly manner. The effects of different ratios (silver nitrate (AgNO3): CF), pH, and incubation time on the synthesis of Ag NPs were also examined. Ultraviolet-visible (UV-Vis) spectra of the synthesized Ag NPs showed a distinct surface plasmon resonance (SPR) peak at 420 nm. Spherical and monodisperse NPs were observed using scanning electron microscopy (SEM). Elemental silver (Ag) was identified in the Ag area peak indicated by energy dispersive x-ray (EDX) spectroscopy. The crystallinity of Ag NPs was confirmed by x-ray diffraction (XRD), and Fourier transform infrared (FTIR) was used to examine the functional groups present in the CF. Dynamic light scattering (DLS) revealed an average size (43.68 nm), which was reported to be stable for 4 months. Atomic force microscopy (AFM) was used to confirm surface morphology. We also investigated the in vitro antifungal efficacy of biosynthesized Ag NPs against Alternaria solani, which demonstrated a significant inhibitory effect on mycelial growth and spore germination. Additionally, microscopic investigation revealed that Ag NP-treated mycelia exhibited defects and collapsed. Apart from this investigation, Ag NPs were also tested in an epiphytic environment against A. solani. Ag NPs were found to be capable of managing early blight disease based on field trial findings. The maximum percentage of early blight disease inhibition by NPs was observed at 40 parts per million (ppm) (60.27%), followed by 20 ppm (58.68%), whereas in the case of the fungicide mancozeb (1,000 ppm), the inhibition was recorded at 61.54%.

Resistance risk assessment of Fusarium pseudograminearum from wheat to prothioconazole

Fusarium crown rot (FCR), primarily caused by Fusarium pseudograminearum, poses significant threats to cereal crops worldwide. Prothioconazole is a demethylation inhibitor (DMI) fungicide used to control FCR. However, the risk of resistance in F. pseudograminearum to prothioconazole has not yet been evaluated. In this study, the sensitivity of a total of 255 F. pseudograminearum strains obtained from Henan Province, China to prothioconazole were determined by the mycelial growth inhibition. The results showed that the effective concentration to 50% growth inhibition (EC₅₀) of these strains ranged from 0.4228 μg/mL to 2.5284 μg/mL, with a mean EC₅₀ value of 1.0692 ± 0.4527 μg/mL (mean ± SD). Thirty prothioconazole-resistant mutants were obtained out of six selected sensitive parental strains by means of fungicide taming. The resistant mutants exhibited defects in vegetative growth, conidia production, and pathogenicity on wheat seedlings compared to their parental strains. Under ion, cell wall, and temperature stress conditions but not osmotic stress, all the mutants exhibited decreased growth rates compared with their parental strains, which was consistent with the control treatment. Cross-resistance test showed that there was a cross-resistance relationship between prothioconazole and four DMI fungicides, including prochloraz, metconazole, tebuconazole and hexaconazole, but no cross-resistance was observed between prothioconazole and carbendazim, phenamacril, fludioxonil, or azoxystrobin. Although no site mutation occurred on Cyp51a and Cyp51b genes, the constitutive expression level of the Cyp51a gene was significantly increased in all mutants. After being treated with prothioconazole, the Cyp51a and Cyp51b genes were significantly increased in both the resistant mutants and their parents. These results suggested that the resistance to prothioconazole of the mutants may be attributed to the changes of the relative expression level of Cyp51a and Cyp51b genes. Taken together, these results could provide a theoretical basis for the scientific use of prothioconazole in the field and fungicide resistance management strategies.

Farmers' perceptions on tomato early blight, fungicide use factors and awareness of fungicide resistance: Insights from a field survey in Kenya

Early blight (EB) caused by Alternaria solani is one of the most devastating tomato diseases in Kenya and is most often managed by application of synthetic fungicides. However, there have been reports from farmers about the declining efficacy of some fungicides. These reports suggest that A. solani populations in Kenya could be developing resistance to some of the commonly used fungicides. In this study, we surveyed 175 tomato fields, sampled in 3 major tomato producing counties in Kenya, to determine the status of EB, management practices, and fungicide use factors that could contribute to development of resistance to fungicides among A. solani populations in Kenya. Data was recorded on farm characteristics, EB prevalence, fungicide usage, and farmers' perceptions on fungicide efficacy. EB was prevalent in 85% of the fields and 90% of the farmers identified it as a major cause of yield loss. Tomato was grown all year round on 60% of the fields with only short fallow periods. All farmers reported that they were relying on fungicides for EB control and none among the cultivars grown was resistant to the disease. A total of 40 fungicide products, representing 20 active compounds with varying FRAC resistance risk levels were in use against EB. Majority (83%) of the farmers were applying fungicides at dosages and frequencies higher than those indicated on labels. Most farmers (81%) indicated that they had observed declines in effectiveness of at least one fungicide, used at EB control. This observation was more with fungicides in the strobilurin and triazole groups. These findings demonstrate that the current tomato production systems in Kenya do not take into account the risk of A. solani developing resistance to fungicides. Enhancing farmers' knowledge of the disease and their ability to properly select and apply fungicides is therefore crucial for effective control of EB and mitigating the high risk of fungicide resistance build up.

Natural compounds derived from Brassicaceae plants as an alternative to synthetic fungicides and their influence on soil fungus diversity

BACKGROUND

The study aimed to develop a new formulation based on active substances of natural origin to protect plant seedlings against fungal pathogens, and to evaluate the effect of this formulation on fungal communities in arable soil.

RESULTS

Coating seeds of common crop plants with a p-coumaric acid (p-CA)-based preparation resulted in a significant reduction in the growth of most of the tested pathogens. When applied to soil, both the p-CA-based formulation and Porter 250 EC had a similar overall effect on soil fungal communities and significantly altered the structure of fungal communities at all of the times examined. Shifts in the fungal community composition concerned less than 2% of the total number of amplicon sequence variants (ASVs). The strongest impact of the formulations on soil microbiota was recorded at the fourth week of treatment. Two ASVs assigned to Botrytis and Chromelosporium, known as plant pathogens, and an unidentified ASV from Diversisporales encompassing the arbuscular mycorrhizal fungi (AMF), were significantly depleted in soil samples treated with p-CA in comparison with Porter 250 EC.

CONCLUSION

The p-CA-based preparation has the potential to be used as an alternative to synthetic fungicides. It shows a similar effect to Porter 250 EC on the organization of soil communities, determining changes in the character of the communities of fungi in general, at any given time. Moreover, p-CA caused a reduction in ASVs belonging to Botrytis and Chromelosporium (plant pathogens) and ASVs of Diversisporales (containing arbuscular mycorrhizal fungi) in comparison with the commercial compound that was analyzed. © 2022 Society of Chemical Industry.

Genetic Structure and Triazole Antifungal Susceptibilities of Alternaria alternata from Greenhouses in Kunming, China

Alternaria alternata is an opportunistic human fungal pathogen and a ubiquitous phytopathogen capable of causing diseases to >100 agricultural crops and ornamental plants. To control plant diseases caused by A. alternata, triazole fungicides have been widely used both in open crop and vegetable fields and in indoor growth facilities such as greenhouses. At present, the effect of fungicide use on triazole resistance development in A. alternata populations is not known. Here, we isolated 237 A. alternata strains from nine greenhouses around metropolitan Kunming in Yunnan, southwest China, determined their genotypes using 10 short tandem repeat markers, and quantified their susceptibility to four triazoles (difenoconazole, tebuconazole, itraconazole, and voriconazole). Abundant allelic and genotypic diversities were detected among these A. alternata strains. Significantly, over 17% of the strains were resistant to difenoconazole, and both known and new drug-resistance mutations were found in the triazole target gene cyp51. Our findings of high-level genetic variation of A. alternata in greenhouses coupled with high-frequency fungicide resistance call for greater attention to continued monitoring and to developing alternative plant fungal disease management strategies in greenhouses. IMPORTANCE Alternaria alternata is among the most common fungi in our environments, such as indoor facilities, the soil, and outdoor air. It can cause diseases in >100 crop and ornamental plants. Furthermore, it can cause human infections. However, our understanding of its genetic diversity and antifungal susceptibility is very limited. Indeed, the critical threshold values for resistance have not been defined for most antifungal drugs in this species. Greenhouses are known to have heavy applications of agricultural fungicides. In this study, we analyzed strains of A. alternata from nine greenhouses near metropolitan Kunming in southwestern China. Our study revealed very high genetic diversity and identified strains with high MIC values against two agricultural and two medical triazole antifungals within each of the nine greenhouses. Our study calls for greater attention to this emerging threat to food security and human health.

Detection and Biological Characteristics of Alternaria alternata Resistant to Difenoconazole from Paris polyphylla var. chinensis, an Indigenous Medicinal Herb

Black spot caused by Alternaria alternata (BSAA) is one of the most common diseases of Paris polyphylla var. chinensis, causing yield losses in China. Demethylation inhibitors (DMIs) have been used to control this disease in China for decades. Some farmers have complained about the decreased efficacy of DMIs against BSAA. The objective of this study was to detect and characterize the resistance of A. alternata against difenoconazole from P. polyphylla var. chinensis during 2018. Of the 22 isolates of A. alternata obtained from Sichuan Province in the southwest of China, 20 were resistant to difenoconazole. Mycelial growth rates and sporulation of the difenoconazole-resistant (Dfn^R) isolates were not different from those of the difenoconazole-sensitive (Dfn^S) isolates. No cross resistance between difenoconazole and tebuconazole or propiconazole was observed. Mutations were identified at gene AaCYP51 of Dfn^R isolates based on the sequence alignment of the Dfn^R and Dfn^S isolates. All of the mutations could be divided into three resistant genotypes, I (K715R + Y781C), II (K715R + D1140G + T1628A), and III (no mutation). The docking total score of the Dfn^S isolates was 5.6020, higher than the resistant genotype I (4.4599) or the resistant genotype II (3.8651), suggesting that the DMI resistance of A. alternata may be caused by the decreased affinity between AaCYP51 and difenoconazole.

Site-directed mutagenesis of the succinate dehydrogenase subunits B and D from Corynespora cassiicola reveals different fitness costs and sensitivities to succinate dehydrogenase inhibitors

Carboxamide fungicides target succinate dehydrogenase (SDH). Recently published monitoring studies have shown that Corynespora cassiicola isolates are resistant to one or several SDH inhibitors (SDHIs) with amino acid substitutions in the SDH B and D subunits. We confirmed, by site-directed mutagenesis of the sdhB and sdhD genes, that each of the mutations identified in the field strains of C. cassiicola conferred resistance to boscalid and, in some cases, cross-resistance to other SDHIs (fluopyram, carboxin and penthiopyrad). Analyses of the enzyme activity and sdhB and sdhD gene expression show that modifications (SdhB_H278Y and SdhD_H105R) that result in a decline in SDH enzyme activity may be complemented by gene overexpression. The SdhB_H278Y, SdhB_I280V and SdhD_H105R mutants suffered large fitness penalties based on their biological properties, including conidia production and germination, mycelial growth, pathogenicity or survival abilities under environment stress. However, fitness cost was not found in the SdhB_H278R, SdhD_D95E and SdhD_G109V mutants. In the evaluation of resistance to boscalid in 2018 and 2019, the frequency of the SdhD_D95E and SdhD_G109V genotypes in the Liaoning and Shandong provinces changed dramatically compared with 2005-2017, from low resistance frequency (0.53% for D95E and 2.53% for G109V) to dominant resistance frequency (17.28% for D95E and 15.38% for G109V). Considering both the fitness and increased frequency of these genotypes, we may infer that the SdhD_D95E and SdhD_G109V mutants will be the dominant resistance mutants in field.

Cross-resistance to the new fungicide mefentrifluconazole in DMI-resistant fungal pathogens

In the European Union (EU), regulation of sterol demethylation inhibiting (DMI) fungicides is tightened due to their suspected endocrine disrupting properties. However, the new DMI fungicide mefentrifluconazole was reported to have high fungicidal activity with minimal adverse side effects. In addition, some evidence suggests inconsistent cross resistance between mefentrifluconazole and other azoles. In this study, mefentrifluconazole and other triazoles were examined for activity to select pathogens sensitive or resistant to DMIs using mycelial growth tests on fungicide-treated culture medium or spray trials using cucumber plants. Cross-resistance was confirmed for all of the fungal species tested but activity levels varied. The sensitivity of Monilinia fructicola from peach to mefentrifluconazole was higher compared to other DMIs. In contrast, the inhibitory activity of mefentrifluconazole was equal or slightly inferior compared to difenoconazole, tebuconazole, propiconazole in Colletotrichum spp., Alternaria alternaria sp. complex and Cercospora beticola isolated from peach and sugar beet, respectively. Similar tendencies (i.e. equal or slightly inferior activity and cross-resistance) were observed for cucumber powdery mildew (Podosphaera xanthii) resistant to triflumizole, myclobutanil, and difenoconazole. Despite cross-resistance to other DMIs, mefentrifluconazole is a promising fungicide for fungal disease control on peach and other crops, with a reportedly more favorable toxicity profile.

Induced expression of CYP51 associated with difenoconazole resistance in the pathogenic Alternaria sect. on potato in China

BACKGROUND

Early blight caused by Alternaria spp. is amongst the most important diseases in potato. Demethylation inhibitor (DMI) fungicides are widely used to control the disease but long-term use may decrease its control efficacy due to fungicide resistance. This study investigated the occurrence of difenoconazole resistance in Alternaria spp. and molecular resistant mechanisms.

RESULTS

EC₅₀ values of 160 isolates to difenoconazole ranged from 0.026 μg mL^-1 to 15.506 μg mL^-1 and the frequency of difenoconazole sensitivity formed a non-normal distribution curve with a major and a minor peak. Isolates with EC₅₀ values of 4.121 and 5.461 μg mL^-1 were not controlled effectively at fungicide doses of 50 and 100 μg mL^-1 . Cross-resistance was observed between DMI fungicides difenoconazole and propiconazole, but not between difenoconazole and other fungicide groups, including boscalid, iprodione, or carbendazim. The CYP51gene was 1673 bp encoding 525 amino acids in length and contained two introns. All sensitive and resistant isolates had the identical amino acid sequence of CYP51, with the exception of one resistant isolate carrying a mutation of R511W. A 6 bp insertion in the upstream region was observed in half of the resistant isolates. In the absence of propiconazole, the relative expression of CYP51 was not significantly different in sensitive and resistant isolates. In the presence of difenoconazole, expression of CYP51 gene was induced significantly in the DMI-resistant isolates but not in the sensitive ones.

CONCLUSION

Induced expression of CYP51 in resistant isolates exposed to difenoconazole is an important determinant for DMI resistance in potato pathogens Alternaria sect. © 2019 Society of Chemical Industry.