Population Structure and Resistance to Mefenoxam of Phytophthora capsici in New York State

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.

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.

Quantitative Genetic Analysis of Interactions in the Pepper-Phytophthora capsici Pathosystem

The development of pepper cultivars with durable resistance to the oomycete Phytophthora capsici has been challenging due to differential interactions between the species that allow certain pathogen isolates to cause disease on otherwise resistant host genotypes. Currently, little is known about the pathogen genes involved in these interactions. To investigate the genetic basis of P. capsici virulence on individual pepper genotypes, we inoculated sixteen pepper accessions, representing commercial varieties, sources of resistance, and host differentials, with 117 isolates of P. capsici, for a total of 1,864 host-pathogen combinations. Analysis of disease outcomes revealed a significant effect of inter-species genotype-by-genotype interactions, although these interactions were quantitative rather than qualitative in scale. Isolates were classified into five pathogen subpopulations, as determined by their genotypes at over 60,000 single-nucleotide polymorphisms (SNPs). While absolute virulence levels on certain pepper accessions significantly differed between subpopulations, a multivariate phenotype reflecting relative virulence levels on certain pepper genotypes compared with others showed the strongest association with pathogen subpopulation. A genome-wide association study (GWAS) identified four pathogen loci significantly associated with virulence, two of which colocalized with putative RXLR effector genes and another with a polygalacturonase gene cluster. All four loci appeared to represent broad-spectrum virulence genes, as significant SNPs demonstrated consistent effects regardless of the host genotype tested. Host genotype-specific virulence variants in P. capsici may be difficult to map via GWAS with all but excessively large sample sizes, perhaps controlled by genes of small effect or by multiple allelic variants that have arisen independently. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Transcriptome reveals BCAAs biosynthesis pathway is influenced by lovastatin and can act as a potential control target in Phytophthora sojae

AIMS

Lovastatin has been indicated to impair growth and development of Phytophthora sojae. Therefore, this study was performed to understand the inhibitory mechanism of lovastatin and investigate the metabolic pathway potentially serviced as a new control target for this plant pathogen.

METHODS AND RESULTS

Whole transcriptome analysis of lovastatin-treated P. sojae was performed by RNA-sequencing. The results revealed that 84 genes were upregulated and 58 were downregulated with more than four-fold changes under treatment. Kyoto Encyclopedia of Genes and Genomes analysis indicated that the branched-chain amino acids (BCAAs) biosynthesis pathway was abundantly enriched. All enzymes in the BCAAs biosynthesis pathway were identified in the P. sojae genome. Moreover, the study found that the herbicide flumetsulam targeting acetohydroxyacid synthase (AHAS) of the BCAAs biosynthesis pathway could effectively inhibit mycelial growth of P. sojae.

CONCLUSIONS

Lovastatin treatment significantly influences the BCAAs biosynthesis pathway in P. sojae. Moreover, the herbicide flumetsulam targets AHAS and inhibits growth of P. sojae.

SIGNIFICANCE AND IMPACT OF STUDY

The present study revealed that BCAAs biosynthesis pathway was influenced by lovastatin treatment and its key enzyme AHAS was identified as a potential new control target, which provides clues for exploring more oomycides to control plant diseases caused by P. sojae.

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.

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.

Pathotype Complexity and Genetic Characterization of Phytophthora sojae Populations in Illinois, Indiana, Kentucky, and Ohio

Phytophthora sojae, the causal agent of Phytophthora root and stem rot of soybean, has been managed with single Rps genes since the 1960s but has subsequently adapted to many of these resistance genes, rendering them ineffective. The objective of this study was to examine the pathotype and genetic diversity of P. sojae from soil samples across Illinois, Indiana, Kentucky, and Ohio by assessing which Rps genes were still effective and identifying possible population clusters. There were 218 pathotypes identified from 473 P. sojae isolates with an average of 6.7 out of 15 differential soybean lines exhibiting a susceptible response for each isolate. Genetic characterization of 103 P. sojae isolates from across Illinois, Indiana, Kentucky, and Ohio with 19 simple sequence repeat markers identified 92 multilocus genotypes. There was a moderate level of population differentiation between these four states, with pairwise FST values ranging from 0.026 to 0.246. There were also moderate to high levels of differentiation between fields, with pairwise FST values ranging from 0.071 to 0.537. Additionally, cluster analysis detected the presence of P. sojae population structure across neighboring states. The level of pathotype and genetic diversity, in addition to the identification of population clusters, supports the hypothesis of occasional outcrossing events that allow an increase in diversity and the potential to select for a loss in avirulence to specific resistance genes within regions. The trend of suspected gene flow among neighboring fields is expected to be an ongoing issue with current agricultural practices.

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.

Cinnamaldehyde inhibits the growth of Phytophthora capsici through disturbing metabolic homoeostasis

Background

Phytophthora capsici Leonian (P. capsici) can cause wilting and roots rotting on pepper and other cash crops. The new fungicide cinnamaldehyde (CA) has high activity against this pathogen. However, its potential mechanism is still unknown.

Methods

In order to gain insights into the mechanism, isobaric tags for relative and absolute quantification (iTRAQ)-based quantitative proteomics was used to analyze P. capsici treated with CA. The iTRAQ results were evaluated by parallel reaction monitoring (PRM) analysis and quantitative real-time PCR (qRT-PCR) analysis. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis was used to speculate the biochemical pathways that the agent may act on.

Results

The results showed that 1502 differentially expressed proteins were identified, annotated and classified into 209 different terms (like metabolic process, cellular process, single-organism process) based on Gene Ontology (GO) functional enrichment analysis and nine different pathways (glyoxylate and dicarboxylate metabolism, fatty acid metabolism and so on) based on the Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis. This study suggested that CA disordered fatty acid metabolism, polysaccharide metabolism and leucine metabolism. Based on PRM analysis, five proteins including CAMK/CAMK1 protein kinase, glucan 1,3-beta-glucosidase, 1,3-beta-glucanosyltransferase, methylcrotonoyl-CoA carboxylase subunit alpha and isovaleryl-CoA dehydrogenase were down-regulated in P. capsici treated with CA. Furthermore, the qRT-PCR analysis showed that the gene expression level of the interested proteins was consistent with the protein expression level, except for CAMK/CAMK1 protein kinase, acetyl-CoA carboxylase and fatty acid synthase subunit alpha.

Conclusions

CA destroyed the metabolic homoeostasisof P. capsici, which led to cell death. This is the first proteomic analysis of P. capsici treated with CA, which may provide an important information for exploring the mechanism of the fungicide CA against P. capsici.

Activity and Resistance-Related Point Mutations in Target Protein PcORP1 of Fluoxapiprolin in Phytophthora capsici

Fluoxapiprolin is a new piperidinyl thiazole isoxazoline fungicide developed by Bayer Crop Science in 2012, but the sensitivity and resistance mechanism of fluoxapiprolin are unclear. In this study, the sensitivities of 130 Phytophthora capsici isolates to fluoxapiprolin were determined, and a unimodal distribution was observed with a mean EC₅₀ value of 0.00043 μg/mL. Nine stable fluoxapiprolin-resistant mutants were obtained by fungicide adaption. The fitness of mutants was similar to or lower than that of the corresponding parental isolate. Seven heterozygous point mutations in the target protein PcORP1 were found in these mutants. These point mutations were confirmed in PsORP1 of P. sojae homologue positions using the CRISPR/Cas9 system. G770V and N835S+I877F do confer high fluoxapiprolin resistance (resistance factor, RF > 1000), and ΔN835, N767I, and N837T+S910C lead to low resistance (RF < 100). P. sojae transformants containing L733W, S768F, S768Y, ΔG818/F819, N837Y, N837F, P861H, L863W, and I877Y in PsORP1 were also associated with high fluoxapiprolin resistance (RF > 100). In conclusion, the resistance risk of P. capsici to fluoxapiprolin is moderate, and multiple point mutations in PcORP1 could confer different resistance levels to fluoxapiprolin.

A combined BSA-Seq and linkage mapping approach identifies genomic regions associated with Phytophthora root and crown rot resistance in squash

Two QTL mapping approaches were used to identify a total of six QTL associated with Phytophthora root and crown rot resistance in a biparental squash population. Phytophthora root and crown rot, caused by the soilborne oomycete pathogen Phytophthora capsici, leads to severe yield losses in squash (Cucurbita pepo). To identify quantitative trait loci (QTL) involved in resistance to this disease, we crossed a partially resistant squash breeding line with a susceptible zucchini cultivar and evaluated over 13,000 F₂ seedlings in a greenhouse screen. Bulked segregant analysis with whole genome resequencing (BSA-Seq) resulted in the identification of five genomic regions-on chromosomes 4, 5, 8, 12, and 16-featuring significant allele frequency differentiation between susceptible and resistant bulks in each of two independent replicates. In addition, we conducted linkage mapping using a population of 176 F₃ families derived from individually genotyped F₂ individuals. Variation in disease severity among these families was best explained by a four-QTL model, comprising the same loci identified via BSA-Seq on chromosomes 4, 5, and 8 as well as an additional locus on chromosome 19, for a combined total of six QTL identified between both methods. Loci, whether those identified by BSA-Seq or linkage mapping, were of small-to-moderate effect, collectively accounting for 28-35% and individually for 2-10% of the phenotypic variance explained. However, a multiple linear regression model using one marker in each BSA-Seq QTL could predict F_2:3 disease severity with only a slight drop in cross-validation accuracy compared to genomic prediction models using genome-wide markers. These results suggest that marker-assisted selection could be a suitable approach for improving Phytophthora crown and root rot resistance in squash.

C-coordinated O-carboxymethyl chitosan Cu(II) complex exerts antifungal activity by disrupting the cell membrane integrity of Phytophthora capsici Leonian

Damage to the cell membrane is an effective method to prevent drug resistance in plant fungal diseases. Here, we proposed a negative remodeling model of the cell membrane structure induced by the C-coordinated O-carboxymethyl chitosan Cu (II) complex (O-CSLn-Cu). FITC-labeled O-CSLn-Cu (FITC-O-CSLn-Cu) was first synthesized via a nucleophilic substitution reaction and confirmed by FT-IR. FITC-labeled O-CSLn-Cu could pass through the fungal cell membrane, as detected by confocal laser scanning microscopy (CLSM) coupled with fluorescein isothiocyanate (FITC)-fluorescence. O-CSLn-Cu treatment led to apparent morphological changes in the membranes of P. capsici Leonian and giant unilamellar vesicles (GUVs) by transmission electron microscopy (TEM). Then, we performed component analysis of the cell membrane from the P. capsici Leonian affected by O-CSLn-Cu with a particular interest in membrane physicochemical properties. Many unsaturated fatty acids (UFAs) and key enzymes promoting UFA synthesis of the cell membrane were downregulated. Similarly, a large number of membrane proteins responsible for substance transport and biochemical reactions were downregulated. Furthermore, O-CSLn-Cu treatments increased plasma membrane permeability with significant leakage of intercellular electrolytes, soluble proteins and sugars, and lipid peroxidation with decreasing membrane fluidity. Finally, aquaporin 10 was proven to be a potential molecular target sensitive to antimicrobial agents according to composition analysis of membrane structure and immunohistochemistry.

Genome-Wide Association Study in New York Phytophthora capsici Isolates Reveals Loci Involved in Mating Type and Mefenoxam Sensitivity

Phytophthora capsici is a soilborne oomycete plant pathogen that causes severe vegetable crop losses in New York (NY) state and worldwide. This pathogen is difficult to manage, in part due to its production of long-lasting sexual spores and its tendency to quickly evolve fungicide resistance. We single nucleotide polymorphism (SNP) genotyped 252 P. capsici isolates, predominantly from NY, in order to conduct a genome-wide association study for mating type and mefenoxam sensitivity. The population structure and extent of chromosomal copy number variation in this collection of isolates were also characterized. Population structure analyses showed isolates largely clustered by the field site where they were collected, with values of F_ST between pairs of fields ranging from 0.10 to 0.31. Thirty-three isolates were putative aneuploids, demonstrating evidence for having up to four linkage groups present in more than two copies, and an additional two isolates appeared to be genome-wide triploids. Mating type was mapped to a region on scaffold 4, consistent with previous findings, and mefenoxam sensitivity was associated with several SNP markers at a novel locus on scaffold 62. We identified several candidate genes for mefenoxam sensitivity, including a homolog of yeast ribosome synthesis factor Rrp5, but failed to locate near the scaffold 62 locus any subunits of RNA polymerase I, the hypothesized target site of phenylamide fungicides in oomycetes. This work expands our knowledge of the population biology of P. capsici and provides a foundation for functional validation of candidate genes associated with epidemiologically important phenotypes.

Sensitivity of Different Developmental Stages and Resistance Risk Assessment of Phytophthora capsici to Fluopicolide in China

Sensitivities of Phytophthora capsici to fluopicolide were investigated in vitro, with results showing that fluopicolide had strong inhibitory activities on each development stage of P. capsici, in particular on the motility of the zoospore. The potential resistance risk for fluopicolide in P. capsici was evaluated. The baseline sensitivities to fluopicolide of 146 isolates obtained from 28 provinces in China were initially determined, and the 50% inhibition of mycelial growth (EC₅₀) distribution was a unimodal curve with a mean of 0.17 μg/ml. A series of fluopicolide-resistant mutants of P. capsici were obtained by fungicide adaptation, and their biological traits were determined. Most of the resistant mutants showed similar favorable fitness in mycelial growth, sporangium and zoospore production, cystospore germination, and pathogenicity compared with their sensitive parents, with few exceptions. Additionally, the cross-resistance result indicated that the sensitivity of fluopicolide did not correlate with other oomycete fungicides, apart from fluopimomide (LH-2010A). These results suggest a moderate to high resistance risk of P. capsici to fluopicolide in China.

Race Characterization of Phytophthora Root Rot on Capsicum in Taiwan as a Basis for Anticipatory Resistance Breeding

Peppers (Capsicum sp.) are an increasingly important crop because of their use as a vegetable, spice, and food colorant. The oomycete Phytophthora capsici is one of the most devastating pathogens to pepper production worldwide, causing more than $100 million in losses annually. Developing cultivars resistant to P. capsici is challenging because of the many physiological races that exist and new races that are continuously evolving. This problem is confounded by the lack of a universal system of race characterization. As a basis to develop a global anticipatory breeding program, New Mexico recombinant inbred lines (NMRILs) functioned as a host differential for Phytophthora root rot to characterize the race structure of P. capsici populations in Taiwan. Using the NMRILs, 24 new races were identified, illustrating the utility and usefulness of the NMRILs for anticipatory breeding. Virulence of P. capsici was observed to be geographically specific and in two virulence clusters. Interestingly, all but two isolates collected in 2016 were the A2 mating type, which is a shift from the predominantly A1 mating type isolates collected prior to 2008. The NMRILs host differential provides an approach for scientists to work together on a global scale when breeding for resistance as well as on a local level for regional gene deployment. Additionally, we propose that the current race numbering system, which has no biological meaning, be supplemented with the virulence phenotype, based on the susceptible NMRILs to a given isolate. This work provides insights into the population dynamics of P. capsici and interactions within the highly complex Capsicum-Phytophthora pathosystem, and offers a basis for similar research in other crops.

Challenges and Strategies for Breeding Resistance in Capsicum annuum to the Multifarious Pathogen, Phytophthora capsici

Phytophthora capsici is the most devastating pathogen for chile pepper production worldwide and current management strategies are not effective. The population structure of the pathogen is highly variable and few sources of widely applicable host resistance have been identified. Recent genomic advancements in the host and the pathogen provide important insights into the difficulties reported by epidemiological and physiological studies published over the past century. This review highlights important challenges unique to this complex pathosystem and suggests strategies for resistance breeding to help limit losses associated with P. capsici.

Analysis of microsatellites from transcriptome sequences of Phytophthora capsici and applications for population studies

Phytophthora capsici is a devastating oomycete that affects solanaceous, cucurbitaceous, fabaceous, and other crops in the United States (US) and worldwide. The release of the P. capsici genome allows for design of robust markers for genetic studies. We identified and characterized microsatellites in the P. capsici transcriptome. A subset of 50 microsatellites were assayed in a diverse set of P. capsici isolates and evaluated for polymorphism. Polymorphic microsatellites were confirmed by fragment analysis, and 12 were used for population characterization of 50 P. capsici isolates from different states, hosts, and mating types. Analysis of genetic relationship among isolates revealed significant geographic structure by state. Our findings highlight the usefulness of these 12 microsatellites to characterize the population structure of P. capsici and potential transferability to closely-related Phytophthora spp. since markers are located in coding regions. Our markers will facilitate genetic characterization and complement phenotypic studies of P. capsici populations, which may assist in deployment of disease management strategies.

Genotypic Diversity of Phytophthora cinnamomi and P. plurivora in Maryland's Nurseries and Mid-Atlantic Forests

Genetic diversity of two Phytophthora spp.-P. cinnamomi (102 isolates), commonly encountered in Maryland nurseries and forests in the Mid-Atlantic United States, and P. plurivora (186 isolates), a species common in nurseries-was characterized using amplified fragment length polymorphism. Expected heterozygosity and other indices suggested a lower level of diversity among P. cinnamomi than P. plurivora isolates. Hierarchical clustering showed P. cinnamomi isolates separated into four clusters, and two of the largest clusters were closely related, containing 80% of the isolates. In contrast, P. plurivora isolates separated into six clusters, one of which included approximately 40% of the isolates. P. plurivora isolates recovered from the environment (e.g., soil and water) were genotypically more diverse than those found causing lesions. For both species, isolate origin (forest versus nursery or among nurseries) was a significant factor of heterozygosity. Clonal groups existed within P. cinnamomi and P. plurivora and included isolates from both forest and nurseries, suggesting that a pathway from nurseries to forests or vice versa exists.

Temporal Genetic Dynamics of an Experimental, Biparental Field Population of Phytophthora capsici

Defining the contributions of dispersal, reproductive mode, and mating system to the population structure of a pathogenic organism is essential to estimating its evolutionary potential. After introduction of the devastating plant pathogen, Phytophthora capsici, into a grower's field, a lack of aerial spore dispersal restricts migration. Once established, coexistence of both mating types results in formation of overwintering recombinant oospores, engendering persistent pathogen populations. To mimic these conditions, in 2008, we inoculated a field with two P. capsici isolates of opposite mating type. We analyzed pathogenic isolates collected in 2009-2013 from this experimental population, using genome-wide single-nucleotide polymorphism markers. By tracking heterozygosity across years, we show that the population underwent a generational shift; transitioning from exclusively F1 in 2009-2010, to multi-generational in 2011, and ultimately all inbred in 2012-2013. Survival of F1 oospores, characterized by heterozygosity excess, coupled with a low rate of selfing, delayed declines in heterozygosity due to inbreeding and attainment of equilibrium genotypic frequencies. Large allele and haplotype frequency changes in specific genomic regions accompanied the generational shift, representing putative signatures of selection. Finally, we identified an approximately 1.6 Mb region associated with mating type determination, constituting the first detailed genomic analysis of a mating type region (MTR) in Phytophthora. Segregation patterns in the MTR exhibited tropes of sex-linkage, where maintenance of allele frequency differences between isolates of opposite mating type was associated with elevated heterozygosity despite inbreeding. Characterizing the trajectory of this experimental system provides key insights into the processes driving persistent, sexual pathogen populations.

Population Diversity and Sensitivity to Azoxystrobin of Alternaria brassicicola in New York State

Alternaria brassicicola is the causal agent of Alternaria leaf spot, a common disease of brassica crops in New York State. New York isolates of A. brassicicola were collected from a variety of brassica crops and locations to evaluate the population diversity and screen for fungicide sensitivity. Isolates were genotyped for 10 microsatellite loci and assayed for sensitivity to azoxystrobin, a quinone outside inhibitor fungicide. The New York State population of A. brassicicola was found to have high levels of genotypic diversity and the population was found to be in linkage disequilibrium. Based on in vitro assays, the effective concentrations of azoxystrobin reducing spore germination by 50% ranged from 0.22 to 14.12 μg/ml. In order to confirm the sensitivity of 47 isolates to azoxystrobin, the cytb gene was characterized and sequenced to determine whether any of the mutations known to confer resistance to azoxystrobin were present. The mutations F129L, G137R, and G143A were not detected in the isolates studied.

Translocation of Oxathiapiprolin in Bell Pepper Plants and Systemic Protection of Plants Against Phytophthora Blight

Production of bell pepper is seriously affected by Phytophthora capsici, the causal agent of Phytophthora blight. Limited approaches are available for effective management of the disease. Oxathiapiprolin is a fungicide recently registered in the United States that suppressed P. capsici and reduced Phytophthora blight on bell pepper significantly in our previous studies. It is unknown whether oxathiapiprolin translocates in bell pepper plants systemically after application. Experiments were conducted to determine uptake of oxathiapiprolin by bell pepper plants and its systemic movement in the plant. Quantification of oxathiapiprolin in plant tissues was conducted by high-performance liquid chromatography (HPLC) that detected the compound sensitively and selectively. Percentage of recovery of oxathiapiprolin from plant tissues was calculated by comparing the quantities in plant tissues determined by HPLC with known quantities of the compound added to the plant tissues. Recovery rates of oxathiapiprolin from pepper plant tissues ranged from 87.0 to 119.3%. When oxathiapiprolin was applied to roots of bell pepper plants grown in hydroculture, the compound was detected in the root within 4 h and in the cotyledon, first true leaf, and second true leaf within 8 h. It was detectable in the top new leaf 48 h after application to the root. In greenhouse studies with bell pepper plants grown in pots, oxathiapiprolin was applied as a soil drench at 100 and 400 μg/ml. The compound was detected in the root within 3 days and in the stem and first true leaf within 6 days when applied at 100 μg/ml. It was detected in the root, stem, first true leaf, and top new leaf within 3 days when applied at 400 μg/ml. Phytophthora blight on pepper foliage was significantly reduced when oxathiapiprolin was applied as a soil drench at 100 or 400 μg/ml under greenhouse conditions. This is the first report indicating systemic movement of oxathiapiprolin in pepper plants that provides useful information for designing fungicide application programs for effective management of the disease.

Insights into the adaptive response of the plant-pathogenic oomycete Phytophthora capsici to the fungicide flumorph

Phytophthora capsici is an important oomycete plant pathogen that causes significant losses worldwide. The carboxylic acid amide fungicide flumorph has shown excellent activity against oomycete plant pathogens. Despite its potential, there remains concern that the sexual reproduction of oomycete pathogens, which results in genetic recombination, could result in the rapid development of resistance to flumorph. The current study utilized an iTRAQ (isobaric tags for relative and absolute quantitation) based method to compare differences between the proteome of the parental P. capsici isolate PCAS1 and its sexual progeny S2-838, which exhibits significant resistance to flumorph. A total of 2396 individual proteins were identified, of these, 181 were considered to be associated with the adaptive response of P. capsici to flumorph. The subsequent bioinformatic analysis revealed that the adaptive response of P. capsici to flumorph was complex and regulated by multiple mechanisms, including utilising carbohydrate from the host environment to compensate for the cell wall stress induced by flumorph, a shift in energy generation, decreased amino acids biosynthesis, and elevated levels of proteins associated with the pathogen's response to stimulus and transmembrane transport. Moreover, the results of the study provided crucial data that could provide the basis for early monitoring of flumorph resistance in field populations of P. capsici.

Loop-mediated isothermal amplification for detection of the tomato and potato late blight pathogen, Phytophthora infestans

AIMS

To design and validate a colorimetric loop-mediated isothermal amplification assay for rapid detection of Phytophthora infestans DNA.

METHODS AND RESULTS

Two sets of loop-mediated isothermal amplification (LAMP) primers were designed and evaluated for their sensitivity and specificity for P. infestans. ITSII primers targeted a portion of the internal transcribed spacer region of ribosomal DNA. These primers had a limit of detection of 2 pg P. infestans DNA and cross-reacted with the closely related species Phytophthora nicotianae. Rgn86_2 primers, designed to improve assay specificity, targeted a portion of a conserved hypothetical protein. These primers had a limit of detection of 200 pg P. infestans DNA and did not cross-react with P. nicotianae. The specificity of the Rgn86_2 assay was tested further using the closely related species P. andina, P. ipomoeae, P. mirabilis and P. phaseoli. Cross-reactions occurred with P. andina and P. mirabilis, but neither species occurs on tomato or potato. Both primer sets were able to detect P. infestans DNA extracted from tomato late blight leaf lesions.

CONCLUSIONS

Two colorimetric LAMP assays detected P. infestans DNA from pure cultures as well as infected leaf tissue. The ITSII primers had higher sensitivity, and the Rgn86_2 primers had higher specificity.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first report of a LAMP assay for the detection of P. infestans, the causal organism of potato and tomato late blight. These assays have potential for immediate utility in plant disease research and diagnostic laboratories.

Population Structure Among and Within Iowa, Missouri, Ohio, and South Dakota Populations of Phytophthora sojae

Phytophthora root and stem rot, caused by Phytophthora sojae, is an economically important disease of soybean throughout the Midwestern United States. This disease has been successfully managed with resistance (Rps) genes; however, pathogen populations throughout the Midwest have developed virulence to many Rps genes, including those that have not been deployed. To gain a better understanding of the processes that influence P. sojae evolution, the population genetic structure was compared among populations using one isolate collected from 17, 33, and 20 fields in Iowa, Ohio, and South Dakota, respectively, as well as multiple isolates from individual fields in Iowa, Ohio, and Missouri. Genotypic diversity was measured using 21 polymorphic microsatellite (simple-sequence repeat) markers. and pathotype diversity using 15 soybean differentials. For all but three of the populations with low sample size, there was a high level of pathotype diversity and a low to moderate level of genotypic diversity among the populations for both comparisons between states and within-field variation. None of the Rps-gene differentials were resistant to all of the isolates. There were 103 unique multilocus genotypes identified in this study and only 2 were identified from the same field. Although no clones were identified in more than one field, pairwise F_ST indicated that some gene flow within neighboring fields does occur but not across the region, including fields from neighboring states. These results suggest that there is a strong probability that each state may have their own or several regional populations, as well as provide further evidence of high diversity within this homothallic pathogen which may be due, in part, to limited gene flow, mutation, or outcrossing, and this likely affects the success of deployment of resistance.

Phytophthora species recovered from the Connecticut River Valley in Massachusetts, USA

Little is currently known about the assemblage of Phytophthora species in northeastern North America, representing a gap in our understanding of species incidence. Therefore, Phytophthora species were surveyed at 20 sites in Massachusetts, with 16 occurring in the Connecticut River Valley. Many of the sampled waterways were adjacent to active agricultural lands, yet were buffered by mature floodplain forests composed of Acer, Platanus, Populus and Ulmus. Isolates were recovered with three types of baits (rhododendron leaves, pear, green pepper) in 2013 and water filtration in 2014. Overall, 457 isolates of Phytophthora were recovered and based on morphological characters and rDNA internal transcribed spacer (ITS), β-tubulin (β-tub) and cytochrome oxidase c subunit I (cox1) sequences, 18 taxa were identified, including three new species: P. taxon intercalaris, P. taxon caryae and P. taxon pocumtuck. In addition, 49 isolates representing five species of Phytopythium also were identified. Water filtration captured a greater number of taxa (18) compared to leaf and fruit baits (12). Of the three bait types rhododendron leaves yielded the greatest number of isolates and taxa, followed by pear and green pepper, respectively. Despite the proximity to agricultural lands, none of the Phytophthora species baited are considered serious pathogens of vegetable crops in the region. However, many of the recovered species are known woody plant pathogens, including four species in the P. citricola s.l. complex that were identified: P. plurivora, P. citricola III, P. pini and a putative novel species, referred to here as P. taxon caryae. An additional novel species, P. taxon pocumtuck, is a close relative of P. borealis based on cox1 sequences. The results illustrate a high level of Phytophthora species richness in the Connecticut River Valley and that major rivers can serve as a source of inoculum for pathogenic Phytophthora species in the northeast.

Interactions of Phytophthora capsici with Resistant and Susceptible Pepper Roots and Stems

Using host resistance is an important strategy for managing pepper root and crown rot caused by Phytophthora capsici. An isolate of P. capsici constitutively expressing a gene for green fluorescent protein was used to investigate pathogen interactions with roots, crowns, and stems of Phytophthora-susceptible bell pepper 'Red Knight', Phytophthora-resistant bell pepper 'Paladin', and Phytophthora-resistant landrace Criollos de Morelos 334 (CM-334). In this study, the same number of zoospores attached to and germinated on roots of all cultivars 30 and 120 min postinoculation (pi), respectively. At 3 days pi, significantly more secondary roots had necrotic lesions on Red Knight than on Paladin and CM-334 plants. By 4 days pi, necrotic lesions had formed on the taproot of Red Knight but not Paladin or CM-334 plants. Although hyphae were visible in the crowns and stems of all Red Knight plants observed at 4 days pi, hyphae were observed in crowns of only a few Paladin and in no CM-334 plants, and never in stems of either resistant cultivar at 4 days pi. These results improve our understanding of how P. capsici infects plants and may contribute to the use of resistant pepper cultivars for disease management and the development of new cultivars.

The Top 10 oomycete pathogens in molecular plant pathology

Oomycetes form a deep lineage of eukaryotic organisms that includes a large number of plant pathogens which threaten natural and managed ecosystems. We undertook a survey to query the community for their ranking of plant-pathogenic oomycete species based on scientific and economic importance. In total, we received 263 votes from 62 scientists in 15 countries for a total of 33 species. The Top 10 species and their ranking are: (1) Phytophthora infestans; (2, tied) Hyaloperonospora arabidopsidis; (2, tied) Phytophthora ramorum; (4) Phytophthora sojae; (5) Phytophthora capsici; (6) Plasmopara viticola; (7) Phytophthora cinnamomi; (8, tied) Phytophthora parasitica; (8, tied) Pythium ultimum; and (10) Albugo candida. This article provides an introduction to these 10 taxa and a snapshot of current research. We hope that the list will serve as a benchmark for future trends in oomycete research.

Evolution of an Experimental Population of Phytophthora capsici in the Field

Populations of the vegetable pathogen Phytophthora capsici are often highly diverse, with limited gene flow between fields. To investigate the structure of a newly established, experimental population, an uninfested research field was inoculated with two single-zoospore isolates of P. capsici in September 2008. From 2009 through 2012, ≈50 isolates of P. capsici were collected from the field each year and genotyped using five microsatellite loci. The same two isolates were also crossed in the lab. High levels of diversity were detected in the research field, with 26 to 37 unique multilocus genotypes detected each year. Through 2012, genotypic diversity did not decline and no evidence of genetic drift was observed. However, during the 2011 and 2012 growing seasons, four new alleles not present in either parental isolate were observed in the field. Selfing (but not apomixis) was observed at low frequency among in vitro progeny. In addition, evidence for loss of heterozygosity was observed in half of the in vitro progeny. These results suggest that recombination, mutation, and loss of heterozygosity can affect the genetic structure observed in P. capsici populations.

Loss of heterozygosity drives clonal diversity of Phytophthora capsici in China

Phytophthora capsici causes significant loss to pepper (Capsicum annum) in China and our goal was to develop single nucleotide polymorphism (SNP) markers for P. capsici and characterize genetic diversity nationwide. Eighteen isolates of P. capsici from locations worldwide were re-sequenced and candidate nuclear and mitochondrial SNPs identified. From 2006 to 2012, 276 isolates of P. capsici were recovered from 136 locations in 27 provinces and genotyped using 45 nuclear and 2 mitochondrial SNPs. There were two main mitochondrial haplotypes and 95 multi-locus genotypes (MLGs) identified. Genetic diversity was geographically structured with a high level of genotypic diversity in the north and on Hainan Island in the south, suggesting outcrossing contributes to diversity in these areas. The remaining areas of China are dominated by four clonal lineages that share mitochondrial haplotypes, are almost exclusively the A1 or A2 mating type and appear to exhibit extensive diversity based on loss of heterozygosity (LOH). Analysis of SNPs directly from infected peppers confirmed LOH in field populations. One clonal lineage is dominant throughout much of the country. The overall implications for long-lived genetically diverse clonal lineages amidst a widely dispersed sexual population are discussed.

Genetically diverse long-lived clonal lineages of Phytophthora capsici from pepper in Gansu, China

Phytophthora capsici causes significant loss to pepper production in China, and our objective was to investigate the population structure in Gansu province. Between 2007 and 2011, 279 isolates were collected from pepper at 24 locations. Isolates (or subsets) were assessed for simple sequence repeat (SSR) genotype, metalaxyl resistance, mating type, and physiological race using cultivars from the World Vegetable Center (AVRDC) and New Mexico recombinant inbred lines (NMRILs). The A1 and A2 mating types were recovered from nine locations and metalaxyl-resistant isolates from three locations. A total of 104 isolates tested on the AVRDC panel resolved five physiological races. None of 42 isolates tested on the NMRIL panel caused visible infection. SSR genotyping of 127 isolates revealed 59 unique genotypes, with 42 present as singletons and 17 having 2 to 13 isolates. Isolates with identical genotypes were recovered from multiple sites across multiple years and, in many cases, had different race types or metalaxyl sensitivities. Isolates clustered into three groups with each group having almost exclusively the A1 or A2 mating type. Overall it appears long-lived genetically diverse clonal lineages are dispersed across Gansu, outcrossing is rare, and functionally important variation exists within a clonal framework.

Resistance to the novel fungicide pyrimorph in Phytophthora capsici: risk assessment and detection of point mutations in CesA3 that confer resistance

Pyrimorph is a novel fungicide with high activity against the plant pathogen Phytophthora capsici. We investigated the risk that P. capsici can develop resistance to pyrimorph. The baseline sensitivities of 226 P. capsici isolates, tested by mycelial growth inhibition, showed a unimodal distribution with a mean EC(50) value of 1.4261 (± 0.4002) µg/ml. Twelve pyrimorph-resistant mutants were obtained by repeated exposure to pyrimorph in vitro with a frequency of approximately 1 × 10(-4). The resistance factors of the mutants ranged from 10.67 to 56.02. Pyrimorph resistance of the mutants was stable after 10 transfers on pyrimorph-free medium. Fitness in sporulation, cystospore germination, and pathogenicity in the pyrimorph-resistant mutants was similar to or less than that in the parental wild-type isolates. On detached pepper leaves and pepper plants treated with the recommended maximum dose of pyrimorph, however, virulence was greater for mutants with a high level of pyrimorph resistance than for the wild type. The results suggest that the risk of P. capsici developing resistance to pyrimorph is low to moderate. Among mutants with a high level of pyrimorph resistance, EC(50) values for pyrimorph and CAA fungicides flumorph, dimethomorph, and mandipropamid were positively correlated. This indicated that point mutations in cellulose synthase 3 (CesA3) may confer resistance to pyrimorph. Comparison of CesA3 in isolates with a high level of pyrimorph resistance and parental isolates showed that an amino acid change from glutamine to lysine at position 1077 resulted in stable, high resistance in the mutants. Based on the point mutations, an allele-specific PCR method was developed to detect pyrimorph resistance in P. capsici populations.

Using Soil-Applied Fungicides to Manage Phytophthora Crown and Root Rot on Summer Squash

Phytophthora crown and root rot (Phytophthora capsici) of summer squash is especially difficult to manage because all commercial cultivars are highly susceptible to P. capsici. Producers have traditionally relied on foliar fungicide applications to control Phytophthora crown and root rot despite their limited efficacy. Soil fungicide applications, including via subsurface drip chemigation, have recently gained interest as a method of improving control of P. capsici infections. In this study, soil drenches and foliar applications of 11 fungicides were compared for control of Phytophthora crown and root rot of summer squash in replicated field and greenhouse trials. Fungicides were applied at 7-day intervals. Incidence (%) of plant death was assessed from 7 to 42 days post inoculation (dpi) in field trials. Crown rot severity was rated on a scale of 1 (no wilting) to 5 (plant death) from 5 to 21 dpi in greenhouse trials. Results of field and greenhouse trials were similar. Plant death of 'Cougar' following inoculation with P. capsici isolate 12889 occurred at all growth stages from first true-leaf to full maturity in field trials. Plant death 42 dpi differed significantly (P ≤ 0.0001) among fungicides and application methods. The fungicide-application method interaction also was significant. Some fungicides were ineffective regardless of application method. In general, soil drenches were more effective than foliar applications at limiting plant death but no treatment completely controlled disease symptoms. Mean plant death 42 dpi was 41% for soil drenches and 92% for foliar sprays. Drenches of fluopicolide, mandipropamid, or dimethomorph limited plant death to ≤10% and prevented yield loss associated with root and crown rot. Foliar applications generally did not reduce plant death compared with the untreated, inoculated control, and were unable to prevent yield loss in field trials. In greenhouse trials, crown rot severity differed significantly (P ≤ 0.0001) among fungicides, application methods, and cultivars when plants were inoculated with P. capsici isolate 12889 or SP98. Crown rot was less severe and disease progress was slower following soil drenches than foliar applications. Some fungicide treatments were more effective on 'Leopard,' which was less susceptible to P. capsici than 'Cougar.' Soil application methods, including soil drench and drip chemigation, should be evaluated when fungicides are registered for soilborne disease control, because these methods provide better control of Phytophthora crown and root rot than foliar application.

Advances in Research on Phytophthora capsici on Vegetable Crops in The United States

Since L. H. Leonian's first description of Phytophthora capsici as a pathogen of chile pepper in 1922, we have made many advances in our understanding of this pathogen's biology, host range, dissemination, and management. P. capsici causes foliar blighting, damping-off, wilting, and root, stem, and fruit rot of susceptible hosts, and economic losses are experienced annually in vegetable crops including cucurbits and peppers. Symptoms of P. capsici infection may manifest as stunting, girdling, or cankers for some cultivars or crops that are less susceptible. P. capsici continues to be a constraint on production, and implementation of an aggressive integrated management scheme can still result in insufficient control when weather is favorable for disease. Management of diseases caused by P. capsici is currently limited by the long-term survival of the pathogen as oospores in the soil, a wide host range, long-distance movement of the pathogen in surface water used for irrigation, the presence of fungicide-resistant pathogen populations, and a lack of commercially acceptable resistant host varieties. P. capsici can infect a wide range of hosts under laboratory and greenhouse conditions including cultivated crops, ornamentals, and native plants belonging to diverse plant families. As our understanding of P. capsici continues to grow, future research should focus on developing novel and effective solutions to manage this pathogen and prevent economic losses due to the diseases it causes.

An Evaluation of Cucurbits for Susceptibility to Cucurbitaceous and Solanaceous Phytophthora capsici isolates

Cucumber (Cucumis sativus) and squash (Cucurbita spp.) production in Michigan is limited by the oomycete pathogen Phytophthora capsici. Cucumber, summer squash, and winter squash fruit were evaluated for susceptibility to five isolates of P. capsici. Detached fruit were inoculated with a 5-mm-diameter culture plug of mycelia and sporangia and were incubated in a laboratory or greenhouse. Lesion and pathogen growth diameters were measured and pathogen growth density was visually assessed. All P. capsici isolates incited rot, with significant differences found among fruit type and pathogen isolate. Straightneck squash (Cucurbita pepo), slicing cucumber, and butternut squash (C. moschata) exhibited more severe symptoms than the other fruit tested. Summer and winter squash seedlings were evaluated in greenhouse experiments, in which P. capsici-infested millet seed (approximately 1 g) were placed on the surface of soilless potting media. Disease severity was visually assessed every 2 days for 14 days post inoculation. Crop type, pathogen isolate, or the crop type-pathogen isolate interaction term were significant for symptom appearance and area under the disease progress curve values. Differences in susceptibility of butternut squash and zucchini cultivars were observed following inoculation with solanaceous isolate 13351. Results from this study can refine management programs for Phytophthora rot.

Sensitivity of Phytophthora capsici on Vegetable Crops in Georgia to Mandipropamid, Dimethomorph, and Cyazofamid

Phytophthora blight, caused by Phytophthora capsici, is a serious disease in vegetable production, and selective use of fungicides continues to be a significant component of disease management programs. The effect of three chemical compounds-mandipropamid, dimethomorph, and cyazofamid-on asexual stages of P. capsici collected from bell pepper and cucurbits in Georgia was assessed in this study. Forty isolates of P. capsici were determined to be sensitive to mandipropamid and dimethomorph based on mycelial growth, zoospore germination, and sporangial production. Concentrations that were 50% effective (EC₅₀ values) of mandipropamid that inhibited mycelial growth, zoospore germination, and sporangial production of the isolates averaged 0.03, 5.70, and 0.02 μg/ml, respectively. EC₅₀ values of dimethomorph in inhibiting mycelial growth, zoospore germination, and sporangial production averaged 0.24, 0.10, and 0.46 μg/ml, respectively. The majority of isolates were either resistant or intermediately sensitive to cyazofamid at 500 μg/ml or lower concentrations based on mycelial growth or sporangial production, although all the isolates were sensitive to this compound based on zoospore germination, with an average EC₅₀ of 0.04 μg/ml. The results indicated that P. capsici populations in Georgia have not developed resistance to mandipropamid and dimethomorph whereas, for the majority of the isolates, certain asexual stages were resistant to cyazofamid.

The oomycete broad-host-range pathogen Phytophthora capsici

Phytophthora capsici is a highly dynamic and destructive pathogen of vegetables. It attacks all cucurbits, pepper, tomato and eggplant, and, more recently, snap and lima beans. The disease incidence and severity have increased significantly in recent decades and the molecular resources to study this pathogen are growing and now include a reference genome. At the population level, the epidemiology varies according to the geographical location, with populations in South America dominated by clonal reproduction, and populations in the USA and South Africa composed of many unique genotypes in which sexual reproduction is common. Just as the impact of crop loss as a result of P. capsici has increased in recent decades, there has been a similar increase in the development of new tools and resources to study this devastating pathogen. Phytophthora capsici presents an attractive model for understanding broad-host-range oomycetes, the impact of sexual recombination in field populations and the basic mechanisms of Phytophthora virulence.

TAXONOMY

Kingdom Chromista; Phylum Oomycota; Class Oomycetes; Order Peronosporales; Family Peronosporaceae; Genus Phytophthora; Species capsici.

DISEASE SYMPTOMS

Symptoms vary considerably according to the host, plant part infected and environmental conditions. For example, in dry areas (e.g. southwestern USA and southern France), infection on tomato and bell or chilli pepper is generally on the roots and crown, and the infected plants have a distinctive black/brown lesion visible at the soil line (Fig. 1). In areas in which rainfall is more common (e.g. eastern USA), all parts of the plant are infected, including the roots, crown, foliage and fruit (Fig. 1). Root infections cause damping off in seedlings, whereas, in older plants, it is common to see stunted growth, wilting and, eventually, death. For tomatoes, it is common to see significant adventitious root growth just above an infected tap root, and the stunted plants, although severely compromised, may not die. For many cucurbit fruit, the expanding lesions produce fresh sporangia over days (or even weeks depending on the size of the fruit) and the fruit often look as if they have been dipped in white powdered confectioner's sugar (Fig. 1). Generally, hyphae do not emerge from infected plants or fruit (common with Pythium infections) and all that is visible on the surface of an infected plant is sporangia.

IMPORTANCE

Phytophthora capsici presents an oomycete worst-case scenario to growers as it has a broad host range, often produces long-lived dormant sexual spores, has extensive genotypic diversity and has an explosive asexual disease cycle. It is becoming increasingly apparent that novel control strategies are needed to safeguard food production from P. capsici and other oomycetes. Considering that P. capsici is easy to grow, mate and manipulate in the laboratory and infects many plant species, this pathogen is a robust model for investigations, particularly those related to sexual reproduction, host range and virulence.

USEFUL WEBSITES

Phytophthora capsici genome database: http://genome.jgi-psf.org/Phyca11/Phyca11.home.html. Molecular tools to identify Phytophthora isolates: http://phytophthora-id.org.