Isolates Of Phytophthora Capsici Research Articles

In silico analysis to understand genetic variability, phylogenetic and phylogeographic relationships between Phytophthora capsici isolates infecting various crops

Phytophthora capsici is a major oomycete-based pathogen causing enormous damage to the productivity of peppers and other major crops from different families worldwide by causing fruit rot, root rot and foliar blight diseases. This study utilized molecular signatures present within ITS1-5.8S rRNA-ITS2 and Ras-related GTP binding protein (Ypt1) genes to evaluate haplotype variability and phylogenetic relationships among P. capsici isolates with significant epidemiological implications. Twelve different evolutionary lineages specific to particular hosts and/or geographical regions and differed by SNPs, and 30 haplotypes with maximum diversity in India and Pakistan, were identified by sequence analysis of the ITS regions. It includes country-specific lineages Pc-I and Pc-II in India with multiple host ranges; capsicum-specific lineages Pc-IV, Pc-V and Pc-VIII from Pakistan; papaya-specific lineages Pc-VI from China, and watermelon-specific lineages Pc-VII and Pc-XI from USA. Many lineages showed cross-border infections, such as Pc-III and Pc-X in India and Pakistan, Cocoa-specific lineage Pc-IX in Brazil and Cote d’Ivoire, and Capsicum-specific multi-continental lineage Pc-XII. Similarly, 11 different SNP lineages and 32 haplotypes with maximum diversity in China were identified for the Ypt1 gene. These findings would be very useful for epidemiological surveillance of P. capsici strains worldwide, understanding population and evolutionary dynamics, developing lineage- and haplotype-specific diagnostic assays and disease-resistant varieties.

Report of Phytophthora capsici (mating type A1) isolates associated with crown‐rot disease of Capsicum annuum in Guatemala

AbstractWilting and crown‐rot symptoms were observed in sweet pepper (Capsicum annuum) plants in field surveys carried out in 2021 across major vegetable‐producing areas of three municipalities of Guatemala. Phytophthora capsici (Peronosporales, Peronosporaceae) isolates were identified in association with these symptoms based upon morphological and phylogenetic analyses using nucleotide sequence information of the Cox2 mitochondrial DNA (mtDNA) region. Biological tests confirmed P. capsici mating type A1 isolates as the causal agents of the disease. Symptoms similar to those observed in the field were obtained after controlled inoculation of C. annuum ‘Tico’ seedlings. Solanum habrochaites ‘LA 404’ and C. annuum ‘Criollo de Morelos’ displayed strong resistance reaction against all Guatemalan isolates. Even though P. capsici is a cosmopolitan pathogen, this is the first molecular confirmation of this pathogen infecting C. annuum in Guatemala.

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.

Morphological characterization of Phytophthora capsici isolates from Jalisco and Michoacán, Mexico

&lt;em&gt;Phytophthora capsici&lt;/em&gt; es el principal fitopatógeno del cultivo de chile (&lt;em&gt;Capsicum annuum&lt;/em&gt;) y de diversas plantas de interés comercial en México. El desconocimiento por parte de los productores sobre la presencia de este patógeno en zonas de cultivo dificulta la prevención y manejo de la enfermedad. El objetivo de este trabajo fue identificar y caracterizar morfológicamente aislados obtenidos de cultivos de cucurbitáceas y solanáceas con síntomas de “marchitez” en Jalisco y Michoacán, México. Los muestreos se realizaron durante 2016 y 2017. Los 41 aislados de &lt;em&gt;P. capsici&lt;/em&gt; obtenidos de plantas enfermas se analizaron por morfología comparativa con base a caracteres sexuales y asexuales. Se caracterizaron 33 aislamientos del cultivo de&lt;em&gt; C. annuum&lt;/em&gt;, seis de &lt;em&gt;C. pepo&lt;/em&gt; y dos de &lt;em&gt;S. lycopersicum&lt;/em&gt;. La mayoría de los aislados presentaron características morfológicas típicas de&lt;em&gt; P. capsici&lt;/em&gt;. Solo un aislado presentó clamidosporas, globosas y terminales (aislado de Queréndaro, Mich). Se registraron 40 aislados heterotálicos y un aislado homotálico. Se determinó la patogenicidad de siete aislados, asociando a&lt;em&gt; P. capsici&lt;/em&gt; como el agente causal de la marchitez para estos aislados y para siete analizados en un estudio previo. Los 27 aislamientos restantes solo se asociaron a la enfermedad. Se sugiere realizar un manejo integrado de este patógeno en las zonas de producción de Jalisco y Michoacán.

Occurrence of stem blight and fruit rot caused by Phytophthora capsici on Chinese cucumber (Trichosanthes kirilowii) in China.

Chinese cucumber, Trichosanthes kirilowii Maxim, is a perennial liana plant belonging to the Cucurbitaceae family and is an important traditional medicine in Chinese herbalism. The root, fruit and seed possess the medicinal value, and also the seeds are edible (Zhang et al. 2019). With increasing demand, the wild resource was domesticated and has been planted in China. Diseases of T. kirilowii have become more prominent with the expansion of cultivated area and have caused the yield reduction (Zhang et al. 2014). The general disease field has caused a yield reduction of 10% -30%, even up to 80% seriously. Since 2017, fields in Luan city, Anhui province exhibited 10 to 30% of plants with stem blight and fruit rot. The two-week seedlings were infected at the basal part of stem and showed water-soaking, then damping off. In older plants, it is common to see stem blight with brown-to-black lesions, stunted growth, and most diseased plants eventual death. On rot fruit, the symptom of water soaked lesions was firstly observed, and then with white mold, eventual rot. More than 60 samples of symptomatic stems or fruit were collected from Luan and Hefei, Anhui province during April to October 2017. The symptomatic tissues were washed, disinfested with 0.5% sodium hypochlorite for 30 s, rinsed twice in sterile water for 1 min, dried and incubated on V8-juice amended with 50μg ml-1 of ampicillin and rifampicin at 25°C. Based on morphological characteristics, more than 80% of the total 98 isolates generated were similar on the level of morphology, and preliminarily identified as Phytophthora species (Erwin and Ribeiro 1996). Three isolates were selected randomly to further observe and identify up to species level. The isolates produced abundant, aerial, white mycelia on V8 agar plates. Sporangia were produced on sporangiophores in 10% V8 liquid culture medium after 4 days at 25°C, mainly obovoid with one papilla, 38.8 to 50.4 μm× 22.8 to 33.5 μm in size. The single mature sporangium immersed in water quickly released 20 to 40 biflagellate motile zoospores. The isolates were further confirmed by amplification and sequencing of two conserved markers, the internal transcribed spacer (ITS) region and cytochrome c oxidase subunit I (COI) region with primers ITS1/ITS4 (White et al. 1990) and OomCoxILevup/Fm85mod (Robideau et al. 2011), respectively. The GenBank Accession Nos. were MN368092, and MN369544 for ITS and COI, respectively. The BLAST search results showed 100% similarity with ITS sequences (KF700090, KC438376) and COI sequences (MH136864, AY129166) of Phytophthora capsici isolates in Genbank. Koch's postulates were performed by testing the pathogenicity of the sequenced isolate on the Chinese cucumber (cv. 'Wanlou9'). On 1-month-old plants, a flap of bark was cut with a sterile scalpel, and the 2×2 cm plug of 5-day-old mycelium was inserted. The flap was then closed and sealed with Parafilm. The agar plug was treated in the same manner as the inoculated plants as the controls. And also, the intact fruit (one month old) was inoculated with 10 μL of zoospore suspension (2 × 105 zoospore/ml) and kept in growth chamber at 25 °C, with 80% relative humidity, and the control was treated with 10 μL of sterile distilled water. Three days after inoculation, the stems and the fruit inoculated with mycelium and zoospores showed water-soaked lesions. After 10 days, the symptoms on the tissues resembled those observed in the field. No symptoms were detected on the controls. P. capsici was reisolated from the diseased tissues but not from the control. This combination of data confirmed that the pathogen was P. capsici. To our knowledge, this is the first report of P. capsici causing stem blight and fruit rot on Chinese cucumber in China.

Dynamic Extreme Aneuploidy (DEA) in the vegetable pathogen Phytophthora capsici and the potential for rapid asexual evolution.

Oomycete plant pathogens are difficult to control and routine genetic research is challenging. A major problem is instability of isolates. Here we characterize >600 field and single zoospore isolates of Phytophthora capsici for inheritance of mating type, sensitivity to mefenoxam, chromosome copy number and heterozygous allele frequencies. The A2 mating type was highly unstable with 26% of 241 A2 isolates remaining A2. The A1 mating type was stable. Isolates intermediately resistant to mefenoxam produced fully resistant single-spore progeny. Sensitive isolates remained fully sensitive. Genome re-sequencing of single zoospore isolates revealed extreme aneuploidy; a phenomenon dubbed Dynamic Extreme Aneuploidy (DEA). DEA is characterized by the asexual inheritance of diverse intra-genomic combinations of chromosomal ploidy ranging from 2N to 3N and heterozygous allele frequencies that do not strictly correspond to ploidy. Isolates sectoring on agar media showed dramatically altered heterozygous allele frequencies. DEA can explain the rapid increase of advantageous alleles (e.g. drug resistance), mating type switches and copy neutral loss of heterozygosity (LOH). Although the mechanisms driving DEA are unknown, it can play an important role in adaptation and evolution and seriously hinders all aspects of P. capsici research.

Resistance of Capsicum annuum genotypes against various isolates of Phytophthora capsici from Java, Indonesia

Abstract. Wartono, Wiyono S, Syukur M, Giyanto, Lestari P. 2019. Resistance of Capsicum annuum genotypes against various isolates of Phytophthora capsici from Java, Indonesia. Biodiversitas 20: 3723-3730. Phytophthora capsici Leonian is one of the pathogenic oomycetes that can reduce chilli production worldwide. In this study, Indonesian local chilli genotypes were evaluated for their resistance to P.capsici on chilli in Java, under controlled conditions. P. capsici used in this study consisted of four isolates derived from chilli plants in Java Island. The chilli genotypes tested were 38 local chilli (Capsicum annuum L.) originated from Indonesia and 3 introduced C. annuum. Inoculation was carried out by drenching 5 ml suspension of zoospore (1 x 104) on the growing medium suitable for the oomycetes around the plant. Disease symptom was measured at 21 days after inoculation. Salicylic acid and b-1.3-glucanase analyses were conducted to 7-week old plant with specific interval. Molecular characterization to complement disease assay was carried out using SCAR primers (OpD04,717-F/D04,717-R). Significant differences were found in genotypes, isolates, and interactions between chilli genotypes x P. capsici isolates. Four tested isolates had different levels of virulence ranging from moderate (CpnsCK1, KdrRM3, WnsbCk) to high (WnsbCK2). The chilli genotypes revealed high partial resistance to P. capsici, as demonstrated by six genotypes resistant to CpnsCK1, two genotypes to KdrRM3, and one genotype resistant to WnsbCK1. Resistance of one of chilli pepper, Ungara was positively associated with an increase of salicylic acid and b-1.3-glucanase. There is no association between the SCAR marker with resistance of chilli to P. capsici suggested these pathogens from Java are probably controlled by another locus which is not detected by this SCAR marker.

Genetic Analysis of Resistance to Multiple Isolates of Phytophthora capsici and Linkage to Horticultural Traits in Bell Pepper

Phytophthora capsici is one of the major pathogens found in pepper production, especially in bell pepper. Due to the high level of genetic diversity of the pathogen, bell pepper varieties with broad genetic resistance are essential for disease management. Criollo de Morelos – 334 (CM334), a landrace that has a high level of genetic resistance to P. capsici, has been used as the resistant source for P. capsici to generate a recombinant inbred line (RIL) population with the susceptible bell pepper cultivar Maor. From the resulting population, quantitative trait locus (QTL) models explaining resistance to each of four isolates of P. capsici were derived from QTL regions on three chromosomes using stepwiseqtl in R/qtl. A single region of chromosome 5 contained major QTL for resistance to each of the four isolates. Two isolate-specific QTL conferring resistance to isolates PWB53 and PWB106 were located on chromosomes 10 and 11, respectively. Both isolate-specific QTL had epistatic interactions with a major QTL on chromosome 5. Using the pepper reference genome and gene annotation, candidate genes for P. capsici resistance within 1.5-logarithm of odds (LOD) interval were identified. Based on functional annotations derived from Arabidopsis thaliana and solanaceous crop databases, multiple candidate genes related to resistance (R) gene complexes or to plant immune system were found under the QTL on all three chromosomes. A comparison of the locations of resistance QTL and previously identified horticultural QTL using the same population revealed tight linkage between resistance to P. capsici and a stem pubescence QTL o chromosome 10. Both candidate genes for P. capsici resistance and the linkages between resistance and horticultural traits could be applied for selection to broad resistance to P. capsici in bell pepper–breeding programs.

Pathogenic and biological characterization of Phytophthora capsici isolates from zucchini and pepper in Southeast Spain

In this work, isolates from pepper and zucchini were tested for pathogenicity on crops commonly cultivated in Mediterranean greenhouses. Also, morphological and growing features and mating types have been studied to find any differences linked with the plant origin and pathogenicity of each isolate. Three isolates were highly pathogenic on all cucurbitaceous and solanaceous hosts tested and caused root rot and wilting. Eggplant and pepper were moderately susceptible, while zucchini, tomato and cucumber were highly susceptible. No root rot symptoms occurred on the fabaceous plants inoculated with Phytophthora capsici, including bean and pea. Moreover, the pathogen was never reisolated from the roots of fabaceous plants. Other isolate of P. capsici was less pathogenic on cucurbitaceous and solanaceous crops, causing only slight root damages. None of the isolates tested produced chlamydospores, and all belonged to A1 mating type. These findings suggest that beans or peas may be a feasible alternative crop for those Mediterranean greenhouses with a history of root rot due to P. capsici.

Baseline Sensitivity and Control Efficacy of Tetramycin Against Phytophthora capsici Isolates in China.

Tetramycin is a new biopesticide that combines high-level and broad-spectrum fungicidal activity, low toxicity, and environmental safety. In this study, 90 Phytophthora capsici isolates obtained from various regions in southern China were characterized for their baseline sensitivity to tetramycin. The protective and curative activities of tetramycin against P. capsici were determined on leaves of pepper, and the control efficacy of tetramycin in greenhouse experiments was also determined. Compared with mycelial growth, the formation of sporangia and the discharge of zoospores were inhibited by lower concentrations of tetramycin, approximately 5 µg ml-1 on V8 media. The frequency distribution curves for the tetramycin sensitivity were unimodal, with mean values for the fungicide concentration that reduced mycelial growth, sporangia formation, and zoospore discharge by 50% compared with the control of 1.18 ± 0.91, 0.64 ± 0.42, and 0.63 ± 0.30 µg ml-1, respectively. In addition, no correlation was observed between tetramycin and other fungicides tested, including mandipropamid, azoxystrobin, mefenoxam, fluazinam, fluopicolide, and famoxadone. Tetramycin exhibited both protective and curative effects against P. capsici in vitro, and its protective activity was better than its curative activity. In greenhouse experiments, tetramycin concentration of 60 and 90 µg ml-1 provided a protective control efficacy of 47.1 to 56.4% and curative efficacy of 43.3 to 52.7%. These results demonstrated that tetramycin could serve as an excellent alternative fungicide to control Phytophthora blight of pepper.

Reaction of Solanum (section Lycopersicon) germplasm to Phytophthora capsici

The use of resistant cultivars is one of the most efficient strategies for reducing the amount of pesticides in agricultural crops. Several species of the Solanaceae and Cucurbitaceae families are susceptible to diseases caused by Phytophthora capsici isolates. Even though sources of resistance have been identified in other host plants, reports of resistance to P. capsici in cultivated and wild tomato species [genus Solanum (section Lycopersicon)] germplasm are yet scarce. The main objective of the present work was to characterize the reaction of 244 Solanum (Lycopersicon) accessions to P. capsici under controlled greenhouse experiments. In the first set of experiments, tomato seedlings were inoculated at the collar area with a 3 mL of a spore suspension (2 × 104 zoospores mL−1). Disease incidence (dead plants/total plants) was assessed 14 days after inoculation. The accessions were separated in five well-defined reaction groups: highly resistant (HR), resistant (R), moderately susceptible (MR), susceptible (S), and highly susceptible (HS). The reaction of a group of the ten accessions with the highest levels of resistance was then evaluated against a collection of five P. capsici isolates. Differential reaction was observed among accessions and isolates. The resistant response was host species-dependent with susceptibility being more often found in S. peruvianum accessions, whereas sources of resistance were identified more frequently among accessions of the cultivated tomato (S. lycopersicum).

Evaluation of a Diverse, Worldwide Collection of Wild, Cultivated, and Landrace Pepper (Capsicum annuum) for Resistance to Phytophthora Fruit Rot, Genetic Diversity, and Population Structure.

Pepper is the third most important solanaceous crop in the United States and fourth most important worldwide. To identify sources of resistance for commercial breeding, 170 pepper genotypes from five continents and 45 countries were evaluated for Phytophthora fruit rot resistance using two isolates of Phytophthora capsici. Genetic diversity and population structure were assessed on a subset of 157 genotypes using 23 polymorphic simple sequence repeats. Partial resistance and isolate-specific interactions were identified in the population at both 3 and 5 days postinoculation (dpi). Plant introductions (PIs) 640833 and 566811 were the most resistant lines evaluated at 5 dpi to isolates 12889 and OP97, with mean lesion areas less than Criollo de Morelos. Genetic diversity was moderate (0.44) in the population. The program STRUCTURE inferred four genetic clusters with moderate to very great differentiation among clusters. Most lines evaluated were susceptible or moderately susceptible at 5 dpi, and no lines evaluated were completely resistant to Phytophthora fruit rot. Significant population structure was detected when pepper varieties were grouped by predefined categories of disease resistance, continent, and country of origin. Moderately resistant or resistant PIs to both isolates of P. capsici at 5 dpi were in genetic clusters one and two.

Genetic diversity, population structure, and resistance to Phytophthora capsici of a worldwide collection of eggplant germplasm.

Eggplant (Solanum melongena L.) is an important solanaceous crop with high phenotypic diversity and moderate genotypic diversity. Ninety-nine genotypes of eggplant germplasm (species (S. melongena, S. incanum, S. linnaeanum and S. gilo), landraces and heirloom cultivars) from 32 countries and five continents were evaluated for genetic diversity, population structure, fruit shape, and disease resistance to Phytophthora fruit rot. Fruits from each line were measured for fruit shape and evaluated for resistance to two Phytophthora capsici isolates seven days post inoculation. Only one accession (PI 413784) was completely resistant to both isolates evaluated. Partial resistance to Phytophthora fruit rot was found in accessions from all four eggplant species evaluated in this study. Genetic diversity and population structure were assessed using 22 polymorphic simple sequence repeats (SSRs). The polymorphism information content (PIC) for the population was moderate (0.49) in the population. Genetic analyses using the program STRUCTURE indicated the existence of four genetic clusters within the eggplant collection. Population structure was detected when eggplant lines were grouped by species, continent of origin, country of origin, fruit shape and disease resistance.

Resistance of Chili Pepper Cultivars to Isolates of Phytophthora capsici

Resistance of one hundred commercialized cultivars of chili pepper to four isolates of Phytophthora capsici was evaluated under controlled environmental conditions. The cultivars are commercialized as resistant (59%) and susceptible (41%) to Phytophthora blight in Korea. Mean disease severities of the cultivars on P. capsici MY-1, KPC-1, JHAI1-7, and KPC-7 isolates were 37, 55, 60, and 74%, respectively. In addition, 38 for MY-1, 48 for KPC-1, 56 for JHAI1-7, and 76 cultivars for KPC-7 showed susceptibility. To P. capsici MY-1, the weakest pathogenicity isolate among them, 59 cultivars represented high resistance. By contrast, only six cultivars showed high resistance to P. capsici KPC-7, the strongest isolate. Furthermore, resistance of most cultivars except for three cultivars was negatively correlated with the virulence of P. capsici isolates. And isolate-specific resistance of the chili pepper cultivars could not be found. Among them, six cultivars showing resistance to all the tested isolates were selected for further study. The development of Phytophthora blight on the six cultivars according to inoculum density ( to sporangia/pot) and incubation temperature (25 to ) after inoculation of P. capsici was tested. Resistance of the cultivars to P. capsici KPC-1 and JHAI1-7, moderately pathogenic isolates, was hardly affected. But to KPC-7 isolate, the highly resistant cultivars showed susceptiblility or moderate resistance when the seedlings were inoculated with inoculum density of sporangia/pot and incubated at 28 to . From these results, it is likely that resistance of chili pepper cultivars to Phytophthora blight is affected by the virulence of P. capsici isolate.

Cross-infectivity and genetic variation of Phytophthora capsici isolates from chilli and black pepper in Vietnam

Phytophthora capsici is an economically significant pathogen of various important annual and perennial crops in temperate and tropical regions. Not much is known about the genetic diversity of this pathogen worldwide. In Vietnam it is the causal agent of devastating diseases on several hosts, including chilli and black pepper. Crossinfectivity of P. capsici isolates obtained from the two hosts was demonstrated. Analysis of forty-six P. capsici isolates based on Random Amplified Microsatellites (RAMS) and Repetitive Extragenic Palindromic (REP) fingerprinting revealed that isolates from black pepper were genetically distinct from isolates recovered from chilli. Twenty-two isolates from chilli clustered into two clonal groups at a DICE similarity level of >85%, whereas twenty-four isolates from black pepper were separated from these chilli isolates at a similarity level of <50%. In general the genetic diversity among isolates of P. capsici from black pepper was greater than that of the chilli isolates. The current study indicated that the P. capsici population infecting chilli and black pepper in Vietnam consists of two separate genetic strains, adapted to chilli and black pepper, despite their morphological similarity and host cross-infectivity. The implications of these findings are discussed.

Genetic diversity of Phytophthora capsici (Pythiaceae) isolates in Anhui Province of China based on ISSR-PCR markers

Phytophthora capsici is a plant pathogenic oomycete that damages numerous crops worldwide. Consequently, interest in research on the genetic structure of this species has grown in recent decades. However, there is little information about P. capsici in eastern China. We investigated the genetic diversity of P. capsici isolates from three large regions of Anhui Province in eastern China based on ISSR-PCR technology. Thirteen random primers were screened and used to amplify DNA from 51 samples. We obtained 158 reproducible ISSR fragments, of which 90% were polymorphic, revealing a high degree of polymorphism among the isolates. Genetic similarity coefficients among all the isolates ranged from 0.56 to 0.94, with a mean of 0.84 based on the ISSR data, indicating a high level of genetic variation in these P. capsici isolates. Cluster analysis using UPGMA indicated that the Anhui isolates were divided into seven groups according to the DNA fingerprints, although there was no correlation between the ISSR group and geographic origin. Isolates from the same location showed no clustering based on the year of sampling. AMOVA partitioned variability among (13.6%) and within populations (86.4%). The gene flow among populations ranged from 2.804 to 4.937, with a mean of 3.545, indicating highly frequent gene exchange. Genetic distances and genetic differentiation were negatively correlated with geographic distances. These results lead us to suggest that this pathogen has considerable evolutionary potential, which will enable it to adapt to and overcome management strategies over time.

Evaluation of phytophthora root rot-resistant &amp;lt;i&amp;gt;Capsicum annuum&amp;lt;/i&amp;gt; accessions for resistance to phytophthora foliar blight and phytophthora stem blight

A mixture of six Georgia isolates of Phytophthora capsici (Leon.), the causal agent of phytophthora blight, were used for greenhouse mass screening of over 700 accessions of Capsicum annuum for both stem blight and foliar blight. From this screening, it was determined that resistance to both forms of the disease were relatively common in the germplasm, but resistance to one form of the disease was not strongly correlated to resistance to the other form. Ten accessions previously shown to possess root rot resistance were tested for resistance to stem rot and leaf blight, and were found to also be highly resistant to these forms of the disease. It appears that single accessions have resistance to foliar, stem and root rot caused by P. capsici, which may simplify breeding for resistance to all three forms of the disease.

Genetic diversity of Phytophthora capsici isolates from pepper and pumpkin in Argentina

Phytophthora capsici is a soilborne oomycete plant pathogen that limits pepper production worldwide. The population structure varies significantly depending on the location (e.g. Peru vs. USA) and little is known about the diversity of P. capsici in Argentina. Our objective was to assess the diversity of P. capsici in Argentina at key pepper production areas. Forty isolates were recovered 2006–2009 from pepper and one isolate from pumpkin at 11 locations. Isolates were assessed for mating type, mefenoxam sensitivity and multilocus single nucleotide polymorphism (SNP) genotype profiles. Ten isolates with identical SNP profiles also were genotyped with amplified fragment length polymorphism (AFLP) markers. All 41 isolates had the A1 mating type and were sensitive to mefenoxam. Genotypic analysis using eight polymorphic SNP markers indicated 87% of the isolates had the same multilocus genotype, which is fixed for heterozygosity at seven of the eight SNP sites. AFLP analyses confirmed these findings, and overall it appears that clonal reproduction drives the population structure of P. capsici in Argentina. The implications for breeding resistant peppers and overall disease management are discussed.

Differences in virulence of Phytophthora capsici isolates from a worldwide collection on host fruits

Phytophthora capsici causes root, crown, and fruit rot of vegetable and tropical hosts. Cucumber, zucchini, tomato, and pepper fruits were inoculated using 6-mm-diameter agar plugs of P. capsici, incubated in clear plastic boxes at room temperature (25 ± 2°C and 100% relative humidity), and virulence was estimated by measuring the lesion diameter, pathogen growth diameter, and pathogen sporulation density three (cucumber, zucchini) or four (tomato, pepper) days later. When isolates were grouped by genetic cluster, significant differences in virulence were observed on cucumber and zucchini, with isolates belonging to genetic cluster five causing larger lesions than isolates from genetic cluster six. On tomato, no significant differences were observed for isolates grouped by genetic cluster, but isolates from vegetable crops were generally more virulent than isolates from tropical hosts. Isolates from fabaceous hosts sporulated better on cucumber fruits than isolates from solanaceous hosts. Isolates from vegetable hosts sporulated better on zucchini than isolates from tropical hosts. No significant differences in lesion diameter were noted on pepper when isolates were grouped by host family of origin or genetic cluster, but differences in pathogen sporulation were apparent by host family. Our findings suggest that isolate characteristics such as host family of origin and genetic cluster membership may be used to guide initial isolate selection for cucurbit fruit resistance screening. Final isolate selection should incorporate the phenotypic and genetic diversity of P. capsici, including isolates with differing virulence to the host organ of interest.

Biological and genetic characterization of Phytophthora capsici mutants resistant to flumorph

Baseline sensitivity to flumorph, a carboxylic acid amide (CAA) fungicide used to control some oomycetes, was examined using 83 Phytophthora capsici isolates, resulting in a unimodal distribution of effective concentration for 50% inhibition of mycelial growth ranging from 0·716 to 1·363, with a mean of 1·033 ± 0·129 μg mL−1. To assess the potential risk of developing flumorph resistance, 13 flumorph‐resistant mutants of P. capsici were obtained using ultraviolet irradiation. Most of these mutants and their progeny had high levels of fitness, including mycelial growth, sporulation and virulence. The resistance to flumorph changed slightly, either increasing or decreasing, after 10 transfers on agar media. Cross‐resistance was found between flumorph and other CAA fungicides (dimethomorph and iprovalicarb), but not between flumorph and non‐CAA fungicides (cymoxanil, metalaxyl, azoxystrobin and cyazofamid). To investigate the genetics of the flumorph resistance, 619 progeny were obtained by self‐crossing and sexual hybridization. Segregation of sensitivity to fungicide was measured as a ratio of sensitive (S) to resistant (R) isolates. Segregation of the progeny, from self‐crossed isolate PCAS1 (flumorph resistant), was 1:15 in the first generation; and 0:1 or 1:15 in the second generation. In sexual hybridization, segregation of progeny was 0:1 and 1:7 for R × R hybridization; and 1:3 for R × S hybridization. Therefore, the resistance of P. capsici against flumorph was controlled by two dominant genes.