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 EC50 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, EC50 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.

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.

A total of 1511 isolates of Phytophthora capsici were collected from farms with no history of exposure to the carboxylic acid amide (CAA) fungicides in 32 provinces in China during 2006 to 2013. All 1511 isolates were assayed for mating type and 403 were assayed for sensitivity to dimethomorph (DMM) and metalaxyl. The DMM EC 50 values ranged from 0Á126 to 0Á339 lg mL À1 . Both A1 and A2 mating types were detected on the same farms in four provinces and with a 1:1 ratio. Most isolates were sensitive to metalaxyl but a few exhibited intermediate resistance or resistance to metalaxyl. The segregation of DMM resistance and sensitivity among 337 progeny obtained from hybridization or self-crossing in vitro indicated that the resistance of P. capsici to DMM is controlled by two dominant genes. Eighteen progeny that were derived from hybridization differed in DMM sensitivity and in fitness. Some progeny were as fit as parental isolates. Given the distribution of mating types and therefore the potential for sexual reproduction, the control of resistance by two dominant genes, and the fitness of hybrid progeny, the risk of P. capsici populations developing DMM resistance in China is substantial.

Phytophthora capsici causes significant losses to vegetable production worldwide. Pyrimorph, a new carboxylic acid amide fungicide, has been registered to control P. capsici in China. A mutation (Q1077K) in cellulose synthase 3 has been reported to confer resistance to pyrimorph. In this study, we measured the competition between pyrimorph-resistant and pyrimorph-sensitive isolates of P. capsici. Mixed zoospore suspensions of resistant (R) and sensitive (S) isolates at five ratios (1R:9S, 3R:7S, 5R:5S, 7R:3S, and 9R:1S) were applied to carrot agar in vitro test (with five successive transfers) and to the soil surface around pepper plants in planta test (with 10 successive disease cycles). The proportion of resistant isolates was measured by a conventional assay in which single zoospore isolates recovered after transfers or disease cycles were grown on agar medium with a discriminatory concentration of pyrimorph. The results were then compared with those of a real-time polymerase chain reaction (PCR)-based method developed here, the results were similar. Both assays showed that the competitive ability of the resistant isolates was similar to or less than that of the sensitive isolates. The real-time PCR assay developed will be useful for high-throughput analysis and monitoring the development of pyrimorph resistance in field populations of P. capsici.