Resistance to the Novel Fungicide Pyrimorph in Phytophthora capsici: Risk Assessment and Detection of Point Mutations in CesA3 That Confer Resistance

Abstract

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.

Studies on Inhibition of Respiratory Cytochrome bc1 Complex by the Fungicide Pyrimorph Suggest a Novel Inhibitory Mechanism

“…In a recent publication [21], pyrimorph-resistant isolates of P. capsici were selected in the presence of the inhibitor and the three most resistant strains share a common mutation (Q1077K) in the CesA3 gene, which is different from the one (G1104V) selected for mandipropamid resistance [19]. However, it remains to be seen whether transfer of the resistant allele to the sensitive parental strain would make the latter pyrimorph-resistant.…”

Activity of the novel fungicide oxathiapiprolin against plant‐pathogenic oomycetes

“…Zoospore suspensions (1 × 10 5 zoospores mL −1 ) were prepared according to the protocol of a previous study 22 and placed in the wells of a concavity slide in 20 μL aliquots. To determine the effect of oxathiapiprolin on zoospore motility, 20 μL of the fungicide was added to the well to produce a final concentration Pest Manag Sci 2016; 72: 1572-1577 of 1 or 10 μg mL −1 .…”

EFFECT OF MYCORRHIZAE AND ACTINOMYCETES ON GROWTH AND BIOPROTECTION OF Capsicum annuum L. AGAINST Phytophthora capsici

“…Excessive application of biocides, mainly chemical, to combat this disease can cause damage to human health and the environment. In addition, this pathogen oomycete can generate resistance even to new fungi such as the pyrimorph (Pang et al, 2013). Alternative handling of this disease comprises biological control using microorganisms that are antagonistic to the pathogenic agent (Segarra et al, 2013).…”