Frequency of Alternaria genotypes resistant to SDHI fungicides in California pistachio orchards determined by real-time PCR.

Real-time PCR methods were developed to quantify the frequency of SDHC-H134R and SDHB-H277Y mutants associated with SDHI resistance in Alternaria populations from pistachio. The linearity of the standard curves demonstrated the applicability in the quantification of the assays. The accuracy and reliability of the qPCR protocols to determine the frequency of mutants in real samples were corroborated. Orchards visibly affected by Alternaria Late Blight were sampled. The frequency of mutants were determined using the qPCR assays, while the frequency of resistant phenotypes was determined using a single discriminatory dose. The statistical analysis showed that the frequencies of the mutation SDHC-H134R determined with the qPCR assay were highly correlated with those estimated with the conventional method. The survey also evidenced that resistance to boscalid is still widespread in California. Results also indicated the possible contribution of other mutations to SDHI resistance. Our results confirmed the prevalence of SDHC-H134R mutants and the occurrence of mutation SDHB-H277Y at low frequencies. The real-time PCR methods developed in this study were able to detect differences in the frequencies of resistant mutants caused by the use of chemical fungicides. Finally, the effects of two fungicide programs on the frequency of mutants resistant to SDHI and QoI fungicides were studied using qPCR assays. The experiments demonstrated that the use of AP and DMI fungicides in the same program reduced the frequency of these mutations in Alternaria populations. The qPCR methods developed and used in this study can be used to track resistance levels in the pistachio orchards on a large scale.