Development of fungicide cross resistance in Helminthosporium solani Populations from California

Helminthosporium solani populations on potato from the Tulelake area in California frequently have been exposed to thiophanate-methyl but not benomyl. Assessment of three H. solani populations found that 182 of 238 isolates were resistant to both thiophanate-methyl and benomyl and 56 isolates were sensitive to both fungicides. None of the isolates exhibited a differential reaction to the two fungicides. The effective concentrations that reduced growth by 50% for resistant isolates of H. solani were approximately 400 mg/liter for thiophanate-methyl and 35 mg/liter for benomyl. Two point mutations were detected in the beta-tubulin gene from H. solani isolates from California; however, only the mutation at the 455-nucleotide position (codon 198) was strongly associated with thiophanate-methyl resistance. This mutation led to a change of glutamic acid to alanine at codon 198. Independent of fungicide resistance, a mutation at the 836-site was detected in all California isolates. A polymerase chain reaction (PCR)-based restriction fragment length polymorphism (RFLP) assay was used to identify thiophanate-methyl resistance in H. solani; resistant isolates exhibited two DNA fragments of 390 and 482 bp, whereas sensitive isolates had three fragments of 62, 390, and 420 bp. Every isolate assigned either as resistant or sensitive by the PCR-based RFLP assay correlated 100% with the results of the assay on fungicide-amended medium.