Pyrichalasin H production and pathogenicity of Digitaria-specific isolates of Pyricularia grisea

SUMMARY Culture filtrates from 72 isolates of Pyricularia, grouped into 13 rDNA types, were analysed via HPLC. Of these isolates, 31 (r9 DNA type) from crabgrass (Digitaria sanguinalis), one (r9 DNA type) from pangolagrass (Digitaria smutsii) and six (r8 DNA type) from Digitaria horizontalis produced 20-280 microg pyrichalasin H per millilitre of culture. These same isolates were pathogenic on five Digitaria species. Interestingly, two isolates, KM-1 and Br 29, which were originally isolated from Digitaria plants, did not produce pyrichalasin H, nor caused blast lesion on Digitaria plants. Because these two isolates were identified as Digitaria pathogens by PCR analysis using Digitaria-specific primers, they are likely to be mutants lacking pyrichalasin H production. Isolates that belonged to the remaining 11 rDNA types did not produce pyrichalasin H and were avirulent to Digitaria plants. Therefore, the virulence of Pyricularia on Digitaria plants correlates with pyrichalasin H production. Pyrichalasin H was also present in spore germination fluid of a crabgrass isolate (IBDS 5-1-1), but not in that of isolates from rice, foxtail millet, finger millet, common millet and wheat. In addition, pyrichalasin H was detected in host leaves infected with IBDS 5-1-1, but not in leaves from other plants infected with compatible Pyricularia isolates. Pretreatment of leaf sheaths of crabgrass with 3 microg/mL pyrichalasin H led to the penetration and colonization by non-host isolates. Overall, these results indicate that production of pyrichalasin H is responsible for the genus-specific pathogenicity of Digitaria isolates.