Evidence for differential host preference in Microdochium nivale var. majus and Microdochium nivale var. nivale

The pathogenicity of Microdochium nivale var. majus and var. nivale was tested on wheat, rye and oat seedlings using both visual disease scoring and quantitative PCR measurements. In an individual inoculation trial at 10 degrees C var. majus and var. nivale were strongly pathogenic towards wheat and rye, with var. nivale causing slightly greater disease in rye. At this temperature only var. nivale caused significant disease of oats. In a further experiment M. nivale was inoculated as a series of mixtures of the two varieties and incubated at 15 degrees C. The ratio of the varieties present in the inoculum and present at harvest was analysed by quantitative PCR and this enabled a coefficient of selection to be calculated for the varieties on each host. M. nivale var. majus showed a weak selective advantage over var. nivale on wheat (0.33 +/- 0.08) and oat seedlings (0.35 +/- 0.016) and M. nivale var. nivale showed a strong selective advantage over var. majus on rye seedlings (0.92 +/- 0.26). The isolates were also compared for sensitivity to benzoxazolinone (BOA), a hydroxamic acid compound derived from rye leaves. M. nivale var majus was found to be significantly more sensitive to BOA than M. nivale var. nivale, indicating a possible mechanism for the selective advantage of var. nivale growing on rye. This is the first substantiated indication of a significant difference in host preference between Microdochium nivale var. majus and var. nivale.