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 tes ted on wheat, rye and oat seedlings using both visual disease scoring and q uantitative PCR measurements. In an individual inoculation trial at 10 degr ees C var. majus and var. nivale were strongly pathogenic towards wheat and rye, with var. nivale causing slightly greater disease in rye. At this tem perature only var. nivale caused significant disease of oats. In a further experiment M. nivale was inoculated as a series of mixtures of the two vari eties 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 th is 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 sensitiv ity to benzoxazolinone (BOA), a hydroxamic acid compound derived from rye l eaves. M. nivale var majus was found to be significantly more sensitive to BOA than M. nivale var. nivale, indicating a possible mechanism for the sel ective advantage of var. nivale growing on rye. This is the first substanti ated indication of a significant difference in host preference between Micr odochium nivale var. majus and var. nivale.