Differences were found in the expression of resistance of perennial ryegras
s, Lolium perenne, cultivars to crown rust, Puccinia coronata Corda, when p
lants were incubated at either 10 degrees C or 25 degrees C. In an extreme
example, all the genotypes of 'Carrick' were resistant at 10 degrees C but
susceptible at 25 degrees C. The variation between cultivars was due both t
o a general increase in expression of susceptibility and the proportion of
plants which became susceptible. A BC1 line containing a stay-green gene de
rived from Festuca pratensis was insensitive to temperature, while a sister
line was sensitive.