Temperature-dependent resistance to crown rust infection in perennial ryegrass, Lolium perenne

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.