Light leaf spot (caused by the hemibiotrophic Pyrenopeziza brassicae) is one of the most important diseases of winter oilseed rape (Brassica napus) in northern Europe, including the UK. In controlled environment and field experiments to study sources of genetic resistance against P. brassicae, R gene-mediated resistance introduced into B. napus slowed growth of P. brassicae, prevented asexual sporulation on living tissue, but did not prevent sexual sporulation on senescent tissue. The resistance did not operate in the manner typical of R gene-mediated resistance against hemibiotrophs. P. brassicae infected the resistant lines but did not elicit an immediate hypersensitive response preventing further fungal growth. Instead, it grew sparsely as sub-cuticular hyphae within green leaves, until a “dark flecking” phenotype associated with collapse of epidermal cells was observed approximately 10 days post inoculation. This resistance may be more durable than that of a typical R gene because it reduces secondary infection by splash-dispersed conidia but does not apply selection by preventing the pathogen from completing its life cycl
