Abstract
Ramularia leaf spot (RLS) is a newly-important disease of barley which is caused when the fungus Ramularia collo - cygni enters necrotrophic development during colonisation of the host. Mutant alleles at the barley MILDEW LOCUS O , mlo , locus confer broad spectrum durable resistance against the powdery mildew fungus, Blumeria graminis f. sp. hordei , but can enhance susceptibility to pathogens with necrotrophic development stages such as R . collo - cygni . Given the importance of mlo in spring barley breeding programmes, identifying loci that mitigate the effect of mlo -mediated susceptibility on necrotrophic disease development is an important target. Mutation of the ENHANCED MAGNAPORTHE 1 ( emr1 ) locus which can affect mlo -associated disease susceptibility, leads to a reduction in RLS symptoms on barley leaves but does not reduce R . collo - cygni accumulation. The effect of emr1 on the transition of R . collo - cygni from endophyte to necrotroph may relate to changes in reactive oxygen species in mutant plants which show reduced sensitivity to chloroplastic superoxide induced cell death and has lower relative chlorophyll content compared to mlo plants.