Outstanding host resistance will resolve the threat from white leaf spot disease (Pseudocercosporella capsellae) to oilseed and vegetable Brassica spp.crops

Field host resistance against the white leaf spot pathogen Pseudocercosporella capsellae was determined across one hundred and seventeen Brassica napus, eleven B. juncea, four B. rapa, five B. oleracea, eight B. fruticulosa and one B. carinata genotypes. Of the one hundred and seventeen B. napus, there were seven lines of B. napus containing weedy crucifer introgression, sixteen lines with B. carinata introgression, fifty six lines of synthetic B. napus from B. juncea into B. carinata and B. rapa into B. oleracea, while remaining B. napus were commercial varieties from Australia. There was wide and significant (P < 0.001) variation across the test genotypes in terms of relative disease development assessed either as % leaves diseased index (%LDI), values ranging from 0 to 68, or % leaves collapsed index (%LCI), with values ranging from 0 to 38. B. rapa subsp. oleifera ATC 95966 Bo was the most resistant genotype to white leaf spot disease (%LDI = 0, %LCI = 0). Very high resistance was observed in all five B. oleracea genotypes (%LDI < 13, %LCI = 10) and of these B. oleracea var. gongylodes Tronchuda with %LDI = 1.0 and %LCI = 2.5 and B. oleracea var. sabellica Kailan with %LDI = 2.4 and %LCI = 2.0 were the most resistant. Eight B. fruticulosa genotypes showed high resistance with %LDI < 15 and <= 10 %LCI. While eight out of twelve B. juncea varieties were highly susceptible, there was a range in %LDI of 5-58 and %LCI of 10-30 and with both Montara and VT 535 G showing best resistance of this species. Amongst B. napus genotypes, more than ten of the Australian commercial cultivars demonstrated comparatively higher resistance to white leaf spot disease than the most resistant genotypes with weedy introgression or synthetic B. napus, with commercial cultivars Oscar and Stubby considered to be highly resistant. These resistances offer unique opportunities both for direct deployment where white leaf spot is severe and for oilseed and vegetable Brassica spp. breeding programs to develop commercial cultivars with effective resistance.