Screening Winter Canola Germplasm for Resistance to Black Leg.

Black leg, caused by the fungus Leptosphaeria maculans, has become widespread in the Southern Great Plains. Two types of resistance to black leg are recognized. Polygenic (field) resistance is expressed in adult plants as reduced severity of basal stem cankers. Major gene (seedling) resistance is expressed as a hypersensitive response to leaf spot on young plants. Seedling resistance is controlled by one or more of nine resistance genes (Rlm) that interact in a gene-for-gene manner with corresponding avirulence genes (Avr) in the pathogen. Entries from the National Winter Canola Variety Trial and the breeding program at KSU have been screened in the field by inoculating plots and evaluating canker severity after swathing. The value and repeatability of the field results have been limited by factors including a prolonged drought that has reduced disease development. The incidence of plants with cankers has been high (50-100%), but canker severity (% stem girdling) has averaged 50% or less and few plants have been killed. Popular, glyphosate-tolerant cultivars have either been intermediate in reaction or among the most susceptible entries. Breeding lines have ranged from resistant to intermediate in disease reaction. Evaluation of disease reaction on seedling cotyledons has been more repeatable than field screening. Efforts have initially focused on characterizing the Avr genes in local isolates (n>100) by inoculating differential cultivars containing known Rlm genes and by polymerase chain reaction. Avr1,2,3 were present in less than 50% of the isolates suggesting Rlm 1,2,3 would not be very effective. Avr4-7,6 were present in over 90% of the isolates suggesting Rlm4 or Rlm7, and Rlm6 would be broadly effective. Additional Avr genes will be characterized and a panel of representative races will be used to inoculate cultivars and breeding lines to identify the presence or absence of Rlm genes.