Development of phoma lesions on oilseed rape leaves inoculated with ascospores of A-group or B-group Leptosphaeria maculans (stem canker) at different temperatures and wetness durations

In controlled-environment experiments, ascospores of both A-group and B-gro up Leptosphaeria maculans were able to infect leaves of oilseed rape and pr oduce phoma leaf spot lesions at temperatures from 5 to 20 degreesC and wet ness durations from 8 to 72 h after inoculation. Lesions formed on leaves i noculated with B-group ascospores had few pycnidia and were darker, smaller and less noticable than the larger, pale grey lesions with many pycnidia p roduced by A-group ascospores. Lesions formed by A-group or B-group L. macu lans on naturally infected winter oilseed rape experimental crops were simi lar to lesions produced by the two groups on inoculated plants. The greates t numbers of lesions were produced with a leaf wetness duration of 48 h and at temperatures of 15-20 degreesC for both A-group and B-group ascospores. As leaf wetness duration and temperature decreased below the optimal value s, the number of lesions decreased. The incubation period, estimated as the time from inoculation to the appearance of the first lesions (t(1)), or th e time to the appearance of 50% of the lesions (t(50)), of B-group was ofte n shorter than that of A-group L. maculans. As temperature decreased below 20 degreesC, the length of the incubation period of both A-group and B-grou p L. maculans increased.