Effects of temperature and wetness duration on infection of oilseed rape by ascospores of A-group or B-group Leptosphaeria maculans (stem canker)

Phoma lesions produced on oilseed rape leaves by B-group ascospores were smaller and less distinctive than lesions produced by A-group ascospores of Leptosphaeria maculans, when leaves were inoculated with ascospore suspensions obtained from infected debris from Poland or the UK. Both Agroup ascospores and B-group ascospores of L. maculans were able to infect leaves of oilseed rape and produce lesions at temperatures from 4 to 20oC and leaf wetness durations greater than 4 h. However, the greatest number of lesions were produced with a leaf wetness duration of 48h at temperatures of 20oC for A-group and >12oC for B-group ascospores. As leaf wetness durations and temperatures decreased below the optimal values, the numbers of lesions produced decreased. For example, very few lesions were produced at 4oC (and only with a leaf wetness duration >48 h) or with a leaf wetness duration of 4h (and only at temperatures >12oC). There was no evidence that the maximum number of lesions produced in relation to number of ascospores inoculated differed between A-group and B-group L. maculans. The incubation period (from inoculation to the appearance of the first lesion) of B-group L. maculans was shorter than that of A-group L. maculans; at 20oC, lesions appeared within 2 days rather than 5 days from inoculation. As temperature decreased below 20oC, the length of the incubation period increased