Factors affecting the incubation period of dark leaf and pod spot (Alternaria brassicae) on oilseed rape (Brassica napus)

Experiments to investigate the factors affecting the incubation period of dark leaf and pod spot (Alternaria brassicae) on leaves and pods of oilseed rape (Brassica napus) were done in controled environment (constant temperatures) and glasshouse conditions (fluctuating temperatures). The length of the incubation period of dark leaf and pod spot decreased as infection and incubation temperatures increased from 6 to 20 degrees C. The incubation period decreased as wetness period increased from 2 to 12 h, as inoculum concentration increased from 80 to 2 x 10(3) spores ml(-1) and as leaf age increased from 4 to 10 days. Asymptotes of leaf age and inoculum concentration, above which the length of the incubation period did not decrease, were 10 days and 2 x 10(3) spores ml(-1), respectively. The shortest and longest incubation periods were 1 and 11 days. The mechanism by which the infection conditions influenced the incubation period of dark leaf and pod spot on oilseed rape seemed to be linked to lesion density. Usually, the length of the incubation period decreased greatly with increasing lesion density.