The timing and sequence of events leading to stem canker disease in populations of Brassica napus var. oleifera in the field

Natural epidemics of Leptosphaeria maculans in cultivated populations of Brassica napus var. oleifera were predominantly monocyclic, involving a sequence of five phases; latent infection of the lamina, leaf lesion expression, symptomless systemic growth down the petiole, latent infection of the stem, and stem canker development. This sequence potentially terminated in severe stem cankers if initiated from ascospore infection of any leaf between a plastochron index of PI = n + 0.5 and n +1.0 until plant growth stage 3.1 of the Harper & Berkenkamp scale. At PI < n + 0.5, infection was rare and at PI > n + 1.0, the infection sequence was usually terminated by leaf abscission. Differential effects of ambient temperature on the rates of progress of infection and leaf development determined whether abscission occurred before infection reached the stem. Based on these interrelationships, a system for predicting the incidence of severe stem cankers is proposed.