Variation in host range, systemic infection and epidemiology of Leptosphaeria maculans

Abstract

The A and B groups (aggressive and non‐aggressive) of Leptosphaeria maculans were compared in studies of host range, infection phenotypes and epidemiology. Isolates of both groups infected a wide range of cruciferous hosts including Brassica napus, B. rapa, B. oleracea, B. juncea, B. carinata. B. nigra, Thlaspi arvense and Raphanus sativus. On cotyledons, B‐group isolates were generally more aggressive than A‐group isolates, causing local lesions and subsequent systemic invasion of the majority of test species. On susceptible stems, A‐group isolates caused cortical lesions; B‐group isolates, if they reached the stem, usually caused pith lesions with no external symptoms. In a susceptible line of B. napus, CrGC5, systemic infections of the leaf and petiole were similar with both groups, but the B‐group rarely entered the stem to form cortical lesions or cankers. At two different field locations, natural epidemics caused by the two groups on oilseed rape leaves were similar in overall pattern, B‐group lesions occurred slightly later but the incidence increased more rapidly and reached a maximum slightly earlier than that of the A group. The two groups differed markedly in stem infection patterns. Infection of the cortex near the base of the stem and the development of a typical stem canker was caused entirely by the A group. However the B group often caused considerable damage to the pith. Superficial chlorotic lesions on stems and inflorescences were mainly attributable to the B group. In view of the stem pith infection by the B group without external symptoms, its importance on oilseed rape may have been underestimated previously.