Pathogenic variation of South Australian isolates of Leptosphaeria maculans and interactions with cultivars of canola (Brassica napus)

Abstract

Cultural characteristics and pathogenicity tests showed that South Australian isolates of Leptosphaeria maculans (Desm.) Ces. et de Not. were highly variable. Isolates were ranked in a continuum based on the severity of disease symptoms on plants, and two of the 40 isolates tested showed very similar characteristics and pathogenicity to a known avirulent isolate. The early stages of infection, on the surface of leaves, through the epidermis and into the mesophyll, were examined histologically, using two canola cultivars (susceptible and resistant) and two isolates (virulent and avirulent). Differences in frequency of penetration into the plant tissue were observed indicating interactions between cultivars and isolates. Up to 6 days after inoculation, hyphae penetrated the stomata more frequently on the susceptible cv. Hyola 42 than on the resistant cv. Dunkeld. This was linked to the lower density of stomata per unit area on Dunkeld compared to Hyola 42.