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

Cultural characteristics and pathogenicity tests showed that South Australi an isolates of Leptosphaeria maculans (Desm.) Ces. et de Not. were highly v ariable. Isolates were ranked in a continuum based on the severity of disea se symptoms on plants, and two of the 40 isolates tested showed very simila r characteristics and pathogenicity to a known avirulent isolate. The early stages of infection, on the surface of leaves, through the epidermis and i nto the mesophyll, were examined histologically, using two canola cultivars (susceptible and resistant) and two isolates (virulent and avirulent). Dif ferences in frequency of penetration into the plant tissue were observed in dicating interactions between cultivars and isolates. Up to 6 days after in oculation, hyphae penetrated the stomata more frequently on the susceptible cv. Hyola 42 than on the resistant cv. Dunkeld. This was linked to the low er density of stomata per unit area on Dunkeld compared to Hyola 42.