Behavior of Alternaria brassicae and its mycoparasite Nectria inventa on intact and on excised leaves of rapeseed

On intact leaves of two cultivars (cv.) of rapeseed, Midas (Brassica napus) and Torch (B. campestris), conidia of Alternaria brassicae germinated at a rate of 12.1% and 19.5%, respectively, at 9 h after inoculation. They germinated usually by producing either germ tubes or secondary conidia. Penetration of leaves by A. brassicae was abundant at 24 h and occurred either with or without the formation of appressoria. Penetration of cv. Torch leaves by the fungus occurred either directly through epidermal cells or indirectly through stomata, while cv. Midas leaves were penetrated almost exclusively through stomata. Blackspot lesions developed within 48 h after inoculation.Conidia of Nectria inventa required at least 24 h to initiate germination and 4 days to parasitize A. brassicae on intact leaves. Therefore, N. inventa did not prevent primary infection of the leaves by A. brassicae. Instead, N. inventa suppressed the vegetative growth and sporulation of A. brassicae on excised rapeseed leaves.