Infection cycle of Alternaria brassicicola on Brassica oleracea leaves under growth room conditions

Abstract

Development of the necrotrophic fungus Alternaria brassicicola was evaluated during infection of three cabbage varieties: Brassica oleracea var. capitata f. alba ‘Stone Head’ (white cabbage), B. oleracea var. capitata f. rubra ‘Langedijker Dauer’ (red cabbage) and B. oleracea var. capitata f. sabauda ‘Langedijker Dauerwirsing’ (Savoy cabbage). Following inoculation of cabbage leaves, conidial germination, germ tube growth, and appressorium formation were analysed during the first 24 h of infection. Differences in the dynamics of fungal development on leaves were observed, e.g. approximately 40% of conidia germinated on Savoy cabbage leaves at 4 h post-inoculation (hpi) while only 20% germinated on red and white cabbage leaves. Leaf penetration on the three cabbage varieties mainly occurred through appressoria, rarely through stomata. Formation of infection cushions was found exclusively on red cabbage. Appressoria were first observed on red cabbage leaves at 6 hpi, and on white and Savoy cabbage leaves at 8 hpi. Conidiogenesis occurred directly from mature conidia at an early stage of fungal development (10 hpi), but later (48 hpi) it occurred through conidiophores. Disease progress and changes in the morphology of leaf surfaces were also observed. At the final 120 hpi measurement point, necroses on all investigated varieties were approximately the same size. Based on detailed light and electron microscopic analyses, an outline of the A. brassicicola infection cycle on B. oleracea leaves under growth room conditions has been proposed. According to the authors’ knowledge, this report represents the most detailed up-to-date description of infection of B. oleracea varieties by A. brassicicola.