Identification and toxicity of Alternaria brassicicola, the causal agentof dark leaf spot disease of Brassica species grown in Thailand

摘要

Fungi, with spores characteristic of the genus Alternaria, were isolated from necrotic lesions on leaves of cabbage, cauliflower, Chinese kale and choi-sum growing in Thailand, and were proved by Koch's postulates to be the causal agents of a disease known as dark leaf spot. All isolates corresponded in morphology to descriptions of Alternaria brassicicola and the identification was confirmed by analysis of the ITS1, 5.8S gene and ITS2 regions of rDNA, the nucleotide sequences of isolates from all four plants being identical to each other and to the published sequence of a known isolate of A. brassicicola. Culture filtrates of isolates of the fungus from each host, grown on a defined medium consisting of Czapek-Dox nutrients supplemented with cations, were toxic to cells isolated from the four host plants. Taking the data overall, filtrates from the cauliflower isolate were significantly less toxic than those from the other isolates. Although the filtrate from the cabbage isolate was most toxic to cabbage cells and that of the choi-sum isolate most toxic to choi-sum cells, filtrates of the Chinese kale and cauliflower isolates were most toxic to cells of plants other than those from which they were isolated.