Distribution and pathogen identification of cassava brown leaf spot in China.

Cassava brown leaf spot surveys were conducted in the main cassava plantation areas of China between 2007 and 2012 in order to understand the distribution of the disease. Cassava plants were damaged by the disease to different degrees in most of the survey sites. Samples were collected and seven strains were isolated from lesions. The mycelium-breaking plus black light induction method was applied for sporulation. Microconidia were formed by means of fragmentation on artificial medium plates. When the leaf was stabbed and inoculated with conidia solution, similar symptoms were formed 14 days later. Morphological characteristics of the specimens and conidia were similar to descriptions of Passalora henningsii infection. The internal transcribed spacer (ITS) regions of rDNA were obtained with primer pair ITS1/ITS4 and deposited in GenBank, which differed by three base pairs from that of the P. henningsii isolate (AF284389). The ITS sequences of related species were downloaded from the NCBI database, and phylogenetic analysis showed that the sequences originating from our strains clustered in the same clade as the AF284389 isolate. Biological characteristics were evaluated in two strains from different sites, which indicated that the optimum conditions for mycelia growth were a temperature of 26° to 28°C, carrot agar medium, pH 6, and continuous dark; cassava leaf juice added to malt extract and cassava leaf juice added to potato dextrose agar were the best media for conidia production. The optimal and lethal temperatures for macroconidia germination were 26° to 28°C, and 60°C for 10 min, respectively.