| Brown Leaf Spot (BLS) of cassava was a worldwide disease, widely distributed over Asia, Africa and America. which seriously affected the quality and yield of cassava. This study focused on the disease occurrence, endangerment, pathogen identification and control technique, which had been rarely studied in China. The main results are as follow:BLS was the most serious fungi disease at cassava plantations in China, which was found at 23 cassava-growing sites in Hai Nan, Yun Nan, Guang Dong and Guang Xi,3 sites was harmed badly by the disease among those sites.Tissue isolation method, dissect needle removing spore method and improved dilution plate isolation method were utilized to isolate the pathogen from lesions.7 strains were obtained by improved dilution plate isolation method and 1 strains was obtained by dissect needle removing spore method, however, none was obtained by tissue isolation method. It showed that tissue isolation method was not suitable method to isolate the pathogen, while improved dilution plate isolation method was more appropriate than dissect needle removing spore method for its convenience and higher efficiency.Pathogenicity assays were performed on cassava leaves by 4 methods, which were agar plug with the fungal mycelium inoculated on wounded detached leaf, agar plug with the fungal mycelium inoculated on wounded living leaf, conidial suspensions inoculated on wounded detached leaf and conidial suspensions inoculated on wounded living leaf. Symptoms which were similar as those of field was recovered only by conidial suspensions inoculated on wounded living leaf. The results revealed that the method of conidial suspensions inoculated on wounded living leaf was appropriated for pathogenicity evaluation.8 strains were identified as Passalora henningsii (Allesch.) R. F. Castaneda & U. Braum by morphalogical characteristic of the conidia, conidiophore and stroma. ITS sequence analysis indicated that the stains had high homology to a Passalora henningsii strain (AF284389) with the identity of higher than 99%. Two strains were selected randomly to study biological characteristics. The results showed that the optimum condition for mycelial growth of the two strains included carrot agar medium, continual darkness,26℃or 24℃, pH 6.0 or pH 5.0 respectively, the optimum nitrogen sources was tryptone, the optimum carbon was D-glucose or D-mannitol respectively. The optimum temperature for macroconidia germination of the two strain was 26℃or 28℃. Lethal temperature for macroconidia was 60℃for 10 min.The resistance of 598 cassava germplasms to Brown Leaf Spot were determined in field. Results were that 6 of them were classified as 0 disease grade(1%),266 of them were classified as 1 disease grade(44.48%).204 of them were classified as 2 disease grade(34.11%). 93 of them were calssified as 3 disease grade(15.55%).20 of them were classified as 4 disease grade(3.34%).9 of them were classified as 5 disease grade(1.51%).The spore germination method was designed to determine the bioactivity of 10 chemical compounds to Passalora henningsii. the representive strain was named CPH-HN01 possessing high pathogenicty. Results indicated that Carbendazim had the best effect with a EC50 of 0.0353 mg/L. Descending order of prohibitive effect was as follow:Carbendazim (Benzimidazole derivatives), Prochloraz(Benzimidazole derivatives). Zhongshengmycin (Agricultural antibiotic), Propiconazole(Triazole derivatives). Chlorothalonil(Substituted Benzenes). Flusilazole(Triazole derivatives), Iprodione(Dicarboximides), Thiophanate-methyl (Substituted Benzenes), Mancozeb(Substituted Benzenes), Procymidone (Substituted Benzenes).6 chemical compounds were applied to control BLS in cassava platation. The results showed that control efficacy of Propiconazole. Carbendazim and Prochloraz were higher than 71.8%. Control efficacy of Chlorothalonil, Flusilazole and Thiophanate-methyl were lower than 58.9%. |
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