| Citrus brown spot disease was first appeared on Emperor mandarin in Australia in1903and then reported in USA, Israel, South Africa, Turkey, Spain, Brazil, Argentina, Greece and Iran. In recent years, the rapid spreading of the disease has attracted serious concern of citrus growers and local government. In2003, the disease was observed on Tangfang mandarin, a local cultivar in Yunnan Province, China and officially reported in2010. Since2007, typical citrus brown spot symptoms were observed on the Gonggan and Ponkan cultivars in Guangdong and Guangxi provinces, respectively. The local people regarded it as ’acute anthracnose’. The pathogen was not identified clearly. In2011, the disease was occurred seriously on tangerine in Wanzhou, Chongqing and the pathogen was identified as Alternaria alternata. This provided theoretic basis for the following analysis.In order to determine the pathogeneticity and genetic diversity of the pathogen, typical infected leaves and fruits were collected from Wanzhou, Beibei of Chongqing, Guangdong, GuangXi, Zhejiang, Spain. Fifty typical small-spored A.alternata strains were isolated. According to the pathogenicity, morphology and molecular analysis, the pathogens were identified. In the study, we found the disease appeared on the hybrids cultivars firstly in China, and the symptoms was similar to those observed on the tangerine of Wanzhou, Chongqing. The results showed as follows:1. Combined whit the symptoms, pathogenicity, morphology and molecular analysis, the pathogen of citrus brown spot on the citrus hybrids cultivars (Amakusa and Fairchild) were determined as citrus brown spot tangerine pathotype. The pathogen isolated from the infected leaves sampled from Gguangdong and Guangxi is also A.alternata by Koch’s postulate.2.ZTGYL-3(tangerine, Chongqing) and SP-36(Clementine, Spain) strains were selected for the biological characteristics and determined factors affecting the mycelia growth and sporulation. The results showed that the two strains could grow on the seven kinds of mediums. Of the seven mediums, PDA, CAA and PCA were favored. The temperature significantly affected the growth of the two strains. They could grow in the range of5℃-35℃. The suitable temperature for their mycelium growth was28℃. The most suitable temperature for SP-36and ZTGYL-3sporulation was20℃and25℃. They could grow in the range of pH4-11. The suitable pH value for their mycelium or colony growth was pH5-9.The colony could not grow once pH value is above11. The most suitable pH value for SP-36and ZTGYL-3sporulation was pH7and pH6. Of the tested different carbon and nitrogen resources in Czapek medium, soluble starch and peptone were the best mediums. Mycelium growth and sporulation were inhibited when they grew on mediums with ammonium chloride and ammonium sulfate. The growth rate and conidial production of Spain strain were significantly superior than the Chongqing strain. According to the strain distribution and pathogencity difference, we speculate the pathogens in older epidemic area might be evolved to adapt to the environment.3. Based on the previous reports on small-spored A.alternata strains related to citrus, EPG, OPA2-1, OP10-2and OPA1-3were selected for genetic diversity analysis of30strains, and recorded the growth rate and colony morphology of part strains. The phylogenetic tree constructed from the four regions showed that no significant correlations among the genetic diversity, samples geographical origin and host varieties were observed. The branch including strains isolated from Ponkon of Wanzhou, Chongqing and Dancy tangerine of Yongjia, Zhejiang could be related with their growth rate. Significant differences in the growth rate and genetic diversity of two strains sampled from Guangxi shoots were also found. |
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