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Two symptoms of Cercospora Leaf Blight of soybean suggests two diseases caused by the same pathogen
E. CHAGAS FERREIRA DA SILVA (1), T. Garcia (2), A. Chanda (3), C. Robertson (2), . Lygin (5), R. Schneider (2) (1) Louisiana State Univ, Agricultural Center, U.S.A.; (2) Louisiana State Univ, Agricultural Center , U.S.A.; (3) Department of Plant Pathology

Considered one of the most devastating soybean diseases in Louisiana, Cercospora leaf blight (CLB) is caused by Cercospora kikuchii, C. flagellaris and C. sigesbeckia. Resistant varieties are not available, and fungicide resistance is widespread. Purple discoloration of leaves exposed to sunlight and leaf blight are the main symptoms of CLB. It is currently assumed that CLB starts with leaf purpling (thought to be a direct accumulation of cercosporin), and leaves become blighted as the disease progresses. Our observations in Louisiana suggested that these symptoms are not correlated. The objective of this work was to examine the relationship between the two CLB symptoms as well as their relationship with fungal growth, cercosporin and flavonoid concentrations =in leaves. Four soybean varieties were planted at two locations. Results showed a low relationship between the two CLB symptoms in Alexandria and Baton Rouge. The pathogen was detected in all leaves (including symptomless); however, blighted leaves had higher fungal biomass than purple leaves. Cercosporin was not detected in symptomless leaves but was detected in low and high concentrations in purple and blighted leaves, respectively. No anthocyanins were found in any leaves. Glyceollin was detected in higher concentration in the intermediate stages of blight symptoms. These results suggest two modes of pathogenesis, which indicates that the same pathogen causes two distinct diseases.