PRODUCTION OF CERCOSPORIN AND COLONIZATION OF SOYBEAN SEED COATS BY CERCOSPORA-KIKUCHII

Isolates of Cercospora kikuchii produced cercosporin in culture and soybean seed coats. C. kikuchii isolates recovered from purple-stained soybean seeds were divided into three groups based on colony coloration on potato-dextrose agar (PDA). Ckcp isolates produced purple coloration, Ckcy isolates a yellow background due to lipids, and Ckcc isolates red cercosporin. Among 72 isolates from 72 seeds studied, 75% were Ckcp, 13% Ckcy, and 12% Ckcc. One isolate representing each group was studied further. The Ckcp isolate produced significantly higher amounts of cercosporin on PDA than either the Ckcy or Ckcc isolates. Production of cercosporin by the three isolates in seed coats was in the order of Ckcc > Ckcp > Ckcy. All isolates inoculated on soybean seeds in culture caused purple stain on the seed coats. The extent of purple stain on seed coats coincided with the extent of colonization by C. kikuchii. A positive relationship (r2 = 0.96) was found between the extent of purple seed coat stain and the amount of cercosporin per 100 mug of seed coat tissue in situ. Extracts from C. kikuchii grown on PDA contained cercosporin, isocercosporin, and lipids. However, lipids were not detected from soybean seed coats inoculated with C. kikuchii or naturally infected purple-stained seed coats, suggesting that the pathogen produced lipids only in culture and that lipids were not involved in seed colonization. This is the first report on the estimation of cercosporin from Cercospora in plant tissues with reference to pathogenesis. The greater occurrence of isolates producing high amounts of cercosporin, and the correlation between cercosporin production and the extent of purple stain or C. kikuchii colonization, indicated a role for cercosporin in facilitating soybean seed coat tissue colonization by C. kikuchii.