Regulation of Cercosporin accumulation in culture by medium and temperature manipulation

The ability to manipulate cercosporin accumulation in specific isolates of Cerospora in culture is a necessary prerequisite for studying the regulation of toxin accumulation at a molecular level. This study defined medium, temperature, and light conditions for maximum and minimum cercosporin accumulation in isolates of C. asparagi, C. beticola, C. kikuchii, C. nicotianae, and C. zeae-maydis. A simple method was developed for the extraction and measurement of cercosporin in cultures of Cercospora spp. grown on solid medium. Of six growth media, malt and potato-dextrose agar were generally favorable for cercosporin accumulation, but the effects of medium and isolate on cercosporin accumulation interacted significantly. The ratio of carbon to nitrogen in a defined medium affected cercosporin accumulation in four of the eight isolates tested but not in any consistent manner. Cercosporin accumulation also was regulated by temperature in four of the eight isolates, higher levels accumulation at 20 C than at 30 C. Two isolates of C. kikuchii accumulated more cercosporin when grown in light than when grown in darkness, but the effect of light interacted with those of medium and isolate. Patterns of regulation of cercosporin accumulation differed markedly among species and even isolates of the same species of Cercospora, making generalizations about the regulation of cercosporin production by envionmental factors of limited use. However, the present study did identify certain isolates for future investigation of cercosporin regulation. Our data also show that screening isolates of Cercospora for cercosporin production under a single set of cultural conditions is unreliable and question the reliability of correlating toxin production in vitro to the virulence of a Cercospora isolate.