Factors Affecting the Production of Elsinochrome Phytotoxin by the Citrus Scab Pathogen, Elsinoë fawcettii

Elsinochrome (ESC), light-activated, nonhost-selective phytotoxin, is produced by many members of the genus Elsinoë and required for full virulence of the fungus. It appears that approaches disrupting the ESC production will likely reduce disease severity caused by Elsinoë pathogens. In this study, the influence of nutritional and environmental factors on ESC biosynthesis by the citrus scab pathogen E. fawcettii was investigated. Light is critical for initiation of ESC biosynthesis. E. fawcettii produced the highest amounts of ESC when grown on potato dextrose agar (PDA) in the light compared to other synthetic media. The fungus synthesized ESC in the presence of high amounts (60 g l) of sucrose, under limiting nitrogen conditions, or in ambient pH; ammonium completely inhibited production of ESC. ESC production was reduced when Ca, Co, Li, cysteine, glutathione, hydrogen peroxide, pyridoxine or -tocopherol was added to PDA; however, addition of Cu, Fe, K, Mg, Mn or Na ions increased ESC production. Addition of antioxidants, such as ascorbate, chlorogenic acid, catechin, or gallic acid into PDA enhanced ESC production substantially. Our results highlight the complexity of regulation of ESC biosynthesis by E. fawcettii at the biochemical and physiological levels.