Mode of action of chitosan and ASM for the control of Cercospora leaf spot on table beet

Cercospora leaf spot (CLS) is the primary disease impacting beet cultivation worldwide. This study evaluated the effectiveness of chitosan (CHT) and Acibenzolar-S-Methyl (ASM) for CLS control. The activities of peroxidase and glucanase, and content of flavonoids, phenolic compounds and betalains were determined spectrophotometrically from leaf samples collected at different intervals after inoculation. The use of CHT (1.0 mg/mL) significantly reduced CLS severity when applied at 1 and 3 days prior to inoculation, while ASM (50 mg/L) resulted in enhanced disease control at 5 and 7 days prior inoculation. CHT-treated plants showed higher peroxidase activity at the time of inoculation. In addition, CHT inhibited C. beticola conidia germination and mycelial growth, suggesting that antibiosis is the primary mode of action of the polysaccharide. Although ASM did not affect C. beticola growth, it increased peroxidase and glucanase activities, suggesting that ASM acts primarily as a plant resistance inducer. Preventative application of CHT and ASM may integrate a CLS management program aiming at suppressing CLS development.