Role of oxalic acid in the Sclerotinia wilt of sunflower

Abstract

Leaves of greenhouse-grown sunflower plants infected with Sclerotinia sclerotiorum frequently exhibit wilting and inter-veinal necrosis 3 to S days after inoculation. A wilt-inducing substance was found in water extracts of hypocotyl, lesions. Chemical tests, thin-layer chromatography and gas-liquid chromatography demonstrated that wilt-active aliquots of purified hypocotyl extracts contained oxalic acid. Wilted leaves from infected plants contained over 15 times more oxalic acid than leaves of healthy plants. Healthy plants fed oxalic acid through the hypocotyl showed foliar symptoms identical to those associated with fungal infections, and the concentrations of oxalic acid in leaves of treated plants were equivalent to those in infected plants. The pH of xylem sap from infected plants taken 2 cm above lesions was one unit lower than sap from healthy plants. Oxalic acid was detectable in sap from infected plants. Oxalic acid moves systemically to the leaves where it apparently accumulates to a “critical” level and elicits the wilt syndrome. A varietal screening technique using oxalic acid and whole leaf cell suspensions was developed which indicated that sunflower cultivars more resistant to Sclerotinia wilt were more tolerant to oxalic acid during their flowering period.