Phenolic Compounds in Stems of Sunflower Plants Inoculated with Sclerotinia sclerotiorum and their Inhibitory Effects on the Fungus

Sunflower plants at I (vegetative), II (closed flower-buds) and III (open flower-buds) growth stages were experimentally inoculated with Sclerotinia sclerotiorum. At the 5th day after inoculation, all plants from I, III and 70 % of II (II_T) did not present wilt symptoms while 30 % of age II plants (II_s) presented severe wilt symptoms in all their leaves and had collapsed. Total soluble phenols were measured in the “lesion” (L) and its “healthy” (H) surrounding zones of all inoculated and control plants. A significant increase in phenols was found in the L and H zones of tolerant plants (I, II_t and III). The corresponding methanolic extracts showed a strong inhibitory effect on the mycelial growth (MG) of the fungus. On the other hand no statistically significant increase took place in the L zones from susceptible plants; their methanolic extracts did not inhibit MG. The chemical analysis of methanolic extracts showed that the isochlorogenic acid was the more inhibitory compound. It significantly accumulated in the L of tolerant plants but did not in those of susceptible ones.