HISTOPATHOLOGICAL STUDIES OF RESISTANCE OF SUNFLOWER (HELIANTHUS-ANNUUS L) TO DOWNY MILDEW (PLASMOPARA-HALSTEDII)

Histopathological studies of the infection of sunflower seedlings by d owny mildew (Plasmopara halstedii) have shown that penetration of root s and the lower part of the hypocotyl occurs for both compatible combi nations (susceptibility) and incompatible combinations (resistance). A fter penetrating susceptible genotypes, the parasite develops intercel lular hyphae and intracellular haustoria, leading to systemic invasion . In contrast, in resistant plants, as soon as colonization develops, hypersensitive-like Reactions occur in the parenchyma, with the appear ance of necrotic zones surrounded by dividing cells. Growth of the par asite is strongly inhibited and most hyphae are blocked before they re ach the cotyledonary node.