Effect of Host-race Combination on Resistance of Sunflower, Helianthus annuus L., to Downy Mildew Plasmopara balstedii

Study of resistance of sunflower, Helianthus annuus L., to downy mildew, Plasmopara halstedii , shows that both susceptible and resistant seedlings are always infected by the fungus, then hypersensitive-like reactions occur in the parenchyma and necroses form around the parasite. Comparison of 21 incompatible combinations (resistance) shows that the extent of the fungus growth differs according to the host-race combination. With type I resistance, the fungus is limited to the roots andthe lower part of the hypocotyl. With the type II resistance, the fungus grows throughout the whole length of the hypocotyl and may sporulate on the cotyledons.