THE RESISTANCE TO POWDERY MILDEW (OIDIUM-LYCOPERSICON) IN LYCOPERSICON SPECIES IS MAINLY ASSOCIATED WITH HYPERSENSITIVE RESPONSE

The cultivated tomato (Lycopersicon esculentum) is susceptible to powd ery mildew (Oidium lycopersicum). Six accessions of three related Lyco persicon species show high levels of resistance (Lindhout et al., 1994 b). The present research aimed at describing the development of O. lyc opersicum on susceptible cv Moneymaker and characterizing the defence response to O. lycopersicrcm in Lycopersicon accessions by histologica l analysis. Spore germination and (primary) haustorium formation in re sistant accessions were as frequent as in the susceptible L. esculentu m cv Moneymaker. A high frequency of necrosis of epidermal cells in wh ich a haustorium was formed appeared to be the major defence response, indicating that resistance to O. lycopersicum in the Lycopersicon gen us was predominantly based on the hypersensitive reaction. However, th e resistance in L. parviflorum was less associated with hypersensitivi ty than in other resistant accessions, suggesting the existence of a d ifferent but still unknown resistance mechanism. In addition, evidence is provided that the level of resistance could depend on the genetic background and the plant age.