SCREENING WILD LYCOPERSICON SPECIES FOR RESISTANCE TO POWDERY MILDEW (OIDIUM-LYCOPERSICON)

Since the late 1980s powdery mildew, designated Oidium lycopersicum, f requently invaded the tomato crop in Western Europe. All commercial cu ltivars are susceptible. To screen for resistance in wild species a re liable and efficient disease test was developed. Young plants with two to three true leaves are inoculated at high relative humidity by spra ying with a freshly prepared suspension of 2 x 10(4) conidia, ml(-1). Symptoms are periodically evaluated according to a scale based on the percentage of leaf area with mycelium. One hundred and twenty seven ac cessions, representing eight wild Lycopersicon species, were screened for resistance to O. Lycopersicum, A large variation in resistance was found between species. L. hirsutum was the most resistant species; L. pennellii was moderately resistant; species of the subgeneric group o f L. esculentum and of the 'peruvianum-complex' were all susceptible. L. parviflorum was classified separately due to a large variation betw een accessions. Except for this species, a low variation was found bet ween accessions within species. High levels of resistance were observe d in four accessions of L. hirsutum, in one of L. parviflorum and in o ne of L. peruvianum. This resistance is characterized by a very low di sease incidence and a strongly restricted mycelium growth and lack of sporulation.