Since the late 1980s powdery mildew, designated Oidium lycopersicum, frequently invaded the tomato crop in Western Europe . All commercial cultivars are susceptible . To screen for resistance in wild species a reliable and efficient disease test was developed . Young plants with two to three true leaves are inoculated at high relative humidity by spraying with a freshly prepared suspension of 2 x 10 4 conidia, ml-' . Symptoms are periodically evaluated according to a scale based on the percentage of leaf area with mycelium .One hundred and twenty seven accessions, 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 of L . esculentum and of the 'peruvianum-complex' were all susceptible . L. parviflorum was classified separately due to a large variation between accessions. Except for this species, a low variation was found between accessions within species . High levels of resistance were observed in four accessions of L. hirsutum, in one of L . parviflorum and in one of L . peruvianum . This resistance is characterized by a very low disease incidence and a strongly restricted mycelium growth and lack of sporulation.

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