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

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(-1). 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.