Screening wild Lycopersicon species for resistance to powdery mildew (Oidium lycopersicum).
With a view to developing a reliable and efficient screening test, young plants with 2 to 3 true leaves were inoculated at high relative humidity by spraying with a freshly prepared suspension of 2 × 104 conidia/ml. Symptoms were periodically evaluated according to a scale based on percentage of leaf area with mycelium. Some 127 accessions, representing 8 wild Lycopersicon species, were screened for resistance to O. lycopersicum [O. lycopersici]. Wide variation in resistance was found between species. L. hirsutum was the most resistant, L. pennellii was moderately resistant and species of the subgeneric group containing L. esculentum and of the peruvianum-complex were all susceptible. L. parviflorum was classified separately owing to wide variation between accessions. Except for this species, low variation was found between accessions within species. High levels of resistance were observed in 4 accessions of L. hirsutum, one of L. parviflorum and one of L. peruvianum. This resistance was characterized by low disease incidence, strongly restricted mycelial growth and lack of sporulation.