The resistance to Oidium lycopersici conferred by ol-2 gene in tomato

The powdery mildew caused by Oidium lycopersici is one of the most destructive diseases in glass-house-grown tomato and is widespreading all over the world. A high level of resistance to O. lycopersici was found in an accession of Lycopersicon esculentum var. cerasiforme at the Department of Biology and Plant Pathology, University of Bari. The genetic analysis of F1, F2 and BC plants indicated that the resistance is conferred by a single recessive gene, designed as ol-2. Studies on the infection process of O. lycopersici on susceptible and ol-2 gene resistant tomatoes were carried out at 24 °C and 90 % relative humidity. Light microscope observations on conidia germination, formation of primary appressoria, elongation of hyphae and sporulation were made on artificially inoculated basal, intermediate and apical leaves. Inoculation was made by shaking mildewed tomato leaves over each test plant. Disease development were assessed by removing the fungal structure from the leaf surface with the ceroidin film technique and by direct observations of stained inoculated leaves. The rate of conidial germination and the appressoria formation was not affected by host genotype. Mycelia growth and sporulation on leaf surface of resistant plant was strongly restricted and influenced by the leaf age. The results indicated that the resistance in ol-2 tomato is postinfectional and is not associated with a hypersensitive response. This work was supported by the MURST and CNR (Paper no. 331)