Initial events in the colonisation of tomatoes by Oidium lycopersici, a distinct powdery mildew fungus of Lycopersicon species

The rDNA intergenic spacer sequence of Oidium lycopersici (ana.; Cooke & Ma ssee 1888, emend. Noordeloos and Loerakker 1989, emend. Mieslerova and Lebe da 1999) was determined and compared with the sequences of other powdery mi ldews. This pathogen was shown to be distinct from other powdery mildew fun gi found on tomato, but it exhibited a close similarity to Erysiphe aquileg iae var. ranunculi. The initial events involved in the germination of conid ia and subsequent formation of the appressorium in the newly described powd ery mildew of tomato, O. lycopersici, were studied by light and scanning el ectron microscopy. Scanning electron microscopy revealed the conidial coat to be smooth to slightly rugose and the appressoria to be multilobed and at tached to the host by a mucilaginous pad of extracellular material.