CHARACTERISTICS AND HOST-RANGE OF TOMATO POWDERY MILDEW

The anamorphic state of a powdery mildew, which first appeared on toma to in the UK in 1987, currently listed as Erysiphe orontii (anamorph O idium violae), was examined microscopically on both tomato and alterna tive hosts. Host range studies were also carried out. In 1993-95, spec ies and cultivars of plants from 26 of the 30 families listed as hosts of E. orontii were inoculated with tomato powdery mildew in the glass house and assessed for susceptibility on a 0-3 scale based on degree o f sporulation and growth. Thirteen families contained species or varie ties that were susceptible and a total of 80 varieties, including many economically important Cucurbitaceae and Solanaceae, supported sporul ation to some extent (susceptibility score 1-3). Necrosis was not a co nstant feature of plants exhibiting resistance, as it could accompany sporulation on some susceptible varieties. Microscopic measurements sh owed that conidial size, conidiophore length, conidiophore foot-cell l ength and width, and the number of cells distal to the foot-cell were remarkably constant between samples of powdery mildew taken from tomat o each year and from the various alternative hosts. Based on the produ ction of conidia singly, the presence of nipple-shaped to moderately l obed appressoria, and occurrence of straight conidiophores, it is conc luded that this fungus on tomato should more correctly be named Oidium lycopersicum until the sexual stage is discovered. The highly polypha gous nature of this powdery mildew would suggest that it represents a potential risk to UK agriculture and horticulture, but the failure to recover any powdery mildew from 120 samples of potato, including varie ties Cara and Maris Piper, on which the tomato powdery mildew could sp orulate in the glasshouse, suggests that environmental factors may pla y a key role in preventing establishment of this powdery mildew on alt ernative hosts on a commercial scale in the field.