Aspects of the epidemiology and control of powdery mildew (Oidium neolycopersici) on tomato in Ontario, Canada

Abstract

The epidemiology and control of powdery mildew (Oidium neolycopersici) of tomato (Solanum lycopersicum L.) was studied in greenhouse, growth chamber, and field experiments at Harrow, Ontario. Greenhouse environmental conditions were more favourable for disease development, with higher disease progress rates than in the field. Symptoms were more severe on older, lower leaves where the air temperature was slightly lower and relative humidity higher than on young leaves at the top of the canopy. A temperature of 22°C with 16 or 24 h of leaf wetness was optimal for disease development in a growth chamber experiment. Early inoculation resulted in significantly greater final disease severity and area under the disease progress curve than later inoculations. Disease progress in greenhouse and growth chamber trials was best described by the Gompertz model, while the logistic model was best for disease progress in field studies and for sections of plants in greenhouse studies. No yield loss occurred in field studies. The best disease control was obtained with acibenzolar-S-methyl (Actigard®), trifloxystrobin (Flint®), JMS-Stylet Oil®, azoxystrobin (Quadris®) and sulphur applied as protectants. Some phytotoxicity occurred with Actigard in greenhouse studies. Serenade® (Bacillus subtilis) was less effective while Kaligreen, Sporodex® (Pseudozyma flocculosa), copper octanoate and fumed silica were ineffective. Quinoxyfen was phytotoxic at the rate applied. Several tobacco cultivars, jimsonweed, American and black nightshade were susceptible to infection by O. neolycopersici. Eggplant, hairy nightshade and bittersweet were less susceptible, while no visible symptoms developed on pepper, potato and tomato ‘Grace’, and only trace levels of sporulation occurred on cucumber.