Technical Report


Evaluation of Grape Powdery Mildew Forecasting Programs Grape (Vitis vinifera 'Chardonnay') Powdery Mildew (Uncinula necator) Public Deposited

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  • Three forecasting programs for scheduling fungicide applications were selected for comparison with the standard Oregon phenology based program. The California (Gubler-Thomas, UC-Davis) program used leaf wetness and temperature early in the year to predict ascospore infection periods and only temperature during the summer to predict conidial infection periods. The New York (Gadoury) program was based on rainfall and temperature. The German (Oi Diag) program incorporated relative humidity along with temperature and rainfall. Treatments were arranged in a randomized complete block design in a block of 'Chardonnay' planted in 1985 on a 7 x 10 ft spacing. Vines were trained to a bilateral cordon with spur pruning. Shoot thinning occurred 12-13 May to provide uniform cane density. Each treatment was replicated on 3 sets of 5 vines. Treatments were applied using a handgun sprayer at 300 psi at a rate of 200 gal water/A for applications between 1 May (budbreak) and 13 May (6" growth). Treatments were applied using a hooded boom sprayer at 300 psi at a rate of 200 gal water/A for all applications after 13 May. Approximately 3.5 gal of spray suspension was applied per 15 vines (150 gal water/A) between 1 May and 13 May, 4.5 gal between 21 May and 28 Jul, and 5 gal (200 gal water/A) for the rest of the applications. Treatments were applied as required by the guidelines for each program. However, additional conditions for stopping programs at or just after verasion were incorporated as requested by Oregon growers. The standard program and the water control were applied on 13 May (6" shoots), 21 May (12" shoots), 2 Jun (prebloom, EL growth stage 17), 16 Jun (90% bloom), 1 Jul, 15 Jul (bunch closure), 22 Jul, 5 Aug, and 12 Aug (verasion). No Botrytis control measures, including leaf removal, were applied to test vines. All programs used one of two fimgicides, Thiolux DF at 3 lb/100 gal water or Rally at 2 oz/100 gal water for each application. Trap plants of 'Cabernet Sauvignon' were placed next to nontreated vines for 24 hour periods within the same block of grapes from 8 May to 3 Jul. After 24 hours of exposure, plants were transferred to a greenhouse several miles away for incubation under conditions favorable for powdery mildew development.
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