Technical Report
 

Evaluation of Grape Powdery Mildew Forecasting Programs

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  • Three forecasting programs for scheduling fungicide applications were selected for comparison with the standard Oregon phenology based program. The UCDavis (California) program used leaf wetness and temperature early in the year and only temperature during the summer. The New York 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 11 yr 'Chardonnay' with a 7x10 ft spacing. Vines were trained to a bilateral cordon with spur pruning. Each treatment was replicated on 3 sets of 5 Vines. Treatments were applied using a handgun sprayer at 300 psi at a rate of 75-150 gal water/A for applications between 14 Apr (budbreak) and 3 Jul (full bloom). Treatments were applied using a hooded boom sprayer at 300 psi at a rate of 200 gal water/A for all applications after 3 Jul. Approximately 1.8 gal of a spray suspension was applied between 14 Apr and 25 Apr (75 gal water/A), 2.2 gal of a spray suspension was applied between 30 Apr and 28 May (90 gal water/A), 3 gal of a spray suspension was applied between 3 Jun and 19 Jun (120 gal water/A), 3.7 gal of a spray suspension was applied between 25 Jun and 3 Jul (150 gal water/A) and 5 gal of a spray suspension was applied (200 gal water/A) for the rest of the applications per 15 Vines. Treatments were applied as required by the guidelines for each program. The standard program and the water control were applied on 6 May (6 inch shoots), 16 May, 28 May (12 inch shoots), 4 Jun, 19 Jun (full bloom), 3 Jul (postbloom), 19 Jul, 26 Jul, 8 Aug, 16 Aug and 5 Sep. No Botrytis control measures, including leaf removal, were applied to test vines. All programs used one of two fungicides, Thiolux DF at 3 lb/ 100 gal water or Rally at 2 oz/ 100 gal water for each application. The field plots experienced several environmental extremes including a freeze (down to 17°F on 2 Feb 1996), a flood (several feet of flowing or standing water on 9-12 Feb), and frost (30°F on 4 May). The frost event impacted the plots the most, causing 50 to 70% of the shoot terminals to dieback. Undamaged secondary lateral buds on the shoots, or at the base of the shoots, continued to grow and produce acceptable vine growth. However, there was a reduction in the number of clusters that developed. Incidence of powdery mildew on leaves was evaluated on 10 Jul, 19 Jul, 30 Jul, 6 Aug, 16 Aug, 27 Aug and 11 Sep by randomly examining 50 leaves from the middle 3 vines of each replicate. Incidence of powdery mildew on clusters was evaluated on 6 Aug, 16 Aug, 27 Aug and 11 Sep by randomly examining 50 clusters from the middle 3 vines of each replicate. Comparisons among treatments for incidence of powdery mildew on leaves and clusters was evaluated by calculating the area under disease progress curves (AUDPQ. ALJDPC was calculated by multiplying the mean incidence from two observation dates by the number of days between observations. Values calculated between each pair of observations are added together to obtain a total AUDPC.
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