Improved Precision of Powdery Mildew Management through Early Detection of Erysiphe (Unicinula) Necator in the Air of Washington Vineyards

Current risk-assessment models designed to facilitate the management of grapevine powdery mildew use vine phenology and prevailing weather conditions to assess disease risk. Inoculum availability and concentration are not model components at this time. The purpose of this study is to devise an reliable, inexpensive, and rapid means of detecting propagules of E. necator in the vineyard air prior to disease onset and, in doing so, add an ?inoculum availability? component to current models. One of our primary challenges was to overcome difficulties inherent in the positive and rapid identification of E. necator propagules using conventional techniques of light microscopy. We have developed primers that when used in PCR reactions can differentiate E. necator from 46 other powdery mildews common in the Pacific Northwest. In our extensive 2006 laboratory studies, we found the primer pairs sensitive enough to amplify the DNA from as few as 100-500 spores placed on glass air sampling rods. Regression analysis revealed a significant (F = 47.3; P= 0.005) relationship [y=1.6*exp(-exp(-(x-74.1)/75.4))] between the numbers of conidia placed on glass sampling rods and successful PCR amplifications with a coefficient of determination (r2) of 0.97. The primers were also used to identify E. necatorin vineyard air samples prior to disease onset. In 2006 E. necator was detected in the vineyard air using Rotorod air samplers situated within the vineyard prior to disease onset. Vineyard air samples were devoid of the pathogen prior to bud burst and prior to the initial ascospore release. The earliest indication of the presence of E. necator in the air occurred during a rain event of 9.9 mm that occurred during the prebloom stage. The presence of airborne ascospores during this rain event was confirmed using a Burkard volumetric air sampler. Subsequent negative sampling results intimated that the vineyard was apparently devoid of E. necatorpropagules, or the concentration of airborne propagules was below the detection thresholds of the sampling method, during incubation and latent periods. The detection of E. necator resumed several days prior to the visual appearance of powdery mildew signs and continued during subsequent disease development. When the stationary Rotorod method indicated the presence of E. necator in the vineyard air for the first time after bud burst, the information was used to initiate a fungicide spray program. In vineyards under high disease pressure, there was good correlation between predicted ascospore release and initial detection in all appellations where air-sampling studies were conducted. In a vineyard under high disease pressure in 2006, the use of sampling-driven mildew fungicide programs resulted in a 1-2 fewer applications, depending upon treatment regime. In a second vineyard trial use of the sampling-driven approach reduced fungicide usage by 86%. We are currently working with a local analytical company interested in providing PCR service to the Washington grape industry.