An important disease of Vitis vinifera production in Oregon and all other commercial growing regions is powdery mildew of grape, caused by the obligate fungal pathogen Uncinula necator (Schwein.) Burril. Grape production can be characterized as a long-term investment in the establishment and maintenance of the vineyard. Establishment times have been reduced with the use of plastic vine shelters, but powdery mildew disease pressure within vine shelters had been an unaddressed issue. Control of the pathogen requires frequent spray applications and costly cultural management of the grape canopy. Industry interest in forecasting programs have shown promise in regulating spray applications to times when they are most effective, or needed. The timing of when to begin spray programs is believed to be a point of weakness in the forecasting programs currently available for grape powdery mildew. The influence of vine shelter use on the development of powdery mildew was investigated in the field during the 1998 and 1999 growing season. Industry standard installations of various brands of vine shelters were tested against modified installations for both incidence and severity of Uncinula necator infection. The industry standard installation of 76 cm high tubes hilled with 8 cm of soil at the bottom to prevent airflow, were effective in reducing the incidence of powdery mildew in both field seasons. Disease reduction was associated with prolonged temperatures above 36��C and the exclusion of infective spores by the artificial barrier created by the vine shelters. The effectiveness of three forecasting programs for predicting the initial spray application was investigated for three seasons. Actual disease onset dates were determined by using trap leaves or plants. The forecasting programs consistently predicted initial spray dates between 31 and 44 days prior to the detection of powdery mildew with the trapping system. Modifications to the existing forecasting programs were attempted to adjust the forecasting programs to more closely predict the actual detected disease onset dates. The UC-Davis program performed the best over the three years of the study, but improvements will be necessary for an adequate forecasting program in the region. Flag shoots were reported for the first time in Oregon. / Graduation date: 2000

During summer of 1998 and 1999, 34 and 10 vineyard sites, respectively, were sampled to assess spider mite pests and associated biological control by phytoseiid mites. Vineyards studied spanned five major valleys in western Oregon where grape production occurs. Leaf samples were taken from site perimeters and centers. One leaf was taken every ten meters of border length, five meters inward from the border to prevent wind-biased or extreme edge effects, while 20 leaves were taken at regular intervals from centers. Variables recorded at each site were: plant age, grape variety, chemical spray information and local vegetation occurring in proximity to vineyards. Sites were categorized as either agricultural or riparian based on what surrounding vegetation type was in the majority. Several parametric and non-parametric tests were used to analyze data, including multiple linear regressions using a computer-based genetic algorithm in conjunction with the AIC criterion to pre-select a subset of explanatory variables. Typhlodromus pyri was the predominant phytoseiid mite and Tetranychus urticae was the most abundant tetranychid mite sampled. High levels of T. urticae were found when predator densities were very low, and low levels of T. urticae occurred when predator densities were moderate or high. Phytoseiid densities were highest in June and July, while T. urticae densities were highest from August to September. The latter's densities were significantly higher in vineyards surrounded primarily by agriculture, while phytoseiid densities were not significantly different between the two categories. Predatory phytoseiids had significantly higher densities on vineyard edges, while T. urticae densities were higher in vineyard centers. Caneberry, cherry and grape habitats appeared to be sources of predator immigration, while no vegetation type consistently served as a short-range or nearby immigration source for spider mites. Due to insufficient data, pesticide information was not included in multiple linear regression models, although certain chemicals used in vineyards can potentially impact mite populations. Impacts of surrounding vegetation type, grape variety, regional location, plant age, and presence of other mites on phytoseiid and T. urticae densities are discussed. / Graduation date: 2001