Evaluation of Management Options For Organic Winegrape Growers
Spider mite presence in the Chardonnay vineyard (Fetzer Vineyards, Mendocino County) first became noticeable in early June 1998. At that time, the plots were delineated and the mite sampling protocol was initiated. Data for an array of viticultural parameters were also collected during 1998. Vine water status was assessed periodically from June through September using porometer readings for stomatal conductance and transpiration readings based upon three leaves per plot. In addition, petiole samples and berry samples were taken, with yield data being collected in early October. Regression analysis was used to determine significance of correlation between mite population and factors such as stomatal conductance, transpiration rate, sugar content, and yield. Comparison of mite infestation and stomatal conductance for 1998 revealed that in situations when vines had heavy mite pressure, stomatal conductance tended to be lower, and vice versa. This inverse relationship between mites and stomatal conductance was significant at the 0.01 level. Linear regression of mite infestation and transpiration rate again indicated a clear inverse relationship between mites and transpiration, with a significance levels of P = 0.021 for 1998. Comparison of mite infestation with sugar content of berries also indicated a significant inverse relationship during 1998, with P = 0.02. Unusual weather conditions (i.e., El Nino) in 1998 appeared to delay maturity throughout the crop. Linear regression of mite infestation and yield for 1998 indicated that the inverse relationship was highly significant (P < 0.01), although no significant relationship for these parameters was delineated (P > 0.05) in 1997. In conclusion, the study conducted at Fetzer Vineyards has clearly demonstrated that mite populations [i.e., Willamette mite (Eotetranychus willamettei)] may be negatively correlated with such viticulture parameters as yield, sugar content, stomatal conductance, and transpiration rate.