REDUCTIONS IN YIELD AND VEGETATIVE GROWTH OF GRAPEVINES DUE TO EUTYPADIEBACK

Eutypa dieback of grapevines is a serious canker disease that slowly k ills vines, but there is little information on yield reduction of decl ining vines. The severity of Eutypa dieback on individual grapevines w as assessed in 1991 and 1992 as the proportion of the vines' spurs kil led or symptomatic. In each year, Eutypa dieback caused a significant reduction in yield of infected vines compared with healthy vines. The linear relationship between disease severity (d) and yield (y), expres sed as a percentage of the yield of healthy vines, was y = 101.0 - 90. 26d (r(2) = 0.70) in 1991, and y = 100.1 - 98.81d (r(2) = 0.79) in 199 2. The slope of this relationship was not significantly different betw een years or among five vineyards of two cultivars (Chenin blanc and F rench Colombard). Yield reduction was primarily due to a diminished nu mber of clusters per vine, while the effects of disease on mean cluste r weight were smaller and not always significant. The effect of diseas e severity on vegetative growth, measured as pruning weight, was signi ficant in two of three vineyards in 1991 and both of two vineyards in 1992. Linear slopes of the pruning weight-disease severity relationshi ps ranged from -29.5% (r(2) = 0.06) to -67.4% (r(2) = 0.46) per unit d isease severity. Yield reductions for whole vineyards, estimated from the regression models, ranged from 30.1 to 61.9%, depending on mean di sease severity. Records for Chenin blanc vineyards in Merced County, C A, revealed a trend of declining yields beginning at 12 yr of age, whi ch closely followed the period of rapid increase of Eutypa dieback. In Barbera vineyards, which are rarely affected by Eutypa dieback, yield s increased up to age 10 and then remained constant.