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Disease Detection and Losses

Reductions in Yield and Vegetative Growth of Grapevines Due to Eutypa Dieback. G. P. Munkvold, assistant professor, Department of Plant Pathology, Iowa State University, Ames 50011; J. A. Duthie(2), and J. J. Marois(3). (2)plant pathologist, Wes Watkins Agricultural Research Center, Oklahoma State University, Lane 74555; (3)professor, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 84:186-192. Accepted for publication 30 November 1993. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/Phyto-84-186.

Eutypa dieback of grapevines is a serious canker disease that slowly kills vines, but there is little information on yield reduction of declining vines. The severity of Eutypa dieback on individual grapevines was assessed in 1991 and 1992 as the proportion of the vines’ spurs killed 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), expressed as a percentage of the yield of healthy vines, was y = 101.0 – 90.26d (r² = 0.70) in 1991, and y = 100.1 – 98.81d (r² = 0.79) in 1992. The slope of this relationship was not significantly different between years or among five vineyards of two cultivars (Chenin blanc and French Colombard). Yield reduction was primarily due to a diminished number of clusters per vine, while the effects of disease on mean cluster weight were smaller and not always significant. The effect of disease severity on vegetative growth, measured as pruning weight, was significant in two of three vineyards in 1991 and both of two vineyards in 1992. Linear slopes of the pruning weight-disease severity relationships ranged from –29.5% (r² = 0.06) to –67.4% (r² = 0.46) per unit disease severity. Yield reductions for whole vineyards, estimated from the regression models, ranged from 30.1 to 61.9%, depending on mean disease severity. Records for Chenin blanc vineyards in Merced County, CA, revealed a trend of declining yields beginning at 12 yr of age, which closely followed the period of rapid increase of Eutypa dieback. In Barbera vineyards, which are rarely affected by Eutypa dieback, yields increased up to age 10 and then remained constant.