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Reduction in Yield and Vigor of Grapevine Caused by Crown Gall Disease. M. N. Schroth, Professor, Department of Plant Pathology, University of California, Berkeley 94720. A. H. McCain, J. H. Foott, and O. C. Huisman. Extension Specialist, Department of Plant Pathology, University of California, Berkeley 94720; Farm Advisor, Cooperative Extension, University of California, San Luis Obispo 93401; and Associate Professor, Department of Plant Pathology, University of California, Berkeley 94720. Plant Dis. 72:241-246. Accepted for publication 6 October 1987. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/PD-72-0241.

The crown gall disease caused by Agrobacterium tumefaciens affected yield and vigor of the grapevine (Vitis vinifera) cultivar Zinfandel. A comparison of berry yield, weight of prunings, and trunk diameters with disease ratings over a 4-yr period revealed that vines with galls covering >50% of the circumference of the crown region at the beginning of the assessment period were less vigorous and yielded fewer berries than vines with no galls. Reductions in berry yield and prunings ranged from 20 to 40% and 10 to 40%, respectively, over a 4-yr period, and trunk diameters (20 cm below forking) averaged 9% less than those of healthy controls. No significant reductions in yield or vine vigor were detected with mildly diseased plants. The incidence and severity of the crown gall disease changed over the period of the test. New galls formed on some previously healthy plants, and galls on diseased plants sometimes rotted and sloughed off without being replaced by new galls. Trunk diameters of plants subjected to continuous heavy disease pressure were significantly less than those of continuously healthy plants. Plant performance in some categories of mildly diseased plants, however, was significantly greater than that of continuously healthy controls. The stripping of berries by starlings was another factor affecting the correlation of disease and yield. Loss of berries was negatively correlated with overall plant vigor and canopy density, with birds preferring to feed on less vigorous vines. We concluded that the crown gall disease has both long-and short-term effects on plant performance as mediated by the environment.