Vector Preference and Inoculation Efficiency as Components of Resistance to Pierce's Disease in European Grape Cultivars

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Abstract
Feeding preference and inoculation efficiency of the leafhopper Graphocephala atropunctata (the blue-green sharpshooter) were evaluated as factors affecting the rate of natural spread of Pierce''s disease (PD) [caused by bacteria] in European grape (V. vinifera) cultivars in California [USA]. Significant feeding preferences for certain cultivars were observed in field counts of sharpshooters and in cage trials, but preference was not correlated with resistance. In young potted grapevines no major differences were noted in the resistance of cultivars to infection following exposure to feeding by infectious leafhoppers. Older foliage of ''Sylvaner'', ''Cabernet Sauvignon'', ''Chenin blanc'', ''Thomposn Seedless'', ''Petit sirah'' and ''Ruby Cabernet'' was more resistant to infection than was young foliage, but this was not true for ''Pinot noir''. Older foliage of ''Flora'', ''Chardonnay'' and ''Mission'' also was more resistant to infection than was younger foliage in greenhouse tests. Exposure of mature vines growing in the field to leafhopper vectors produced infections that spread much less rapidly from the point of inoculation in some cultivars than in others. In some cultivars fewer infections persisted through the following dormant season than in others. Inoculations by leafhoppers in April-June produced more persistent infections in all cultivars than did inoculations made in July or early Aug. A theoretical model of the probability of infection using conservative estimates of vector density, infectivity and transmission efficiency predicted much higher infection levels than those commonly observed under the assumed conditions. Adjusting predicted disease incidence for the persistence of infections through the dormant season as a function of inoculation date produced more realistic rates of disease spread and differences in PD incidence among European grape cultivars.