Abstract
Laboratory bioassays were used for rapid initial evaluation of the susceptibilities of 26 grape rootstocks to the two known California biotypes of grape phylloxera. An indicator of the population growth rate on each rootstock, the egg multiplication index (EMI), was calculated for each biotype from the bioassay results and was used to define five categories of host plant suitability. This index can be used as an objective criterion for rating the suitability of roots as hosts of grape phylloxera. Populations of the two biotypes grew at approximately equal rates on all but two rootstocks, AXR#1 and 41 B. Tests of 15 other colonies on several of the rootstocks did not suggest that there are other biotypes. Three rootstocks, AXR#1, VR O43-43, and 41 B, and own-rooted cultivars of <i>Vitis vinifera</i> should not be used because they support high populations of one or both phylloxera biotypes.
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