Grape phylloxera, Daktulosphaira vitifoliae Fitch (Hemiptera: Phylloxeridae), is an important pest that forms galls on the foliage and roots of Vitis species. The effects of grape phylloxera on grape root have been studied extensively. This study investigated the effects of the foliar form of grape phylloxera, which is a persistent pest of North American native Vitis species and cold-hardy hybrid wine grapes between Vitis vinifera L. and Vitis riparia Michx. For a susceptible variety, ‘Frontenac’, in commercial Minnesota vineyards, there were reductions in cluster weight due to foliar phylloxera infestations in 2017 (one site; P = 0.05) and 2018 (three sites; nonsignificant). Reductions in cluster weight can be economically important to growers. For four grape varieties (susceptible and resistant) evaluated in the greenhouse, there were numerical reductions in photosynthetic rate and localized photosystem II efficiency in 1 of 2 yr. Resistant varieties had less of a reduction on conductance due to grape phylloxera infestation than susceptible varieties in 1 of 2 yr. The degree to which this occurs in resistant varieties may be related to the formation of fewer adaxial stomata; however, this hypothesis requires additional research. These results add important information to better understand how foliar phylloxera injury affects wine grape photosynthesis and yield. The results serve as a foundation for investigating variety-specific responses to foliar phylloxera through the use of advanced phenotyping technologies. This knowledge will lead to a better understanding of the effect of the pest on the diversity of cold-hardy grape varieties grown in the Midwestern United States.

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