| Potato leafhoppers (Empoasca fabae Harris) can be pests in Michigan vineyards, but grapevine responses to infestations have not been well documented. In order to rectify this, experiments were conducted on potted grapevines at the whole-vine and single-leaf levels. Potted and fruitless Pinot gris (Vitis vinifera L.) grapevines grafted to rootstock 1103 Paulsen (V. berlandieri Planch. x V. rupestris Scheele) were infested with various levels of potato leafhopper (PLH) nymphs for seven days. Infestation severity was directly related to the leaf symptoms of cupping and discoloration and inversely related to both shoot and leaf growth. Leaf and root biomass tended to decline in response to infestation. Damage thresholds, defined as the number of insects necessary to cause damage to the plant (Pedigo et al. 1986), were determined for leaf growth, shoot growth and leaf symptoms. A threshold of 0.5 PLH was found for leaf cupping, leaf size, shoot length and internode length; a threshold of 1.0 PLH was found for leaf discoloration and leaves per vine; a threshold of 3.0 PLH was found for internodes per vine and 4.5 PLH for root fresh mass, root dry mass and total vine dry mass. Although PLH infestations caused decreases in shoot growth, leaf growth and biomass accumulation, the vines were able to tolerate low infestation levels without measurable decline in growth. In addition, the vines were able to recover for some growth reductions during the post-infestation period. In order to investigate the impact of PLH infestation on grapevine leaf photosynthesis, PLH nymphs were caged on 2.5 cm2 portions of grapevine leaves and gas exchange was measured before and after the infestation period. PLH infestation level was inversely proportional to carbon assimilation (A), transpiration (E) and stomatal conductance (Gs) and directly proportional to internal CO2 concentration (Ci). Decreases in A were correlated with decreased Gs and increased Ci and thus reductions in A were due to both stomatal and non-stomatal limitations. Damage thresholds existed for A, E, Gs and Ci at most leaf positions. Threshold levels for the four leaf positions from apex to base respectively were: 1, 2, 2 and 8 for A; 1, 1, 2 and none for E; 1, 1, 2 and 8 for G s; and none, 4, 4 and 8 for Ci. These studies show that: (1) PLH infestations reduce shoot growth, leaf growth and biomass accumulation of potted Pinot gris grapevines; (2) PLH infestations reduce A, G s and E and increase Ci of grapevine leaves; (3) damage thresholds exist for growth and photosynthetic performance of potted Pinot gris grapevines. |
