PHOTOSYNTHESIS AND TRANSPIRATION OF RIESLING X SYLVANER GRAPEVINE LEAVES AS AFFECTED BY EUROPEAN RED MITE (PANONYCHUS-ULMI KOCH) (ACARI, TETRANYCHIDAE) FEEDING

The effects of feeding damage by the European red mite, Panonychus ulm i on the gas exchange of grapevine leaves (Vitis vinifera L.) were inv estigated. At mite infestation levels up to 3500 mite-days per leaf (c orresponding to a peak density of 66 mites/leaf), plants infested with P. ulmi showed no decrease in the pnotosynthetic rate. The same infes tation level reduced transpiration only by 8.7 %. Stomatal and mesophy ll conductance were not affected by P. ulmi feeding. The results obtai ned in this investigation lead to the conclusion that the empirically defined economic injury level for P. ulmi of 2-5 mites per leaf, actua lly used by the viticulturists in Central Europe, is far too low.