GRAPEVINE LEAF GAS-EXCHANGE, PLANT-GROWTH, YIELD, FRUIT-QUALITY AND CARBOHYDRATE RESERVES INFLUENCED BY THE GRAPE LEAFHOPPER, EMPOASCA-VITIS

The impact of the grape leafhopper, Empoasca vitis, on leaf gas exchan ge, plant growth, yield, fruit quality and carbohydrate reserves of th e grapevines, Vitis vinifera L., was studied. Gas exchange was measure d on the discolored (red) and the green parts of infested main leaves and on leaves from uninfested vines. Photosynthesis and mesophyll cond uctance were severely reduced on main leaves showing leafhopper feedin g symptoms. The stomatal conductance of the red leaf section of infest ed main leaves was lower than on undamaged control leaves. Additionall y, the red leaf section of infested main leaves showed lower transpira tion rates when compared to the green parts of the same leaves and to undamaged control leaves. Gas exchange processes of lateral leaves wer e not affected by leafhopper feeding. Leafhopper-load on main leaves w as correlated to visual damage symptoms. At 71.8 leafhopper-days per l eaf up to 40% of the main leaf area of the infested plants was discolo red from the borders towards the center. Lateral leaves showed no feed ing symptoms. Shoot diameter, pruning weight and carbohydrate reserves in the wood were not affected by leafhoppers. Lateral leaf area growt h was significantly stimulated on plants infested by leafhoppers. No d ecrease in yield and fruit quality with leafhopper-loads up to 71.8 le afhopper-days per leaf were observed.