IMPACT OF APPLE RUST MITE (ACARI, ERIOPHYIIDAE) FEEDING ON APPLE LEAFGAS-EXCHANGE AND LEAF COLOR ASSOCIATED WITH CHANGES IN LEAF TISSUE

The impact of the apple rust mite, Aculus schlechtendali (Nalepa), on net CO2 exchange, transpiration rate, and leaf color of field-grown Jo nagold and Golden Delicious apples was investigated. Apple rust mite f eeding causes leaf browning. Changes in leaf color were measured to as sess the cumulative leaf damage. Significant negative relationships we re found between cumulative leaf damage and single-leaf net CO2 exchan ge as well as transpiration rate. The same trends were observed on bot h varieties, but the effect of apple rust mite feeding was more severe on Jonagold than on Golden Delicious. Leaf tissue injury was analyzed by cryoscanning electron microscopy and light microscopy. The picture s show that apple rust mites penetrate epidermal cells with their styl ets, causing multiple puncture wounds. On heavily infested leaves, app le rust mite feeding causes desiccation of the epidermis and the spong y parenchyma. The resulting malfunction of the stomata and problems in gas exchange within the spongy parenchyma are likely to be the main r eason for the reduction of gas exchange.