Interaction of Euonymus scale (Homoptera: Diaspididae) feeding damage and severe water stress on leaf abscission and growth of Euonymus fortunei

Rooted cuttings of Euonymus fortunei were grown in a 2x2 factorial design experiment with or without euonymus scales and/or water stress. Infested plants and water stressed plants abscised leaves whereas uninfested, unstressed plants did not. There was significant interaction between scale infestation and water stress, with a synergistic effect on leaf abscission. Both scale infestation and water stress reduced the root weight, but there was no resulting change in the root/shoot ratio. Naturally infested leaves on plants growing outdoors had a higher solute potential (ψ_s) and a lower pressure potential (ψ_p) than did uninfested leaves with the same water potential (ψ_W). Infested leaves may therefore be more prone to wilting. However, scale-damaged leaves also have increased diffusive resistance and lowered transpiration rates, suggesting that scales impair normal function of guard cells. The possible contribution of these factors to the heightened susceptibility of infested leaves to abscission following water stress is discussed.