Sunburn Susceptibility of ‘Golden Delicious' Apple in Ohio as Affected by Several Rootstocks in the 2003 NC-140 Apple Rootstock Trial

Rootstocks greatly influence tree vigor and thus canopy volume being able to provide shade against excessive solar radiation. Therefore, they have a great indirect effect on sunburn damage of apple fruit. Weak trees with sparse foliage development are likely to bear many sunburned fruit, while vigorous trees suffer little damage. In this trial, the incidence of three types of sunburn (sunburn necrosis, sunburn browning and photooxidative sunburn) on ‘Golden Delicious’ apple on 23 rootstocks (Bud.62396, B.9, CG.16; CG.3041, CG.5935, CG.6210, G.16, JM.1, JM.2, JM.4, JM.5, JM.7, JM.8, JM.10, JTE.4, JTE.6, M.9, M.9-T337, M.26, PiAu.362, PiAu.51-1, PiAu.51-4, and PiAu.5683) was evaluated. Largest trees and low incidence of sunburn necrosis (<2%) and sunburn browning (<4%) were found on PiAu.5683, JM.5 and PiAu.51-4. Trees grafted onto JTE.6, PiAu51-1, JM.1, JM.4, JM.10, M.9, M.9-T337 and Bud.62396 were of low vegetative vigor and better exposure of fruit to the direct sunlight resulting in higher incidence of sunburn necrosis (>3%) and sunburn browning (>6%). Close relationship between tree vigor and the incidence of photooxidative sunburn was not found. Even though sunburn browning and sunburn necrosis often occurred on larger fruit whereas photooxidative sunburn on smaller ones, close correlation between fruit size and sunburn incidence were not established. Fruit position on a cluster (king vs. side fruit) or fruit number per cluster did not seem to have influence on sunburn incidence.