Adjusting Spray Programs to Suppress Fruit Russet and Minimize Phytotoxicity Risks for Apples
Many apple growers in Northeastern United States noted an unusually high incidence fruit russet and/or phytotoxicity damage during the 2013 growing season. Factors that contribute to fruit russet include spring frosts, chemical phytotoxicity (e.g., from copper or captan ), powdery mildew infections, and post - bloom activity of Aureobasidium pullulans. A. pullulans is a yeast-like fungus that is the most common epiphyte on apple trees and that regularly contributes to russet on russet-prone cultivars such as Golden Delicious (Andrews et al., 2002; Heidenreich et al., 1997). Other causes of russet have been identified, but are not well understood. For example, fruit directly exposed to sunlight have been shown to develop more russet than shaded fruit (Noé and Eccher, 1996). Wet weather during the immediate post-bloom period may contribute to “physiological” russet (Creasy, 1980), but it is not clear whether those conditions cause russet directly or via development of large populations of A. pullulans.
Identifying the cause of russet on affected fruit can be difficult in situations where multiple factors might have been involved because russet from various causes often results in similar injury patterns on fruit. Russet on fruit may not be visible until several weeks after it was initiated, and that time delay can further complicate attempts to pinpoint the cause. Despite the ambiguities inherent in determining causes of russet on specific fruit, apple growers should recognize that some management practices can reduce risks while other practices may increase risks for fruit russet or foliar phytotoxicity.