Petal fall. Photo: Tara Baugher, Penn State
Scarf skin and russet occur during the first 40 days of fruit development, starting at petal fall. Apple fruits grow rapidly after fertilization, and the outermost layers must also grow very rapidly to keep up with the growth of the expanding fruit. As the peel expands to keep pace with the rapid fruit growth, skin cells that have been arrested or damaged lose the elasticity needed to keep up with the rapidly growing fruit tissues beneath. Early tissue damage can result in more extensive apple skin defects and diminish the market value of the fresh fruit.
Several sprays of a proprietary mix of gibberellins (GA4+7) at 10-day intervals starting at petal fall (PF) are known to reduce the severity of scarf skin and russet. GA4+7 is effective for reducing skin defects due to unfavorable environmental conditions, and less so against chemical causes.
Four sprays of GA4+7 at 10-day intervals are made. Enhance absorption by applying sprays at 100 gallons per acre, under slow drying conditions. If possible, apply sprays with slow drying conditions (early morning, or late evening) with an expected minimum drying period of six hours. Gibberellins should not be applied with surfactants or other spray additives, and preferably not tank mixed with other crop protectant chemicals.
Gibberellins, particularly GA7 inhibit flower bud formation and can reduce the potential crop in the following year. When GA4+7 is used for managing skin disorders of biennial varieties, growers should also use a program of NAA or ethephon sprays for promoting return bloom. See the Penn State Tree Fruit Production Guide for details.
For detailed information, please review the article: Apple Skin Disorders: Scarf Skin and Russet.