Comparison of standard calendar GA₄₊₇ plant growth regulator programs to applications timed to rainfall events for management of scarf skin, a fruit finish disorder of apple

Scarf skin is a fruit finish disorder of apple that leads to reduced grade quality and rejected crop loads. While not established as a disorder specifically to certain cultivars, the manifestation of the disorder is more apparent on red-skinned cultivars such as ‘Gala’ and ‘Stayman’ at harvest where its whiteish cast is held in contrast with the redness of the mature fruit. The disorder afflicts fruit during the first forty days after petal fall as the fruitlet is developing, though symptoms of the disorder may not be clearly visible until many months after. The exact cause(s) of apple scarf skin are not well defined, although it is known that the presence of the disorder can be influenced by environment, fruit size, and some pesticides. Management currently relies on four applications of the plant hormones gibberellins A4 and A7 (GA₄₊₇) during the first forty days after bloom. Here, we describe our attempts to use a commercialized product (ProVide 10SG) starting at petal fall, applications made after or before rainfall events to ease rainfall’s putative influence on scarf skin development. We compared our programs to the standard program over four years in research orchards and a commercial orchard but observed no consistent reduction in scarf skin severity or incidence of out-of-grade fruit, in part due to the year-to-year variability in which scarf skin manifests. Rainfall, at the thresholds defined, does not appear to be as significant a factor influencing scarf skin development for which a PGR management program can be designed.