FOLIAR S-ABA APPLICATION DOES NOT REDUCE SUNBURN IN 'GRANNY SMITH' APPLE

Sunburn is the major cull factor for apples (Malus ×domestica) under the Mediterranean-type climate of the Western Cape production region in South Africa. Up to 40% of the crop may be lost. Recent research suggests that the biologically active form of abscisic acid, S-ABA, may decrease sunburn in apple. We evaluated the effect of S-ABA on sunburn, general fruit quality and photochemistry in ‘Granny Smith’ apple during the 2010/11 to 2012/13 seasons. S-ABA was applied at various concentrations (250-1000 mg/L) and timings (from November until harvest in March) to a mature ‘Granny Smith’ orchard. A representative scaffold branch was strip picked at commercial harvest on both sides of the tree giving rise to samples of at least 50 fruit per tree. The application of S-ABA had no effect on sunburn incidence and severity. There was a significant reduction in fruit weight, fruit size, juice titratable acidity and total soluble solids concentration with repeated applications of S-ABA. This was probably due to the reduction in photosynthesis caused by stomatal closure in response to S-ABA application. Starch conversion was not affected by S-ABA. A negative effect of S-ABA on green peel color was noted for some applications while pink blush development and stem-end russet were not affected by S-ABA application. At the timings and rates applied, S-ABA does not seem to decrease sunburn in apple under South African conditions.