The influence of low light on apple fruit abscission

SummaryRemoval of the stigma and half the style from flowers at the pink or early stages of bloom caused 98% of the fruits to drop by 15 days after full bloom (AFB). Two or three consecutive days of 92% artificial shade (92% polypropylene shade material) reduced fruit set to 83-93% when trees were shaded 14, 21 and 28 days AFB. However, two or three days of artificial shade applied 8, 35, or 42 days AFB did not influence fruit drop. After three days of artificial shade, fruit stopped growing on the last day of shading and up to six days after shading and dropped 7-12 days later. Naturally occurring fruit drop in the period from 15 to 40 days AFB appeared to be related to 2-4 days of cloudy weather. Low light from cloudy periods 3-4 days long were calculated to be equivalent to 2-3 days of 92% artificial shade. If 1-2 days of sunlight separated the two or three days of applied artificial shade, less thinning occurred. Two days of artificial shade induced more fruit drop than NAA, ethephon, or carbaryl + oil thinning sprays. When carbaryl + oil were applied on the first day of artificially shaded trees, more thinning occurred than if no shade was applied or if trees were exposed to one day of full sun before the thinner was applied. When fruit were 20 mm in diameter, shading the whole tree for three days caused 98% fruit abscission, but if a limb was left in the sun 70% of its fruit were retained on that limb whilst a shaded limb on the shaded part of the tree retained only 5% of its fruit. Shading only one limb for three days caused 45% fruit abscission on that limb without affecting the fruit set on the rest of the tree.