Chemical and Defoliation Effects on Fruitset of \"Cox orange\" and \"Elstar\" Apple Cultivars

Abstract

Thinning trials were conducted in the apple orchards of Klein Altendorf experimental station near Bonn, Germany, using 7 year old CV, \'Cox orange\' in the year 2001 and 8 year old \'Elstar\' apple trees in 2002. The objective was to reduce the
number of fruits per tree, yield, improve fruit quality, overcome alternate bearing and to investigate how photosynthesis and transpiration are affected by one of the chemical thinners (i.e. Azolon). At a dose of 10 mg a.i/Li Ethrel (Flordimex) thinned better than at 5 mg a.i /Li in 2001 while at 3 m g a.i/Li in 2002 it achieved the required fruitset by thinning before June drop 62% and 15% after June drop compared to 49% of the unsprayed control and had 71% of the fruits in the class 1 fruit diameter size range of 75 to 90 mm while yield per tree was reduced by 31% by Ethrel at 10 mg a.i /Li but not Ethrel at 5 mg a.i/Li which had no effect on yield. Mean fruit weight was unaffected by the treatments. The foliar urea based fertilizer Azolon fluid at 75 mg a.i/Li applied at full bloom, resulted in 61% fruitset before and 17% after June drop in 2002, slight leaf yellowing, scorching and epinasty 7 days after treatment application which disappeared 14 days later but probably started earlier and a slight reduction in leaf photosynthesis and transpiration rates. However, Azolon fluid (i.e. methylene urea, or urea formaldehyde) yielded smaller fruit than Ethrel (Flordimex), but larger fruit than the untreated control trees, without affecting fruit yield. The partial and gradual defoliation of selected primary spur leaves reduced
fruitset to 50% before and to 14% after June drop. These results provide evidence that reductions in photosynthetic rate or photosynthetic leaf area during the period of flowering is sufficient and can successfully induce thinning with a commensurate efficacy to chemical thinners.

Keywords: Thinning, growth regulators, growth hormone regulators, defoliators, orchards, apple, azolon, foliar fertilizer, Elstar, Cox Orange fruit set, fructification, fruit drop, defoliation, flowering

Discovery and Innovation Vol. 19 (4) 2007: pp. 280-284