The Use of ABA and AVG to Manage Fruit Set and Cropload of European Pear

‘Comice’ and ‘d’Anjou’ pear trees are "shy" to bear fruit, the latter only typically during the initial 10–15 years from planting, despite profuse bloom. Poor fruit set of ‘Comice’ has been attributed to a short ovule viability period (1–2 d).  The mechanisms limiting fruit set of ‘d’Anjou’ have not been elucidated.  We tested the hypothesis that inhibition of ethylene via amino-ethoxy-vinyl-glycine (AVG) would improve ovule viability, and, hence, fruit set of 'Comice'.  For ‘d’Anjou’, we used AVG to gain biological insights regarding flowering and fruit set.  Forty and 80 ppm AVG (ReTain, Valent) were applied at 80% bloom or 2 weeks after full bloom (wafb) to ‘d’Anjou’ and ‘Comice’ trees and compared to untreated controls.  AVG markedly improved ‘d’Anjou’ fruit set and yield in proportion to rate, but only at the 2 wafb timing.  At 2 wafb, 80 ppm increased tree yield ~3-fold compared to controls.  Fruit from ReTain-treated trees, however, were significantly smaller than control fruit, a function, in part, attributed to the higher fruit densities induced by AVG.  Interestingly, the number of seeds per fruit did not statistically differ between AVG and control fruit, implying that fertilization alone does not limit ‘d’Anjou’ fruit set.  Fruit set and yield of ‘Comice’ were significantly improved in response to rate at both application timings relative to controls. At 80 ppm, the 2 wafb timing produced the highest yields.  ‘Comice’ fruit size was reduced by AVG ~15%, but still attained commercially acceptable size.  Return bloom of either cultivar was not affected by AVG, irrespective of rate or timing.  Contrary to the fruit-setting habits of ‘Comice’ and ‘d’Anjou’, ‘Bartlett’ is precocious and productive.  Hand thinning is required to achieve commercial fruit size in the Pacific Northwest (PNW).  Current thinning agents do not consistently thin ‘Bartlett’ in the PNW.  We, therefore, tested the postbloom thinning efficacy of abscisic acid (ABA) (ProTone, Valent) on whole-canopies of ‘Bartlett’ pear trees.  Rates of 0, 125, 250, and 500 ppm were applied 21 dafb.  Photosynthesis was significantly, negatively related to rate for a 2-week period following application.  A highly significant, rate-dependent fruitlet abscission response was achieved by ABA; 125 ppm reduced fruits per tree to levels equivalent to the hand-thinned control.  ABA at 250 ppm severely reduced fruits per tree and 500 ppm removed all fruit.  Yield and final fruit size at harvest did not significantly differ between the control and 125 ppm ABA.