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Prohexadione-calcium (ProCa) is used routinely in orchards to control vegetative growth and to reduce the shoot blight phase of fire blight. This communication reports on multiple-year applications of ProCa with special emphasis on treatment effects on fruit set, fruit size, and return bloom. Increased fruit set was confirmed from high rates of ProCa above 125 mg·L-1. The increase in fruit set was attributed primarily to a reduction of abscission during June drop rather than an increase in initial set. ProCa decreased fruit weight in some instances. Part of this reduction could be attributed to increased fruit set. However, the large reduction in fruit weight at harvest could only be explained by a direct effect of ProCa when used at high rates. When ProCa was applied as a concentrate spray at 250 mg·L-1, terminal growth was reduced comparable to the application made as a tree row volume dilute spray (Ix). However, fruit set was increased when the spray volume in which ProCa was delivered was reduced to 4x. A range of ProCa rates was used on 'Mutsu'. An initial application rate of 42 mg·L-1 followed by similar subsequent rates controlled growth comparably to higher initial and total rates, yet low rates had no effect on fruit weight or return bloom. Low rates of ProCa appear to be effective at controlling vegetative growth yet appear to have minimal side effects. High rates, especially those intended to reduce fire blight, come with the risk of increased fruit set and reduced fruit size and return bloom. [ABSTRACT FROM AUTHOR]

Prohexadione-calcium (ProCa) has emerged as one of the most important management tools that an orchardist has available to control vegetative growth and to reduce the incidence and severity of fire blight, it has also been implicated in increased fruit set on treated apple trees. This investigation was initiated to confirm the effects of ProCa on fruit set and to evaluate different thinning strategies that might be used to appropriately thin treated trees. ProCa increased fruit set when applied at petal fall at initial rates of 125 or 250 mg·L-1 in three of the four experiments described in this article. Thinners were applied before, at the time of, and after application of ProCa. In all experiments, chemical thinners did not reduce fruit set to the same crop load level on ProCa-treated trees as they did on untreated trees, it was concluded that a different and more aggressive chemical thinning strategy must be used on trees that were treated with ProCa. Fruit size was reduced on ProCa-treated trees. This reduction was usually, but not always, related to increased fruit set. ProCa increased the number of pygmy fruit on 'Delicious' apple trees. [ABSTRACT FROM AUTHOR]

Proheaxadione-calcium (ProCa) was applied to 'Spencer' apple trees at rates between 250 to 750 mg·L-1 after harvest but before leaf fall. The following spring terminal growth was reduced linearly from early petal fall through the growing season to leaf fall. ProCa was applied after harvest and before leaf fall at 500 mg·L-1 to 'Spigold' apples. The carryover effect on terminal growth persisted for about 2 weeks after bloom. A combination of 500 and 82.5 mg·L-1 ProCa on 'Spigold' in the fall and spring, respectively, reduced terminal growth greater than the individual treatments for about 3 weeks after petal fall. Recent report have shown that ProCa can induce physiological resistance to fire blight and apple scab when applied near petal fall. These data support the suggestion that ProCa may be used as a fall application and the carryover effects may result in early growth control. Potential benefits of the carry-over effects of ProCa for early-season suppression of fire blight and apple scab are discussed. [ABSTRACT FROM AUTHOR]