Observations on the Relationship Between Crop Load and Return Bloom in `Honeycrisp' Apple

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  • 1 1Michigan State University, Department of Horticulture, Clarksville Horticultural Experiment Station, Portland, MI, 48815
  • | 2 2Michigan State University, Horticulture, East Lansing, MI, 48823

The `Honeycrisp' apple has unique characteristics favored by consumers that has provided exceptional return to growers. This cultivar also has some traits that challenge plant management. There appears to be a strong inhibitory effect of crop load on flower initiation and thus annual cropping. We studied the relationship between fruit load, established by post-bloom hand and chemical thinning, and effect of ethephon and gibberellin (GA4+7) on flower initiation and thereby annual cropping. Initially, return bloom (RB) was related to previous season's crop load in three thinning studies on 3- and 9-year-old `Honeycrisp'/M 9, Pajam 1 trees. The RB density was rated 1–10 on trees (n=172), which produced 0-60 kg of fruit/tree. Return bloom ratings (RBR) on the 3-year-old trees ranged from 0–9. Percentage of trees with RBR >5 for previous season's yield of <5 kg, 5–10, 10–15, and 15–20 was 70, 9, 2.5, and 0, respectively. There was dramatic inhibition of flowering at a crop load of >5 kg/tree. In the second study (9-year-old trees), crop load ranged from 15–60 kg/tree (n=24). RBR for trees in the 30–40 kg/tree class ranged from 0–8 with high variability. Thirty-one percent of trees with crop load between 20–30 kg had RBR of 5–8, and 26% between 0–5. Twenty-one percent of trees in the 30–40 kg/tree class flowered and all but one had a RBR of 5 or less. Yields ranged from 22–81 kg/tree in the third study (n=60); crop load was normally distributed among trees. Flower initiation was almost completely inhibited. Fifty percent of the trees did not flower; the remainder had a RBR of <1, i.e. <10%. In the ethephon/GA study, RBR ranged 8–10 on trees producing <12 kg/tree, then decreased rapidly to <2 for yields of 25–50; greatest variation at 20–40 kg.

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