Use of prohexadione calcium (Apogee or Kudos 27.5 WDG) can dramatically reduce shoot length of apple when used in a season-long program. Untreated shoots (left) vs. a season-long prohexadione calcium program (right).

I received multiple calls and e-mails regarding use of prohexadione calcium (P-Ca; Apogee or Kudos 27.5 WDG). Most of these inquires were related to managing bitter pit of ‘Honeycrisp’ apple with P-Ca. A team of research and extension faculty at Cornell University conducted multi-year studies and are recommending use of 6 oz. of Apogee or Kudos per 100 gallons of water at pink bud stage to reduce bitter pit incidence. Inexplicably, different rates (higher or lower), later timings, or split applications were either ineffective or seemed to exacerbate bitter pit development. This observation is surprising, as P-Ca is a dose dependent chemistry as it relates to shoot growth and development. So why/how does this work? The authors of the above publication indicate that, “the short answer is that the mode of action is not clear. We have collected and are in the process of analyzing additional data not reported here, and work will continue in 2019.”

Some of you are asking if I would recommend this management practice in the southeastern US. At present, I cannot endorse this recommendation as it has not been evaluated in the southeastern US or adjacent states and the mode of action is unclear. I can however, recommend use of prohexadione calcium (P-Ca) to manage tree vigor. Season-long P-Ca programs can be an effective strategy to manage vegetative growth of apple. Season long programs can: 1) reduce shoot length, 2) improve light distribution in the canopy (related to flower bud development and fruit quality), 3) increase fruit set (which can be a negative consequence), 4) improve pesticide coverage, and 5) reduce pruning costs. Additionally, early applications of P-Ca are an effective method to reduce shoot blight incidence.

2019 Trial: Managing vegetative growth with prohexadione calcium and root pruning

A trial was conducted at the Mountain Horticultural Crops Research and Extension Center in 2019 on excessively vigorous ‘Golden Delicious’/ ‘M.111’ trees. Treatments included an 1) untreated control, 2) root pruning (Figure 1.), and 3) a season long P-Ca program.

Figure 1. Root pruning was imposed ~2′ from the trunk on 8 April and 10 May 2019.

P-Ca applications began on 22 April and were continued on ~21 day intervals up to 8 July 2019. The first two applications were applied at 24 fl. oz./ 200 GPA and the remaining applications were at 16 fl. oz./ 200 GPA, for a season total of 96 fl oz/A (5 applications). Visible effects of P-Ca treatment were evident during the growing season, and factors influenced by P-Ca were evaluated at harvest and during dormancy (Figure 2).

Figure 2. Effect of season long P-Ca applications on vigor and shoot length. Untreated control (left) and P-Ca treated (right).

P-Ca increased yield by 49% compared to the untreated control (Table 1). This block had a relatively weak return bloom, so this was beneficial. Additionally, P-Ca treated trees had a higher crop density (# of fruit per unit trunk cross-sectional area) when compared to root-pruned trees. Since fruit set was increased, average fruit weight was reduced by 19% on P-Ca treated trees, relative to the control. This illustrates the need to utilize a more aggressive thinning program in blocks that are subjected to a season-long P-Ca program, in some instances.

Table 1. Treatment effects on ‘Golden Delicious’ harvest in 2019.

Following harvest, we evaluated effects on shoot length, pruning weight, and pruning time (Table 2). P-Ca significantly reduced shoot length and the pruning weight. Specifically, P-Ca treated trees reduced shoot length by 44% and pruning weight by 63% compared to the control trees (Figure 3). There were no significant differences between treatments for time required for pruning.

Table 2. Effect of treatments on shoot length, pruning weight, and pruning time of ‘Golden Delicious’ in 2019.

Figure 3. Effect of P-Ca on internode length. Control (left) vs. P-Ca (right).

P-Ca can be a useful tool for management of apple vegetative growth. Root pruning had no effect on any measured response. This may be due to the distance of the root pruner from the trunk (2 ft). The goal with root pruning is to remove >50% of the root volume, as apple trees have a phenomenal ability to compensate for significant root mass reductions. Apparently, our root pruning treatments did not remove enough root mass to be effective. Future work will be done to evaluate more severe root pruning treatments on vegetative growth and development of apple.

Additional P-Ca Application Information:

See page 84 and 85 in the 2020 Integrated Orchard Management Guide for Commercial Apples in the Southeast.