Controlling Apple Tree Vigor

There are many reasons to control an apple tree’s vegetative vigor. Some of these reasons include: 

• Overly vigorous trees take longer to prune and have more internal shading that reduces fruit coloring.  
• Dense canopies require more sprays and are harder to cover adequately with pesticides.  
• Trees planted too close together on overly vigorous rootstocks may also be a problem.  
• Overly vigorous trees produce more succulent shoot growth.  These succulent shoots are more susceptible to fire blight infection.

Prohexadione-Calcium (Apogee)

Prohexadione-calcium (Apogee) is a PGR that interferes with the production of gibberellins in the plant. Gibberellins are plant hormones involved in shoot elongation. Inhibiting gibberellin production decreases shoot growth. The effect of a single application of Apogee lasts only 2 to 3 weeks, depending on the inherent vigor of the tree and the time of the season. Once Apogee has been applied to an orchard block, repeat applications at 2-week intervals are typically required until the season of maximum shoot growth has passed. When a repeat application is missed, there may be a “rebound” period when shoot growth resumes at a rate that appears to exceed that of untreated trees.

A beneficial effect of Apogee is that trees are less susceptible to fire blight. While the number of infections does not appear to be affected, the rate at which the infections spread is reduced so that pruning out fire blight strikes in susceptible cultivars becomes more practical. See table below.

Using Apogee to Control Apple Tree Vigor