California apple growers struggle to gain quality and optimal size for acceptance in a marketplace.

Growers thin chemically and by hand to achieve those goals, but with increasing labor costs they would prefer to do much of the job with chemicals. Unfortunately, chemical thinning is as much an art as a science, but researchers like San Joaquin County Farm Advisor Joe Grant and consultant Mike Devencenzi of Lodi, Calif., have spent countless hours researching chemical thinning strategies, trying to take the mystery out of a practice where so many factors can affect efficacy.

Devencenzi told growers and consultants at the Central California Apple Symposium in Stockton, Calif., recently do not expect to chemically thin in just one pass.

“Take a little bit off and then come back,” he said. He and Grant both said carefully evaluate previous hand or chemical thinning before moving to another thinning.

“Work with someone who has experience in chemical thinning,” said Devencenzi.

Remove bees or spray only when bees are not foraging, said the consultant.

Heat, said Devencenzi, can increase thinning potential with chemicals, and cool will delay thinning. Optimum thinning temperature range is 70 to 75 degrees. He recommends applying chemical thinning agents early in the morning, just prior to daybreak and before bees are foraging.

If a second thinning application is necessary, Devencenzi suggests treating only the top half to third of the trees in an orchard. “Consider girdling to help the return bloom next year,” he added.

Develop guidelines

Grant and other University of California apple experts have developed guidelines and chemical recommendations for thinning.

  • Cool, wet conditions may precondition leaves for greater absorption of thinning agents, leading to increased activity.

  • Cool, wet or humid weather prolongs drying, giving great activity. High temperatures following application can cause mild tree stress and increase thinning activity of some chemicals.

  • Tree stress from any source (low nitrogen, lack of water, root damage, heavy crop in previous year, shading within canopy) may increase thinning response to applied chemicals.

  • Young, excessively vigorous trees are easier to thin than older, moderately vigorous trees.

  • Over-thinning may occur when natural post bloom fruit drop is heavy, particularly on Gala and Granny Smith.

  • Chemical rates should be adjusted based on bloom density, pollination conditions and fruit set.

  • Proper timing, choice of chemical and rate varies dramatically according to variety. Most varieties are thinned at petal fall or shortly after fruit set.

  • Sufficient water (150 to 250 gallons per acre) should be used to wet without runoff. Field experience with low-volume application is lacking and is not recommended.

  • Remove flowering weeds or cover crops before spraying to avoid damage to bees.

The University of California Statewide Integrated Pest Management Web site has detailed chemical recommendations for each variety.

Grant recommends “aggressive, but progressive chemical thinning.”

e-mail: hcline@primediabusiness.com