Apple Re-Thinning Advisory for May 15, 2012

Posted: May 15, 2012

Growers are advised to carefully check their orchards prior to making another thinning application. In Adams County, it is ten days since a stress event caused by three days of cloudy weather and we are beginning to see a new wave of fruit drop.
Dr. Jim Schupp, Penn State Fruit Research and Extension Center Pomologist and Center Director

From May 6 to May 8, we measured three days of strong carbon deficits (see Carbon Deficit Model posted May 14). At the Adams County Orchard Meeting on May 9th,  I recalled Ross Byer's research on the effect of shading on apple fruit drop, and suggested that growers wait, before re-thinning, to see if this didn't cause additional natural drop in their orchards. I think the effects of this are being expressed now. I'm seeing quite a strong drop, some from my thinners and some from the carbon deficit (see photo). This wave of drop is occurring about 10 days after the stress event, which indicates a probable causal relationship.

Some of this drop may be caused by previous chemical thinning also. Whatever the cause,  growers are advised to check fruit set in their own orchards carefully before applying an additional thinner.


Contact Information

James Schupp
  • Professor of Pomology
Phone: 717-677-6116