Plant & Pest Advisory» All Fruit Articles


As I See It: Apple Thinning, Disease, and Peach Freeze Damage

-Win Cowgill, Agricultural Agent

This past weekend brought bright sun and warmer temperatures by Sunday. Many North Jersey Growers were doing more apple thinning including myself. Sunday was used to cover all apples with a very late petal fall insecticide for plum curculio (PC), fungicide and Strep. We saw the first PC activity on Monday morning at Rutgers Snyder Farm.

Our Galas and Suncrisp have tons of bloom yet on one year wood and still too much fruit even with two applications of thinners already. I added Streptomycin to the mix on Sunday with rain forecast Sunday night, Monday - Wednesday.

Gala and Suncrisp both had too much fruit on my trees on the older two year old spurs and on the one year wood as well. Fruit sizes ranged from 12-14mm.

The Cornell MaluSim Carbohydrate Model for apple thinning showed a nice deficit (good thinning response) for Sunday night – Tuesday so I applied straight Maxcel at 75 ppm or 48 ounces/100 plus surfactant. I did not add Sevin as Sevin + Maxcel under cloudy weather can be very strong and cause over-thinning. Time will tell. Note the effects of cloudy weather, 3+ days of clouds can cause thinning.

MaluSim Carbohydrate Model 

We are locked in a rain event for a few days.

The model showed good thinning from the 13th through the 16th at both at the Rutgers Snyder Farm and at RAREC in Upper Deerfield. Any thinning done during this time should have good response. Thinners need at least 2 hours drying time, 6-8 is better.

Dr. Jim Schupp, Penn State, cautions to check fruit carefully before adding more thinner. The following is from the Penn State Fruit Times Newsletter (
“Growers are advised to carefully check their orchards prior to making another thinning application. In Adams County, PA 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. Growers are advised to carefully check their orchards prior to making another thinning applica- tion. 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 grow- ers 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. 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.” 

Rescue Apple Thinning with Ethephon 

Once fruit reaches 20 mm only Ethephon will have a thinning effect. 
See our Rutgers/UMASS fact sheet,

Late-season “Rescue” Thinning with Ethephon F-129-R 2009 at

Disease: Fireblight and Apple Scab - Check New Trees

Check the NEWA models for fireblight forecast carefully. Any open blooms; check Galas and newly planted apple trees that may now be blooming. They need to be covered for Fireblight. Remember 24 hours before or after (wetting event) with Streptomycin or Oxytetracycline to be effective.

Peach Freeze Damage

Some cultivars in some blocks are showing freeze damage. Fruit is failing to size. Check your fruit carefully by slicing the fruit across the pit. In cold injured fruit the pit will be brown and not green/white. Damaged fruit may also feel spongy.