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As I See It- Heat Stress, Sunburn, Bitter Rot on Apples

-Win Cowgill

The past week of extreme temperatures in the 90’s has caused injury to many apples. Some cultivars are more sensitive than others.
I wrote about the different materials that can be used for sunburn in the 6/19/12 Fruit Plant and Pest Edition. Dr. Atanass Atanassov, our North Jersey IPM program associate, has seen sun and heat injured fruit all over north and central NY this past week. Damage has ranged from sunburn necrosis to skin browning, and was seen on multiple varieties including gala, McIntosh, Mcoun, Empire, Honeycrisp and Suncrisp.

We know from work done in Washington State that fruit surface temperatures can be 20-25 degrees hotter than the air temperature. So if the air temperature is 95F, fruit surface temperatures can reach 125-130F. This can cause three types of injury:
1) Sunburn necrosis
2) Sunburn browning
3) Photo-oxidative sunburn

All of these can make the fruit unattractive and unsalable. It can cause fruit to drop prematurely and it can make the fruit more susceptible to bitter rot infection.

Solutions- apply one of the sunscreen materials available: Surround or Pureshade

I have used Surround Clay for the past 6 years as a sunburn material on Honeycrisp with good success. We have also evaluated Pure Shade and it has worked well. Pure Shade was purchased by the Surround people. Surround is an inhert Kolin clay, Pureshade is calcium carbonate. Both work by coating the fruit to reflect the sun and keep the skin surface cooler.

The secret with these materials is to get them applied prior to the heat events. For Surround apply 50lbs/100 gallons, tree row volume dilute the first application. You then layer it on with subsequent applications at 25lbs/100. It may take 2-3 applications for full coverage. It will wear with rainfall. It also has the added benefit of repelling Japanese beetles on Honeycrisp apples and may assist with BMSB as well. Pureshade creates more of a dull film on the fruit and will also wear with rainfall.
Other recommendations to relieve heat stress, excerpted from from Mike Fargione’s, Cornell, Fruit Alert, based on Dr. Rosenberger's, Cornell extension Plant Pathologist, suggestion:
  • Irrigate Trees liberally before heat waves
  • Use overhead irrigation during heat waves to avoid heat stress damage (mist cooling)
  • Apply Pristine fungicide ahead of heat stress- this strobularian fungicide has been show to reduce plant stress
  • Do not apply CaCl or other surfactants during the heat events.

One major concern with heat stressed or heat damaged skin is infection with Bitter Rot.

                                          Photo Credit: Jon Clements, UMASS

Dr. Rosenberger believes that sunburn predisposes fruit to bitter rot. Honecrisp is very prone to bitterrot.
We do recommend maintaining Captan or Pristine in every cover spray until harvest to prevent bitter rot, follow the label rates and restrictions for both materials.

You can listen to Dr. Rosenberger online describe the issue of bitter rot following heat stress at: Dr Rosenberger on Bitter Rot
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