Sunscald and heat stress in apples could be an issue in the next week or two

Apply apple sunscald protective materials right now prior to environmental conditions that lead to fruit damage.

Back in mid-July 2013, some very high temperatures and low humidity levels led to some early sunscald, sometimes called sunburn, damage in apples. The forecast for the next 10 to 14 days calls for environmental conditions that again could potentially cause sunscald.

Sunscald can occur in all apple growing regions, but is more pronounced in conditions with high temperatures and clear skies. It is enhanced when soil moisture levels are less than adequate. Fruit in the southwest quadrant of the tree is most likely to be affected. Heavy crops can cause branches to bend over and suddenly expose fruit. Light-skinned apple varieties are most sensitive to sunburn. Sensitivity might be associated with low calcium concentration in the fruit. Symptoms begin as white, tan or yellow patches on the sun-exposed side of the fruit.

Sunscald results from heat stress to the fruit leading to injury of the affected cells. The transpiration of water from the apple fruit helps cool fruit on the tree and water stress conditions can lead to higher levels of sunscald. Most of Michigan’s apple growing regions are in a bit of a water-deficit situation and blocks without irrigation could have higher incident of sunscald in the coming two weeks (as of Aug. 22, 2013). The forecast is calling for a significant shift in our weather pattern – much higher temperatures and clear skies. The greatest likelihood of sunscald occurs when fruit are suddenly exposed to high temperatures and intense sunlight such as when the weather rapidly shifts from being cloudy and cool for many days to sunny and warm.

There are products recommended by Michigan State University Extension you can use to prevent sunscald, but they need to be applied before potential sunburn weather – in other words, right now. These include:

  • Raynox
  • Surround
  • Vapor Gard
  • Purshade
  • DIFFUSION® Light Management from Wilbur-Ellis
  • and others

These products need to be applied before potential sunburn weather and all with 100 gallons of water per acre. Some of these products need reapplication after several days as fruit grow and rain washes the product off. Some suggested rates from are:

  • Raynox at 2.5 gallons per acre
  • Surround at 20 to 50 pounds per acre
  • Vapor Gard at 1 gallon per acre
  • Purshade at 2 gallons per acre
  • DIFFUSION® at 2 gallons per acre

Again, all with 100 gallons of water per acre.

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