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Too much heat can lead to sunburned apples

Fruit can develop ugly brown spots even if not exposed to full afternoon sun

3 green apples on a counter
These Granny Smith apples are all off the same tree. The apple on the left grew on the north side of the tree. (Photo: Debbie Arrington)

Another side effect of a red-hot July: Sunburned apples.

Although they need full sun to grow and bear fruit, apple trees can get too much sun -- and absorb too much heat, leading to sunburned apples. Those apples develop dark brown splotches and other scarring.

It’s a problem in Sacramento as well as Washington, the Apple State. No state grows more apples than Washington, which is why that state has devoted a lot of resources researching apple problems.

Sunburn is a major issue for Washington growers, according to Washington State University.

“Growers often lose more than 10% of their apples to sunburn unless they have used some means of protecting their fruit from sunburn damage,” says the university’s Crop Protection Guide. “Apple fruit are susceptible to sunburn because they have a much higher thermal mass (the ability of a material to absorb and store heat energy) than leaves and are not able to dissipate this heat as effectively as leaves.”

The apples hanging on the tree can absorb more heat than their tree’s leaves. In fact, fruit temperature can be 20 to 30 degrees higher than the surrounding air temperature, say the researchers. It’s that high heat – and not necessarily direct sun – that can cause sunburn.

Those brown splotches – called sunburn browning – are caused by a combination of the sun’s UV rays and high temperatures. They appear when the fruit surface temperature has reached 115 to 120 degrees F. Fruit nearing harvest is most susceptible.

If the fruit temperature tops 125 degrees – even for only 10 minutes – cells start to die in the fruit, resulting in sunburn necrosis. That results in black or dark brown lesions, regardless if the fruit was exposed to direct sunlight. “This type of sunburn can be exacerbated by low humidity,” say the WSU researchers.

Sunburned apples are edible; the damage is mostly cosmetic and can be trimmed off. But WSU researchers found sunburn also changed the apple’s chemistry and taste. Sunburned apples tend to be firmer, have a higher sugar content and less acid. They may taste a little sweeter, but the lack of acidity also made them taste bland – not a good characteristic, especially for pie apples.

WSU recommended providing a little filtered shade to apple trees that tended to produce sunburned fruit, especially on their south- and/or west-facing sides. That shade could be as simple as protective netting.

“Protective netting may be deployed above the orchard canopy or draped directly over apple trees and has proven to be effective at reducing sunburn incidence, as well as conferring other benefits such as protecting against hail damage, reducing wind stress, and potentially excluding some invasive insect pests and birds,” say the researchers.

To protect against sunburn, Washington growers depend on “evaporative cooling” – basically, misting trees with extra water to cool them down. It’s expensive – and impractical for backyard apple growers.

And yes, there is sunscreen for apples. Growers may spray products that put a thin coat of clay particles, calcium carbonate or UV-blocking wax over the fruit. But those products tend to leave a residue film over apples that can be hard to wash off.

For more on apples and sunburn: http://cpg.treefruit.wsu.edu/environmental-fruit-protectants/apple-sunburn/ .

And for how to protect trees and shrubs from sunburn: http://ipm.ucanr.edu/PMG/GARDEN/ENVIRON/sunburn.html .





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Dig In: Garden Checklist

For week of March 3:

* Celebrate the city flower! Catch the 100th Sacramento Camellia Show 3 to 6 p.m. Saturday, March 2, and 10 a.m to 5 p.m. Sunday, March 3, at the Scottish Rite Center, 6151 H St., Sacramento. Admission is free.

* Between showers, pick up fallen camellia blooms; that helps cut down on the spread of blossom blight that prematurely browns petals.

* Feed camellias after they bloom with fertilizer formulated for acid-loving plants.

* Camellias need little pruning. Remove dead wood and shape, if necessary.

* Tread lightly or not at all on wet ground; it compacts soil.

* Avoid digging in wet soil, too; wait until it clumps in your hand but doesn’t feel squishy.

* Note spots in your garden that stay wet after storms; improve drainage with the addition of organic matter such as compost.

* Keep an eye out for leaning trunks or ground disturbances around a tree’s base, a sign of shifting roots in the wet soil.

* Fertilize roses, annual flowers and berries as spring growth begins to appear.

* If aphids are attracted to new growth, knock them off with a strong spray of water or insecticidal soap. To make your own “bug soap,” use two tablespoons liquid soap – not detergent – to one quart water in a spray bottle. Shake it up before use. Among the liquid soaps that seem most effective are Dr. Bronner’s Pure-Castile Soaps; try the peppermint scent.

* Pull weeds now! Don’t let them get started. Take a hoe and whack them as soon as they sprout.

* Prune and fertilize spring-flowering shrubs after bloom.

* Cut back and fertilize perennial herbs to encourage new growth.

* Make plans for your summer garden. Once the soil is ready, start adding amendments such as compost.

* Indoors, start seeds for summer favorites such as tomatoes, peppers and squash as well as summer flowers.

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