Apple sunburn may worsen with climate change, researcher says

By DAN WHEAT

Capital Press

WENATCHEE, Wash. -- As much as 8 to 10 percent of apple crops are lost most years to sunburn, and it is likely to become more frequent and severe because of climate change, a leading industry sunburn expert says.

Larry Schrader, professor emeritus in horticulture and plant physiology at the Washington State University Tree Fruit Research and Extension Center in Wenatchee, and Jozsef Racsko, associate professor of horticulture at Ohio State University, recently finished the first comprehensive review of sunburn in apples.

"Sunburn of Apple Fruit: Historical Background, Recent Advances and Future Perspectives," will be published soon in Critical Reviews in Plant Science, a research journal in Oxfordshire, United Kingdom.

Many growers are probably prepared for sunburn this season which can begin in the middle to end of June in Washington when apples reach golf ball size.

"In my view, there's a lot of evidence that we are going through some climate change. I don't call it global warming because it affects different areas differently," Schrader said.

Apples were damaged by freeze after early warmth this spring in the East, and if trends continue higher temperatures in fruit growing regions will increase sunburn in coming years, said Schrader.

He retired at the end of 2010 after working on apple sunburn for the last 16 years of his 44-year career. Sunburn is the chief cause of apple cullage responsible some years for upward of $100 million of lost fruit, he has said.

"Sunburn has become more critical as we've moved to more high-density plantings of smaller trees where there's less shade canopy," said Don Elfving, also WSU professor emeritus in horticulture.

Varying symptoms of sunburn in apples are referenced in research papers from the 1800s, but a key contribution of Schrader's work was identifying three types of apple sunburn -- necrosis, browning and photo-oxidative -- with varying causes and symptoms but related to skin temperature and amount of light. Solar radiation, temperature, relative humidity, tree canopy density, fruit size and wind interact in complex ways to cause sunburn.

Schrader invented RAYNOX, a sunscreen for apples, and RainGard that helps cherries repel water and reduce splitting after rain. Both are patented and sold commercially.

RAYNOX and other sunburn suppressants are about 50 percent effective. Sun blocks (kaolin and calcium carbonate products) leave a white film on fruit which is difficult to remove and of which consumers are suspicious. RAYNOX, as a sunscreen, absorbs rather than reflects ultraviolet-B radiation, doesn't leave a visible film and is edible, Schrader said.

RAYNOX Organic for organic growers and RAYNOX Air for aerial application were marketed last year.

Overhead sprinklers for evaporative cooling lowers fruit temperature on the trees but doesn't block or absorb UV radiation, he said. Evaporative cooling is most effective used with RAYNOX, he said.

Bags and sun shades don't have much future because they are too expensive. Sunscreens will be improved, but the best hope is genetic modifications -- breeding apple varieties for tolerance and resistance to sunburn, Schrader said.