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By Krisy Gashler
  • Cornell AgriTech
  • School of Integrative Plant Science
  • Agriculture
  • Plants

Ultraviolet light, which has been used successfully to suppress fungal powdery mildew in grapes, strawberries and cucumbers, can also destroy the bacteria that causes devastating fire blight in apples, according to new research from Cornell plant scientists.

In laboratory and orchard field tests conducted over two years at Cornell AgriTech, two nighttime applications of UV light to apple trees in bloom achieved the same effect as the strongest available biopesticides and antibiotics. The UV light even killed strains of antibiotic-resistant Erwinia amylovora, the bacteria that causes fire blight. The results were published Aug. 22 in the journal Phytopathology.

“Fire blight can kill an entire orchard block in one season, and growers regularly use antibiotics and biopesticides to control the disease.  Consumers and regulators are becoming increasingly uncomfortable with antibiotic use, and in organic production, antibiotics are prohibited,” said Kerik Cox, study co-author and associate professor in the Plant Pathology and Plant-Microbe Biology Section of the School of Integrative Plant Science (SIPS). “Our study has shown, for the first time, that UV light can kill fire blight bacterium just as effectively, and it’s residue-free, it’s organic-approved and it doesn’t harm the apple tree in any way.”

First author of the paper is Isabella Yannuzzi, a graduate student in Cox’s lab. Other co-authors are Alexandra Davidson, an undergraduate at Hobart and William Smith Colleges, and David Gadoury, recently retired senior research associate in SIPS.

Scientists first discovered that sunlight could prevent microbial growth in the late-19th century. Since the 1980s, short-wavelength UV light has been used regularly to kill germs in the medical field. The technology has grown and is now used to treat wastewater, fish tanks and home air. Gadoury led the pioneering group of researchers at Cornell AgriTech who began applying UV light to treat plant diseases. They have discovered that UV light can kill 95% of the fungus that causes powdery mildew in basil, roses, grapes, strawberries, rosemary and cucumbers.

Shortwave UV-C light works by damaging the DNA of infectious organisms like fungi, bacteria and viruses, and is far more effective when used at night: In lab settings, the researchers found that when they exposed Erwinia amylovora to UV light at night, it was more than twice as effective as exposure during daytime. This is because during the day, longer-wavelength blue light – which the sun emits alongside UV rays – works to repair the damage caused by UV rays. Without that natural repair mechanism, the bacteria can’t survive the UV-C assault.

“This works the same way with other plants and other microbes: If they get exposed to UV rays, the blue light exposure can actually repair the damage that’s caused. It’s kind of a constant balance of burning and fixing,” Cox said. “For these tiny organisms, even a little damage is enough to kill them if they don’t get blue light exposure for at least four hours.”

UV light is not a panacea, and further research is needed, Cox said. For example, hemp plants burn easily under this type of exposure, he said. Future research will explore whether UV light treatment affects the quality of apples in long-term storage, whether fruits – not just blossoms – can be treated with UV light and whether UV light could be applied to harvested apples to control germs.

The research was supported by the USDA Organic Agriculture Research and Extension Initiative, the New York State Apple Research and Development Program, and Cornell AgriTech.

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