Spraying viruses on plants is the last thing gardeners typically want to do, but University of Florida (UF) researchers say that just might be the solution to one of the world's most persistent plant diseases. Fields tests conducted at UF show that a mixture of viruses that infect and kill plant bacteria, called bacteriophages, can protect tomato plants from tomato spot, a disease that has developed resistance to widely used antibiotic and chemical sprays. The researchers say similar virus cocktails can be used to protect against other bacterial plant diseases, providing a sustainable alternative to pest-control agents that could become environmental hazards.

"These bacteriophages are organisms that are already present in the environment, so there appears to be no environmental risk," said Tim Momol, an associate professor of plant pathology at UF's Institute of Food and Agricultural Sciences. "They also seem to work better than the standards growers are using against tomato bacterial spot."

At UF's Quincy-based North Florida Research and Education Center and its Plant Science Research and Education Unit in Citra, in North Central Florida, the researchers treated several small plots of tomatoes with a mixture of viruses known to prey on two strains of the bacterium that causes tomato bacterial spot: Xanthomonas campestris pv. vesicatoria.

One of the costliest tomato diseases the Eastern United States and the Caribbean, bacterial spot can cut a field's production in half.

Farmers have traditionally had only two ways of stopping it - or any other bacterial crop disease. One method involves spraying a field with antibiotics, which can kill bacteria but at the risk of fostering antibiotic-resistant bacteria strains. The other method entails squirting plants with a solution containing copper, a toxic metal that kills bacteria. The copper in the sprays is too diluted to damage plants, but there is a possibility that decades of spraying could lead to a harmful buildup of copper in the soil, the researchers say.

Florida's strains of Xanthomonas developed a resistance to antibiotics decades ago, and most strains also have developed a tolerance to copper, reducing its effectiveness. "As a result of the prevalence of copper-tolerant strains (of bacteria), the efficacy of copper has been significantly reduced, and we need something to replace it," said Jeff Jones, a professor of plant pathology and co-author of the study. In the study, tomato plants were infected with a strain of Xanthomonas known to remain susceptible to copper spray. Fields treated with copper were slightly more productive than untreated fields, producing roughly 400 pounds more fruit per acre. Fields treated with the bacteriophage brew fared better, outperforming the untreated fields by about 7,600 pounds per acre.

That's good news not only for tomato growers, Momol said, but for any farmer whose fields are plagued by bacterial disease. He said it may be possible to find bacteriophages to combat virtually any of the costly bacterial diseases that affect hundreds of crops around the world. The idea of using viruses to stop these diseases is not new. As early as the 1930s, scientists began looking for ways to effectively apply bacteriophages to crops. But the bacteria in those early trials proved highly adaptable, developing resistance to bacteriophage sprays within a few months.

In recent years, however, researchers have adopted a new tactic: attacking bacteria with an ever-changing mixture of viruses known to infect them. This approach can kill an entire population of bacteria before it develops resistance to any single virus. "It's like coaching a football team," Momol said. "You start out with a certain lineup of players and as some become ineffective, you replace them with other players." The fragility of the viruses also posed a problem for researchers. Viruses are easily damaged by ultraviolet light, which made it difficult to effectively use them during daylight hours.

But the UF team solved that problem by adding a couple of household ingredients - skim milk and sugar - to the virus mixture. "Skim milk acts as a kind of suntan oil to protect the bacteriophage from UV (ultraviolet) rays," Momol said. "Even with that protection, we've found that it's best to apply the virus just before sunset or very early in the morning, which gives it a chance to do its work before UV levels rise." Quincy-based tomato grower Tommy Smith said he has seen "very positive results" after trying the bacteriophage mixture on a field where tomato bacterial spot had made an appearance.

Smith used the bacteriophage as part of a separate experimental field test; phage-based antibacterial sprays have yet to be approved for widespread use in agriculture. "We had some really amazing results in terms of stopping the spread of the bacteria and bringing down bacteria levels on the whole field," he said. "I was extremely impressed." The UF researchers conducted their field tests in conjunction with Lee Jackson, a researcher at Omnilytics, a Salt Lake City company that developed the phage-based antibacterial spray used in the test.

The research was funded by the U.S. Department of Agriculture.