Washington, D.C.—Bacterial canker, a vascular infection, can wreak havoc on tomato plants, with symptoms ranging from wilted leaves to the collapse of an entire plant. Clavibacter michiganensis, the seedborne pathogen that causes the disease, can be challenging to treat because it spreads easily. "This pathogen is really problematic, especially in greenhouses," said microbiologist Eric Déziel, Ph.D, at the Institut national de la recherche scientifique (INRS) in Quebec, Canada.
One treatment strategy may come from other bacteria. For a study published this week in Applied and Environmental Microbiology, an ASM journal, he and his collaborators identified bacterial strains that offer a twofold benefit to tomato plants. These strains both treat tomato bacterial canker—some by delaying the onset of infection up to a week—and promote the growth of the plant. Most studies, Déziel said, focus on either the treatment or the boost. "We were able to do both," he said.
For about 20 years, researchers in Déziel's lab have focused on identifying biological solutions for agricultural problems. A few years ago, during the COVID-19 pandemic, tomato growers alerted them to the problem of tomato canker in greenhouses. Déziel had never heard of it.
He and his research team began at home, testing 500 strains in the lab's collection for those that would antagonize C. michiganensis. They tested strains collected from strawberries, lettuce, apples and other foods, taken from leaves, roots and other parts of the plant. Notably, "none of them had been previously isolated from bacteria-infected plants or fruits," Déziel said. "We were hoping that some of them would have some activity against the pathogen."
Once they'd identified a few dozen promising strains, they narrowed the field by limiting their search to those that promoted plant growth, in addition to antagonizing C. michiganensis. They first tested those strains in bacterial cultures, then in living plants infected with bacterial canker. The experiments yielded 3 bacterial strains that demonstrated a dual benefit for tomato plants. One of those, Pantoea agglomerans SO16PY, originally isolated from a healthy tomato plant leaf, delayed the onset of canker in infected plants by up to a week and reduced the disease severity. The other 2, both strains of Pseudomonas marginalis bacteria, similarly restricted the development and severity of the disease.
Bacterial canker, Déziel said, can be a significant problem for organic farmers. Although they can buy seeds guaranteed to be free of the pathogen, it may still find its way into a greenhouse. "It will typically start in one part, and then the workers will spread it by working on the actual plants," he said. Today's treatment solutions are limited, Déziel added, and said the few commercial options available show poor efficacy.
The 3 beneficial bacteria might be useful in the future, Déziel said, but the delivery method would require further research and development before the microbes could be harnessed for commercial use. "We need to find a way to stabilize the bacteria so that their shelf life is long enough to be actually used in a greenhouse," he said. But if they could do that, he said, they offer a sustainable, environmentally friendly way to improve plant health in agriculture.
