The study, published this week in Applied and Environmental Microbiology, an open-access journal of the American Society for Microbiology, was led by microbiologist Eric Déziel of the Institut national de la recherche scientifique (INRS) in Quebec, Canada. Déziel describes C. michiganensis as a genuinely problematic pathogen, particularly in greenhouse settings where dense planting, shared equipment, and constant human handling create ideal conditions for transmission. Once the bacterium gains a foothold in a facility, it typically establishes itself in one section before workers inadvertently carry it from plant to plant as they prune, tie, and harvest. The result is a creeping wave of infection that is notoriously difficult to contain with conventional tools.

What makes the new research distinctive is its dual focus. Most studies of biological control agents, Déziel notes, tend to concentrate on a single objective: either suppressing a pathogen or stimulating plant growth. The Canadian team set out to find microbes capable of doing both at once. Their screening strategy was deliberately broad and systematic, drawing on a resource the laboratory had been building for roughly two decades. For about twenty years, researchers in Déziel’s group have pursued biological solutions to agricultural problems, accumulating a large collection of bacterial strains isolated from a wide variety of food plants and environments.

The opportunity to apply that collection to tomato canker arose a few years ago, during the COVID-19 pandemic, when tomato growers brought the problem of bacterial canker in greenhouses to the lab’s attention. Déziel admits he had never heard of the disease at the time, but the growers’ concerns were enough to launch an intensive search for answers. The team began at home, so to speak, testing some 500 strains from their own collection for the ability to antagonize C. michiganensis in laboratory assays. These strains had been isolated from strawberries, lettuce, apples, and other foods, and had been recovered from leaves, roots, and other plant parts. Crucially, none of them had previously been isolated from bacteria-infected plants or fruits, meaning the researchers were working with microbes drawn from healthy contexts rather than from diseased tissue.

The initial screen was designed to cast a wide net. In antagonism assays, candidate bacteria are grown alongside the pathogen, allowing researchers to observe whether the candidate produces compounds, competes for resources, or otherwise interferes with the pathogen’s growth. From the 500 strains tested, a few dozen showed promising activity against C. michiganensis. That shortlist then faced a second, more demanding filter: the strains had to do more than fight the pathogen. They also needed to promote plant growth, a property that matters enormously for any biocontrol agent intended for real-world agriculture, since growers are far more likely to adopt a treatment that delivers a visible agronomic benefit in addition to disease protection.

The evaluation proceeded in stages, moving from simple bacterial cultures to living plants. This progression is essential in biocontrol research because activity observed on a laboratory plate does not always translate to the complex environment of a plant’s surfaces and internal tissues. When the team tested their shortlisted strains on tomato plants infected with bacterial canker, the experiments ultimately yielded three bacterial strains that demonstrated a clear dual benefit. One of these, Pantoea agglomerans SO16PY, originally isolated from the leaf of a healthy tomato plant, proved especially interesting: it delayed the onset of canker symptoms in infected plants by up to a week and reduced the overall severity of the disease. The other two strains, both members of the species Pseudomonas marginalis, similarly restricted both the development and the severity of the disease in infected plants.

The significance of a week’s delay should not be underestimated in commercial greenhouse production. Bacterial canker moves quickly through a crop, and every day of slowed progression gives growers additional time to detect the outbreak, remove affected plants, and adjust handling practices to limit the spread that occurs through workers’ hands and tools. Reduced disease severity, meanwhile, translates directly into preserved yield and plant vigor. Combined with the growth-promoting effects observed in the same strains, these microbes appear to function as what researchers sometimes describe as plant probiotics: living allies that shift the balance of the plant’s microbial environment away from pathogens and toward health.

The practical context for this work is particularly important for organic growers. Déziel points out that bacterial canker can be a significant problem for organic farmers, who have fewer chemical options available to them. Although organic producers can purchase seeds that are certified or guaranteed to be free of the pathogen, the bacterium may still find its way into a greenhouse through contaminated equipment, infected transplants, or residual inoculum in the facility. Once inside, the disease follows its familiar pattern of localized establishment followed by mechanical spread through routine work. Today’s treatment solutions are limited, Déziel adds, and the few commercial options that do exist show poor efficacy, leaving growers with largely preventive measures and little recourse once infection takes hold.

That gap between need and available tools is precisely where the three beneficial strains could eventually make a difference. The researchers emphasize, however, that considerable development work remains before these microbes could be harnessed for commercial use. The central challenge is formulation and delivery: the bacteria must be stabilized in a product form that maintains their viability and effectiveness over time. As Déziel explains, the team needs to find a way to stabilize the bacteria so that their shelf life is long enough for the product to be actually usable in a greenhouse. A biocontrol agent that loses potency within days of manufacture is of little value to a grower who needs to apply it on a schedule determined by the crop and the disease, not by the laboratory.

If that stabilization challenge can be solved, the payoff could be substantial. The three strains identified in this study represent a sustainable, environmentally friendly approach to improving plant health in agriculture, one that works with the natural antagonisms among microbes rather than against them. For an industry grappling with a seedborne pathogen that spreads easily and resists conventional treatment, bacteria that can both hold the disease at bay and help the crop grow stronger offer a genuinely new kind of tool. The work also underscores the value of long-term microbial collections and of listening to growers: a problem brought to the lab’s attention during a pandemic has now produced candidate biocontrol agents isolated from healthy tomato leaves and other food plants, strains that had been sitting in a freezer for years waiting for the right question to be asked of them.

Subject of Research: Identification of beneficial bacterial strains that suppress tomato bacterial canker and promote tomato plant growth

Article Title: Beneficial bacteria support tomato growth & suppress disease

Article References: Beneficial bacteria support tomato growth & suppress disease. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: tomato, bacterial canker, Clavibacter michiganensis, biological control, Pantoea agglomerans, Pseudomonas marginalis, plant growth promotion, greenhouse agriculture, seedborne disease, microbiome, organic farming, Applied and Environmental Microbiology