Greenhouse Diseases and Best Practices for Managing Them

(Clockwise from left) Enhanced tolerance to Fusarium shown by variety RZ 1600 compared to the highly susceptible Guintolli, leaf symptoms positive for Ralstonia, and healthy plants at a nice spacing with emitters that keep foliage dry. | BioWorks Inc. courtesy of J Kerkoff, and University of Florida IFAS Plant Diagnostic Center

It’s the beginning of the year, and we talked with three experts to see just what has their antennae twitching regarding emerging or developing disease problems. They also shared new (and old is new again) methodologies and
best practices for handling disease in general.

Ralstonia

Ralstonia remains a significant concern for greenhouse growers, particularly in floriculture crops.

“Ralstonia continues to be a problem, and that’s because we continue to have detections of it every year. We think of Southern wilt as being a solanaceous crop problem, which it is in the ground and in the South, but it’s not coming in on tomatoes. We’re seeing Ralstonia on floriculture crops like geranium coming in from out of the country and being grown here in our greenhouses. We continue to have detections from propagules from offshore,” says Carrie Lapaire Harmon, Ph.D., Extension Plant Pathologist and Director of the Plant Diagnostic Center at the Department of Plant Pathology, University of Florida.

USDA regulates Ralstonia solanacearum race 3 biovar 2 (Rs R3bv2) and watches for contamination. APHIS has updated its framework for the Ralstonia Exclusion Program. When a detection is made, it’s a problem.

“There will probably be a stop sale. Often there’s destruction of plant material. Ralstonia is one that we want our growers watching for all the time,” says Harmon.

Prevention strategies for Ralstonia include careful inspection, quarantine procedures, and proper disposal of infected material.

“Don’t accept stuff that looks iffy; don’t propagate stuff that looks iffy. The potential cost of crop destruction probably outweighs throwing away a couple of flats of suspect-looking seedlings,” says Harmon. Sometimes, propagation supplies are limited, and you may feel you can’t just throw out a batch. “Your next step, if you can’t keep it out and you can’t decline it, is to consider quarantining. Know exactly where it is and keep a really close eye on it. Our good growers are already doing this, and so I want to say, good job! Keep doing that.”

Fusarium

While issues with Fusarium are certainly not new, the pain it’s causing growers is increasing. “There’s always something new on the disease front, I think,” says Michael Brownbridge, Ph.D., Sr. Technical Services Manager at BioWorks. “In both field and greenhouse vegetable production, certainly in the last 12 months, Fusarium has really emerged as public enemy number one for the greenhouse vegetable sector.” Fusarium is challenging. Prevention and exclusion are the best hope for managing the disease. Even more frustrating, chemical control after introduction is often ineffective.

“Fusarium was first identified as the major culprit in the decline of greenhouse peppers a couple of years or so but incidence was limited. However, in 2024 it was significantly more geographically widespread in North America, and a lot of greenhouses ended up pulling crops out before the end of the growing season. They had to pull plants two, three, four, or five months early, so they lost a lot of production,” says Brownbridge. But what is causing a surge? “I suspect there’s no one thing that tipped the scales in the disease’s favor. Rather, a combination of resistance, environmental conditions, and certain varieties (of plants) being more susceptible than others.”

Tomato Brown Rugose Fruit Virus (ToBRFV)

When asked about emerging disease problems for greenhouse growers, Dr. Anissa Poleatewich at Mycorrhizal Applications shared her insights.

“In greenhouse vegetable production, there have been several new viral plant diseases reported. One example is the tomato brown rugose fruit virus, which was first reported in Canada in 2019. For tomato hydroponic growers it’s been a big issue, especially for the Canadian growers. Viruses can be really tricky because they’re difficult to control, and prevention is really your best strategy.”

The disease spreads through infected propagative plant parts and locally by contact. It can remain viable in seeds, plant debris, and contaminated soil for months.

This virus was first detected in Israel in 2014 and in greenhouse outbreaks in California and Arizona a few years later. The disease has spread widely and was detected in Australia in January 2024. According to the Animal and Plant Health Inspection Service (APHIS), “Once in a production system, this virus cannot be controlled. The only options in these cases are destruction of the plant material and disinfection of the greenhouse.”

Changes in How to Conduct Disease Management

Fungicide resistance is an ongoing challenge and it’s important to keep an eye out for known developments of resistance. Monitoring goes hand in hand with communication, and keeping in touch with Extension personnel in your region is a good start. Many Extension offices put out annual reports on products they see working right now and products they see developing resistance. It changes constantly and even by region.

Fungicide resistance is often related to application practices rather than true resistance. “Most of the time, it’s actually that we haven’t done a great job of rotating modes of action, and our coverage, or the rate that we’re applying at, hasn’t been high enough,” says Harmon. She explains that sometimes growers will be tempted to dilute a product to make an expensive product last. Then, it doesn’t perform as it should, and there are escapees or survivors. “It’s not always that the organism is resistant to it, but that we didn’t kill it. It would have died, but we didn’t apply enough product or get coverage sufficient enough to actually contact the organism. Shortening up intervals, higher application rates, higher product concentrations, and good coverage matter.”

While biological controls aren’t exactly new, there is continual development and release of new products to the market. And combinations of active ingredients, or in this case organisms, are growing as a solution. “Rather than having a product based on one biocontrol agent, you have multiple biocontrol agents,” says Poleatewich. “And the trick is just because there’s more doesn’t necessarily mean it’s better. The key to having these combination products is that the company really knows and can explain what each of those components is doing and that they work together.” She advises growers to critically evaluate new products. “Interact with the company and its technical support to understand how their product works and how those organisms in combination were selected.”

Although using biologics isn’t brand new, they are becoming more cost-competitive, more effective, and more broadly applicable. “If you do the math, biologicals deliver great value in production, considering all the protection and growth benefits they can bring to a crop, their essential role in resistance management, and the labor savings that can be realized through their use. They’re very cost-competitive. We’re also getting better at using these materials more efficiently,” says Brownbridge.

Understanding of microbial performance in different growing media has improved as well. “We have a better understanding about how to use biologics more effectively and efficiently in different growing substrates and how those substrates affect microbial activity.”

Starting early is key for effective use of biologicals. “A critical part, I think, that is increasingly recognized as being essential to success, is that biologicals need to be applied in propagation,” says Brownbridge. It’s a sign of changing times. He tells us of conversations with older growers who mention they wish they could still get some of the ‘good stuff’ that was on the market a few years ago. Those nuclear options aren’t on the table anymore, making early intervention and preventative strategies crucial to crop health over the full growing season.