Rooting out disease

Features - Disease Control

If you’re growing under cover, either for propagation or for tender plants, make sure your greenhouse conditions aren’t hospitable to root diseases.

May 4, 2022

All photos from Adobe Stock

Plant pathogenic fungi is one of the most common causes of root impairment in greenhouse-grown crops. Accurately diagnosing what pathogen has invaded your production area is paramount in both managing the problem and preventing future incursions.

Controlling fungal root disease is a challenge largely due to the ubiquitous character of certain pathogens. In greenhouses, pathogens proliferate in plant debris and can be introduced into the growth medium by soiled hands, tools, flats and colonized transplants. University of Maine’s Alicyn Smart, a plant pathologist and director at the school’s plant disease diagnosis laboratory, points to fungus gnats and shore flies as another vector for root-damaging disease.

These annoying insects are attracted to damp locations where fungi are apt to flourish, Smart says. While adult fungus gnats are more of a nuisance, their larvae feed on plant roots and decaying organic matter. Feeding damage creates wounds that allow soilborne pathogens — including pythium and fusarium — to enter and kill plants. The larvae themselves may also carry deadly pathogens, making swift eradication even more important for growers.

“Both fungus gnats and shore flies can move pathogens around the greenhouse, which is something that people don’t often think about,” Smart says. “Gnats can deposit their spores into roots and stems.”

Pythium is commonly associated with excessive nutrient levels or ammonium toxicity. With a genus comprised of more than 100 species, pythium can lead to any number of diseases, among them crown and stem rot and damping-off.

Damping-off affects seeds and new seedlings by rotting stem and root tissues at and below the soil surface. Though infected plants may germinate problem-free, within a few days they can become mushy and water-logged. Species of fusarium, phytophthora and rhizoctonia can cause damping-off, but greenhouses with poor soil draining and high humidity are more likely to invite pythium into the grow space.

“Pythium is not a true fungi, it’s a water mold,” Smart says. “With damping-off, another symptom for growers to look out for is when the plant flops over at the seedling stage.”

Where disease finds a home

Pythium is a classic root nibbler that can kill plants outright, says Mary Hausbeck, a professor of plant, soil and microbial sciences at Michigan State University.

Non-uniform crop height is a telltale sign of a pythium invasion. Soil with high soluble salt content is vulnerable to the pathogen, Hausbeck notes. Being a water mold, pythium will spread more rapidly when soil is saturated, or if crews don’t adjust watering routines after a series of overcast days.

Like pythium, phytophthora favors excess moisture and nitrogen fertility. Though less common than its fellow water-loving cousin, species of phytophthora are more aggressive and ultimately quite harmful to crops.

“I’ve never witnessed a situation with phytophthora that was not highly destructive,” says Hausbeck. “A grower may see a problem they think is pythium, but it’s not responding to treatment how they thought it would. That’s when we find out it’s phytophthora.”

Both phytophthora and pythium are oomycetes based on their algae-like structure and preference for wet, humid conditions. Cultivated soils are a common home for fusarium, a fungus carrying at least 10 different species known to cause root rot.

Catching the problem early

When combating any pathogen, catching the warning signs early is key to staving off plant death. Fusarium rot appears as reddish-brown lesions on the primary root two to three weeks after planting. Affected areas may gradually become brown, with symptoms extending up the main root to the soil surface.

Generally, growers should check their roots regularly, either during watering or when examining foliage.

“Growers should actually take plants out of containers to look at roots,” Smart says. “They should also know what healthy roots look like. Good ones will be white and fleshy, while diseased roots may be brown or degrade in your hands. If you have any previous issues with root disease, you should look at them more often.”

As symptoms develop over time, confirming an accurate diagnosis can be problematic, explains Hausbeck.

“In phytophthora and fusarium, you’ll see a wilt in the plant,” she says. “What throws people is if they have a [container] with three plants with one wilted and two that aren’t. That’s root rot that hasn’t yet moved on to the other plants. The best course is to remove the wilted plant and all soil associated with the root system, then treat preventively moving forward, because your other plants may have an infection that’s not visible yet.”

How can owners eliminate disease and, just as critically, prevent it from returning? Janna Beckerman, a professor of botany and plant pathology at Purdue University, says too many growers choose to simply dispose of a few infected plants rather than taking a more merciless approach.

“That will just allow the pathogen to persist and require additional fungicide applications,” says Beckerman. “If you have to cull, be ruthless. The best greenhouses I’ve been in are absolutely ruthless in their sanitation.”

All photos from Adobe Stock

When and how to use fungicides

Fungicides are vital tools for managing disease when combined with proper vetting of products and their classifications. One method of categorizing fungicides is by their modes of action — or the way the fungicides affect a fungus. Site-specific fungicides react with one very precise biochemical process, called the target site. Multi-site products have multiple modes of action, impacting many target sites and interfering with the metabolic process of fungi.

Rotating fungicides, which requires alternating products with different modes of action, is crucial in limiting the amount of time a pathogen is exposed to any single product, says Beckerman. For best results, growers should also avoid consecutive applications of fungicides within the same chemical class.

“When you do use fungicides, make sure to apply the recommended dose as listed,” Beckerman says. “Skipping a spray is the worst thing you can do, because it allows the population to come back up.” Once the disease is destroyed, an increased focus on cultural controls can help stave off a potentially disastrous return. General practices for effective cultural controls include inspect incoming plant material, avoid overwatering and control insect pests and weeds.

Sanitation best practices can be as simple as keeping a watering hose off any surface that collects soil. Smart suggests cleaning reusable trays with one part bleach and nine parts water.

“After removing the disease, sanitation and good cultural practices will alleviate reliance on fungicides,” says Smart.

Disinfect tools, benches and walkways. Steam-cleaning trays and pots are additional measures in keeping your production space free of root-ravaging pathogens, Beckerman says.

“There’s dozens of products you can use,” she adds. “Just follow the label correctly, and all will be effective against most pathogens.”

Many labs conduct a serological test to uncover harmful organisms. Meanwhile, disease resistant plant varieties can save growers from pathogen-inflicted pain.

Douglas is an Ohio-based freelance writer. This article originally appeared in sister publication Greenhouse Management. For greenhouse news, visit