Deadhead Common Diseases

In our “Taking Out The Top Five” article, Greenhouse Grower asked the technical experts from our industry’s leading chemical companies for tips on insect prevention, diagnosis and treatment.

This month, we’re using that same mold, but we’re now seeking out advice to tackle some of the most common diseases growers face worldwide. Keep in mind diseases do vary with climate, geography and crop. This top five list of common plant infections includes botrytis, powdery mildew, pythium, phytophthora and fusarium.

“If any growers are making a diagnosis for the first time, it is highly recommended that growers make clear notes and take pictures of the symptoms that were found,” advises Vijay Choppaktla, plant pathologist and director of research for BioSafe Systems. “Send infected plant samples or pictures to the nearest plant disease diagnostic laboratory for disease confirmation.” These services can be provided to growers at a minimal cost.


As a lover of humidity, botrytis is initially noticed as a general blight (a browning and collapse of tissue) on the leaves and flowering parts of a plant, says Nancy Rechcigl, a Syngenta field technical manager who specializes in ornamentals. “The telltale fuzzy spores are not produced until humidity reaches 85 percent or greater.”

Jeff Dobbs, director of technical service at OHP, Inc., also has some advice: “If conditions look favorable, be prepared to increase ventilation to keep flowers and leaves dry and space them out.”

Plants that have moisture (specifically condensation) on their foliage for more than four hours at a time are the ones most at risk of botrytis, which can be common with warm days and cool nights.

“Before you begin any concentrated removal of infected plants and plant parts from the growing area to prevent additional spread, be sure to first get a fungicide spray on first,” Rechcigl says. “When you dispose of the material, make sure the bag or container you put the waste in can be closed before you walk through the rest of the greenhouse. Using open dumpsters allow spores to dislodge and blow around freely.”

Also remember to use a fungicide application after any pruning has been done. And dead plant tissue also serves as an ideal host for botrytis reproduction.

“Growers that are proactive rather than reactive reduce disease pressure by applying fungicides starting the first week,” says John Schwartz, market manager for BioSafe Systems turf and ornamental division. “With this practice, growers actually eliminate mounting pressure every time they apply.”

BASF’s Steve Larson agrees. Larson is the national ornamentals account manager for BASF. He believes many growers are starting too late with preventative treatments.

“Prevention is much cheaper than curative,” he says. “Once you see symptoms, the disease is established and you may end up throwing away some plants.”

Powdery Mildew

Easily identifiable by the white powdery spore growth on the upper surface of the leaves, powdery mildew can sometimes be mistaken for downy mildew infection when the mildew happens to develop first on the underside of the leaves, says Syngenta’s Rechcigl.

Unlike botrytis, free moisture on plants does not favor powdery mildew, but actually limits its development. It does however enjoy a relative humidity of 70 percent or higher and cooler temperatures, 62 to 72 degrees. When the temperature reaches 85 degrees and above, you will see powdery mildew stop developing.

“High night humidity is congenial for establishment of powdery mildew,” says Choppaktla of BioSafe Systems.

By the time growers react to powdery mildew, there is already plant injury that cannot be reversed, says Schwartz of BioSafe Systems. “This also weakens the plants and leaves them susceptible to other diseases and insect injury.”

Emphasizing a “detective frame of mind,” any chance Schwartz has to “get a down and dirty view” of his plants with the Bausch & Lomb 10X magnifying lens his grandfather passed onto him is his best defense against a full blown epidemic.

Choppaktla agrees and recommends that scouting be done every three days to at least once a week.

Rechcigl says it’s good to start a preventative fungicide program on a 14-day schedule when environmental conditions are conducive to disease development. “If powdery mildew does break into your crop and you have an active infection, you’re going to want to reduce that interval down to seven days.”


When it comes to pythium, don’t rely on visual observations only, Larson says. Send suspect samples to a qualified pathologist.

“Pythium is one of the most common causes of root rot or damping off, but it usually doesn’t leave many clues as to its identity,” says Dobbs of OHP, Inc.

Since pythium attacks a plant’s root system that results in a plant wilt, growers can mistakenly respond to this visual sign by additional irrigation. “Additional water is the last thing a pythium plant needs,” adds Larson.

Pythium symptoms can commonly misguide growers to believe they have another root disease on their hands like fusarium, phytophthora and rhizoctonia, and a broad spectrum formula fungicide will be necessary, Choppaktla says.

Ideal conditions for pythium include poor water aeration, water logged media and cool temperatures. Allow plants to dry down between irrigations. It’s important to also monitor the level of soluble salts in the growing media. “High soluble salt in the growing media can injure roots and make them susceptible to infection,” says Rechcigl. “If you’re using a lot of liquid fertilizer on a constant basis, it’s important to check that routinely and leach when necessary, because salts can build up in the media and cause root burn.”

Lastly, water sanitation can play a role in the introduction of pythium, especially for those growers using recycled water sources (e.g., ponds). The filtration/sanitation system may need to be improved or if one is not in place, one should be considered.

Pythium and phytophthora belong to the same class of fungi-like organisms called Oomycetes. They thrive in familiar conditions and produce symptoms that are very similar to each other, says Choppaktla. This commonality reinforces the importance of lab diagnosis in order to accurately treat the disease. There are also test kits available for growers to make accurate, on-site identification, says Dobbs.


A disease common in chrysanthemums and cyclamen, fusarium can overwinter in the soil, festering in the waste of infected plants, says Dobbs. Once it has established itself, it can be extremely difficult to completely remove fusarium from the soil.

“Know your crop,” stresses Larson. If the crop has shown a receptiveness to this disease in the past, start early and consider a preventative “sprench” (spray/drench) program that gets down to the crown of the plant.

Unfortunately, once the symptoms of a vascular disease like fusarium appear, it may be too late for the plant. It can cause root, crown and stem rot by growing into the vascular system and blocking movement of water and nutrients into the upper canopy.

When further investigating the symptoms, cut into the stem, says Rechcigl. “You’ll notice a reddish-brown streaking in the vascular tissue that’s very indicative of a fusarium infection.”

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