The Notorious Oomycetes

The oomycetes (Oomycota), also known as water molds, are a group of pathogens that play a large role in the decomposition of plants and the breakdown of organic matter. While this scenario is perfect for the great outdoors, it can cost greenhouse growers millions of dollars in yearly sales.

Just as late blight (an oomycete, Phytophthora infestans) caused the devastating Irish potato famine that killed millions of people, oomycetes such as Phytophthora, Pythium and downy mildew leave a wake of destruction in their path when they run rampant in the greenhouse.

Good Water Management Is Critical

The name “water molds” is a bit of a misnomer. Although several species of oomycetes thrive in wet conditions, many do not. Where they were once classified as fungi, recent research shows they are more closely related to algae.
“Water management is basic to controlling many of these pathogens,” says Janna Beckerman, associate professor of botany and plant pathology at Purdue University. “Some species of Pythium and Phytophthora spread because of spores that swim across free water left on leaf surfaces, or they multiply in prolonged, wet conditions.”

The trick is to find a balance between enough water for healthy growth and not enough water for healthy pathogens. Adequate greenhouse ventilation, ample air circulation and balanced humidity levels go hand-in-hand with water management for control of oomycete pathogens. Proper plant spacing ensures that air flows freely through the canopy. Opening the vents to reduce relative humidity and turning on the heat at the end of the day keeps condensation from accumulating on plant surfaces when nighttime temperatures drop.

Cultural Controls

Sanitation is critical. Removing dead leaves, spent flowers and dead or dying plants halts the spread of disease and eliminates sources of inoculum. Weed control is also important. Rooted cuttings should be free of disease symptoms before transplanting. Choose a well-drained media with good porosity. A balanced fertility programs strengthens plants and bolsters their natural defenses.

How And When To Use Chemical Controls

“You need a good foundation of cultural management in place to control these diseases, but the unfortunate reality in the greenhouse is cultural controls can’t do everything,” Beckerman says. Aaron Palmateer, associate professor of plant pathology at the University of Florida agrees. “Like it or not, there is no way around using chemical controls in monoculture situations. They are also necessary for pathogen control in mist houses where conditions can be very favorable to disease,” he says.

Chemical controls work best if used prior to when disease conditions are at their prime. Palmateer suggests starting applications two to three weeks before diseases are most likely to occur, becoming more aggressive with treatments as conditions ripen, and then tapering off applications after the danger has passed.

“Pay attention to labeling and make sure your diagnosis is correct before you treat your plants,” Beckerman says. “A lot of money is wasted on products that aren’t labeled to control a certain disease. If your product is labeled for that disease and you are not getting control, contact your lab to find out if there is a history of resistance.”

Palmateer encourages growers to use FRAC (Fungicide Resistance Action Committee) group codes to help with rotating chemicals to avoid resistance. “If you have products with different FRAC codes, you know it is safe to use them in rotation,” he says. “Growers also need to be aware of the residual activity for the products they are using. An important part of rotating chemicals and managing resistance is being able to gauge how long you can wait to put on another application.”

Growers are lucky nowadays to have a lot of good chemistry at their disposal for treating oomycete pathogens. Among these products is Pageant® Intrinsic, which provides excellent protection across a number of different foliar, crown and root pathogens. Pageant is one of the first crop protection products in the ornamental market to include “plant health” on the label. In addition to disease control, it helps plants weather periods of high stress in certain situations.

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