Greenhouses: Pythium in Poinsettias

Greenhouse growers commonly have problems with Pythim root and stem rot in poinsettias. The following is an article on the subject.

Pythium root and stem rot is a common disease problem in poinsettias, and is often difficult to control. While there are numerous Pythium species that infect a wide variety of greenhouse ornamentals, Pythium aphanidermatum is the most common pathogen found on poinsettias in late summer and fall. This fast-growing species is favored by warm conditions, and can cause extensive root and stem rot very quickly. Successful disease management requires an integrated program with cultural and chemical components.

Cleanliness. Greenhouse sanitation is an important part of any disease management strategy, especially with soilborne diseases like Pythium root rot. Any soil particle or tiny piece of rotted root can potentially harbor the thickwalled, resistant spores called oopsores, which allow the pathogen to lie in wait until environmental conditions favor pathogen growth. Washing greenhouse benches and floors between crops helps to reduce the amount of pathogen material that persists from crop to crop. Keep hose nozzles off of the greenhouse floor, use clean pots and tools, and store potting media carefully to avoid contamination with soil or plant debris. Monitor for fungus gnats and shoreflies, insects that can carry Pythium from floor to bench. These seemingly mundane housekeeping chores are important in an integrated approach to Pythium root rot management.

Healthy plants. Plants that are under stress from over-fertilization or over-irrigation are more vulnerable to Pythium diseases. Cultural practices that promote overall crop health will help reduce Pythium problems, since healthy plants will produce more roots and the loss of a few roots due to disease will have less of an impact on plant growth.

Protection. Fungicides are another important tool in the battle against Pythium. Several products, such as etridiazole (Truban, Terrazole), thiophanate methyl + etridiazole (Banrot), fosetyl Al (Aliette), propanocarb (Banol), and mefenoxam (Subdue Maxx), are effective in protecting roots from Pythium infection. Resistance to mefenoxam is common among Pythium isolates (Dr. Gary Moorman and colleagues at Penn State University found more than one third of P. aphanidermatum isolates tested were insensitive to this fungicide), so rotating applications of different materials will maximize Pythium disease control. It is important to note that these compounds will not cure plants with extensive root or stem rot – wilted plants should be discarded.

Remember, it takes more than one weapon to win the fight against Pythium – use all of these strategies to maximize disease control.

Information reprinted from "Managing Pythium in Poinsettias" by Karen Rane, Director, University of Maryland Plant Diagnostic Laboratory in the Greenhouse TPM/IPM Weekly Report, University of Maryland Cooperative Extension,Central Maryland Research and Education Center, October 12, 2007