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Greenhouse Updates: Mar 31, 2020

Pythium 101
March 31, 2020

Damping off and root rot are common problems to watch for in spring greenhouse crops. Pythium is one of a handful of pathogens that can cause both diseases. Pythium species tend to have broad host ranges and all greenhouse crops are susceptible to infection under the right circumstances. You may have grappled with this menace, but how well do you really know it? Here are some of the basics of Pythium biology:

Despite displaying many of the same behaviors as fungi, Pythium is not a “true fungus”. It belongs to a group of organisms known as Oomycetes (or Oomycota). Oomycetes were widely believed to be fungi until approximately 40 years ago, when molecular studies revealed that they are more closely related to algae. This group is home to several important plant pathogens including Pythium, Phytophthora, and the downy mildews.

Pythium spreads by means of zoospores, which have flagella (moveable tails) that allow them to swim through soil moisture. The zoospores are drawn to potential host plants by substances naturally produced by growing roots. The zoospore swims to the root and forms a cyst on the surface. From this safe little structure, the organism penetrates the root and proceeds to grow throughout the tissue while robbing host cells of important nutrients.

Many Pythium species also form oospores, reproductive structures that may be seen in infected roots and are an important diagnostic feature under the microscope. These oospores have thick walls and are designed to resist adverse environmental conditions such as desiccation. In this way, Pythium can survive in contaminated soil for years after the original host plant is gone.

Due in part to the zoospore stage, Pythium root rot is most severe in wet soils. It is often most troublesome in the spring, when cool, overcast weather slows plants’ uptake of moisture from potting media. Growing “on the dry side” whenever possible is an important tactic for Pythium management. Since Pythium also thrives when nitrogen is abundant, excess fertilization should be avoided as well.

Because they are not true fungi, the Oomycetes are not affected by most of the fungicides used for pathogens such as Botrytis and Alternaria. Maneb and azoxystrobin are two exceptions to this rule. Most products available for Pythium prevention and management are specific either to the Oomycetes or to Pythium itself.


- Angela Madeiras, Extension Educator and Diagnostician, UMass Extension Plant Diagnostic Lab