Diseases caused by Pythium are widespread throughout the ornamental industry. Almost every ornamental tree, shrub, bedding plant, foliage crop, and grass species is susceptible to Pythium at some point in its life. Geraniums, poinsettias, snapdragons, and grasses are especially susceptible.

Pythium is common in germinating seedlings, causing considerable losses due to seedling disease. Additionally, Pythium root rot can persist throughout all stages of crop production attacking the fine feeder roots and inhibiting the plant’s ability to take up nutrients and water. This results in stunted growth and delayed flowering.

How Does Pythium Spread In Greenhouses and Nurseries?

Pythium is a water mold and is more closely related to algae than to fungi. Other closely related pathogens include downy mildews and Phytophthora. Some isolates of Pythium are aggressive and can cause crown rot and rapid plant death, so quick action is needed once the disease is identified.

The spores of Pythium can spread long distances in the wind and windblown rain, but the pathogen is routinely introduced into greenhouses and nurseries by contaminated stock plants, media, or irrigation water. Once introduced, inoculum can spread by splashing irrigation water and through cultural practices such as potting, pruning, and handling plant material.

Diseases caused by Pythium can be fatal, so early detection is critical, especially in propagation. Depending on the Pythium species, the host plant, and the environmental conditions, there can be several types of disease.

Post-emergence damping off occurs below the soil where the pathogen attacks the emerging root or hypocotyl before full emergence and results in poor stands.

Post-emergence damping off results in rotten discolored stems at or near the soil line and root decay. Affected seedlings often appear shriveled brown, collapsed, or stunted. Some species such as Pythium aphanidermatum and Pythium myriotylum can cause an aerial blight where the pathogens move up into the plant canopy causing a watery rot of leaves petioles and stems.

Effects of Pythium in Greenhouses and Nurseries

Plants infected later in production often appear stunted and develop slower than healthy plants. The leaves appear chlorotic (yellow) and may show signs of nutrient deficiency and wilt, especially during the heat of the day. Plants with Pythium root rot will likely not respond to fertilizer and should be examined and tested for the disease.

Infected roots appear discolored (brown to black) and water-soaked. Often when pulling on affected roots, the outer layer readily peels away from the root, as opposed to snapping or breaking, and leaves behind a bare strand of tissue. Under favorable disease conditions, Pythium can move upward in the plant further rotting the stem and crown.

Pythium inoculum can increase from undetectable to high levels within a few days under the right environmental conditions. This is due to the rapid and massive production of sporangia and zoospores from infected plant tissues when free moisture is present, often leading to disease outbreaks very quickly, sometimes literally overnight. The spores of Pythium can spread readily in the wind, so they can be naturally introduced into the greenhouse or nursery.

The Spread of Pythium in Greenhouse and Nursery Settings

Windblown rain that contains spores of the pathogen and splashing water from overhead irrigation often contribute to outbreaks of Pythium. A further complication is that Pythium can produce oospores, another type of spore that can remain viable for years. To have samples tested for confirmation of disease, consult your local Harrell’s rep for a recommended diagnostic facility.

Wet conditions ultimately favor Pythium, so the presence of free moisture and water-saturated conditions are the most important environmental factors. Temperatures that are not favorable for plant growth tend to favor the development of disease. Some Pythium species thrive and cause disease at high temperatures (> 86°F), while others grow well at low temperatures (< 60°F). These temperature preferences determine which species are problematic in the greenhouse or nursery at different times of the year.

Managing Pythium

Cost-effective disease management starts with excellent sanitation practices in production areas of the nursery or greenhouse.

-        Carefully inspect plugs or newly introduced plant material and reject if any symptoms are present.

-        Clean and remove soil from walkways and beneath benches and sanitize hard surfaces in the growing area with disinfectants.

-        Remove all diseased plant tissue and dispose offsite.

-        Taking a preventative approach with fungicide applications is highly recommended.

Several fungicides are currently labeled for use on ornamentals for controlling Pythium. Due to a high potential for resistance, it’s very important to rotate fungicides from different chemical classes. When applying any pesticide always refer to the manufacturer’s label for recommended rates and application intervals.

The biofungicide and plant symbiont Obtego® can be applied to cuttings and bare root plant material as a dip or as a substrate mix, chemigation, drench, or spray application offering an additional mode of action for control of Pythium.

If you have any questions on any of these products or on all things Pythium, your local Harrell's rep is here to help.