6/27/2023 - By Dr. Aaron Palmateer

Just like the people who enjoy the warm and sunny weather that summer brings, plants also thrive during this time of year. Warmer temperatures with frequent rain events are favorable for plant growth, but these conditions are also highly favorable for disease-causing pathogens, most notably for phytophthora. Those conditions make preventative disease management a priority, especially for commercial greenhouse and nursery operations with a high density of plant material.

Phytophthora Plant Disease

Some plant pathogens, especially those that thrive during warm and wet weather, are introduced into the greenhouse or nursery through the water. In fact, some of the costliest plant diseases are caused by a group of fungal-like pathogens called Oomycetes. The common name for Oomycetes is “the water molds.” The most infamous is Phytophthora infestans, which was the causal agent of the Irish potato famine.

The name of the genus Phytophthora is derived from Greek and means phyto (plant) and phthora (destroyer), giving some inclination to the severity of plant diseases caused by species of Phytophthora.

More like fungi than other pathogens (i.e., bacteria, nematodes, and viruses), the water molds are more closely related to brown algae than true fungi. Though they have the capability of being introduced into a greenhouse or nursery through water (i.e., irrigation water, splashing, windblown rain), most species have adapted to living in moist environments by producing spores with flagella that enable them to swim. Thus, the spores can move readily in free water within the soil and on plant surfaces, often resulting from irrigation, rain, fog, or dew.

How to Spot Phytophthora

Phytophthora plant disease species are known to cause root rots, stem and crown rots, cankers, and aerial blights. Warm, humid, and wet conditions are highly favorable for Phytophthora, and we often see symptoms of the disease above ground. For example, Phytophthora blight on Spathiphyllum (peace lily) begins on the leaves and flowers, which become necrotic and blighted. And the stem-like petioles become discolored and wilted as the disease progresses downward. In advanced stages, dark brown to black leaf petioles collapse, resulting in death, however, the roots of diseased plants often remain healthy.

Prevention of Phytophthora Plant Disease

Wet conditions favor Phytophthora, and overwatering and saturated conditions often lead to disease. Both field-grown and containerized plants are more likely to be affected by Phytophthora in nurseries prone to flooding. Due to the moist environment, propagation houses and plants on mist benches are highly susceptible. Sanitation is a must in production areas of the nursery or greenhouse:

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

-          Space plants apart to allow as much air movement as possible to hasten leaf drying after irrigation or rainfall.

-          Sanitize hard surfaces in the growing area with disinfectants.

-          Remove all diseased plant tissue (especially leaf tissue) and dispose of it offsite.

Like many plant pathogens, Phytophthora is a soilborne organism that resides in the ground or natural field soils, making the use of ground cloth and other physical barriers important. Any area that is exposed to bare ground can potentially be a source of Phytophthora, especially when prone to flooding. It’s not uncommon to see plants that have fallen over where the leaves touch bare ground or standing water, become infected and then when stood upright serve as the source of inoculum for an outbreak of Phytophthora blight.

Chemical Control of Phytophthora Plant Disease

Your Harrell's Representative can help you to formulate the perfect plan for prevention in your plants. In the below table, you can find a few product recommendations and application rates/intervals so you can start your prevention regimen today.

Also, consider the powerful new fungicide Harrell's ProtectMAX® Fluoxastrobin SC (FRAC 11) for your rotation.

Final Thoughts

The disease can move swiftly through a plant canopy and it’s not uncommon to see significant damage occur over a single night or two. Thus, fungicides are highly recommended for managing Phytophthora plant disease and a preventative program should be in place, especially during the summer months when conditions are most favorable for diseases.

When applying any pesticide, always refer to the manufacturer’s label for recommended rates and application intervals.

As always, if you have any further questions regarding disease, diagnosis, product recommendations, or prevention strategies, you can reach out to your local Harrell's Rep for assistance.