Pythium root rot in the greenhouse

Don’t let this root nibbler eat away at your profits. Follow these tips to better control Pythium root rot in your greenhouse.

Pythium crown and root rot is a troubling disease for many growers and at times may seem unavoidable and tough to control. Pythium is a water mold and “nibbles” the feeding roots of plants, resulting in stunted growth and death. Root rot disease is favored by growing conditions that are “too wet,” such as when media does not drain quickly or when weather doesn’t allow rapid drying.

Pythium can be introduced into a greenhouse via plant plugs or other pre-finished plant material. This pathogen can also be a greenhouse “resident” that hibernates on dirty plant containers, benches, hoses and greenhouse walkways, ready to become activated by the right plant and weather conditions. Although Pythium can be a problem on many annuals and perennials, it seems to favor certain crops, for example geraniums and poinsettias, more than others.

Sanitation is especially important in limiting root rot. First, use a pressure washer with soap and water when cleaning walkways, benches, etc. Second, treat cleaned surfaces with a disinfestant to remove any remaining “unseen” problems such as Pythium. Conditions that favor good plant growth and minimize stress make the plant less vulnerable to attack by a root rot. If you’ve done everything right and still find yourself with a Pythium problem, choosing the fungicide tools that work can minimize your losses.

Root rot must be detected and identified early while fungicide drenches can be most effective. If Pythium has a significant head start, the root system of some plants will be too rotted and the fungicide drench won’t be able to rescue them. If timely fungicide drenches have been made using appropriate rates and root rot continues to be a problem, it is time to take a hard look at the products used. If Subdue MAXX has been the only or primary fungicide used over the years for Pythium control, it is possible that the Pythium has become resistant and is no longer affected by this fungicide. To know for sure, however, the Pythium present in your greenhouse can be tested. Some diagnostic clinics offer this service. To avoid the development of resistance, rotate among the different active ingredients available among fungicide products.

Generally speaking, fungicides that control Pythium disease do not control Rhizoctonia and Thielaviopsis root rots. A fungicide such as Banrot 40WP is a mixture of two different active ingredients and targets all three root rots. Banrot can be a good choice when you need to make a treatment quickly and don’t have time to determine which root rot pathogen is the culprit. If Pythium is diagnosed as the problem, fungicides that are specific for Pythium can be used and include Subdue MAXX (also available as Mefenoxam 2), Truban, Terrazole and Aliette. Some greenhouse growers who have struggled with Pythium problems have determined that Subdue MAXX does not control the disease and have had to rely on other fungicides. Pythium should be tested each year root rot is a problem to determine whether Subdue MAXX can be part of an effective fungicide program.

Pythium graph

Healthy vs. diseased
Untreated healthy control plant (left) versus untreated diseased control plant (right).

Healthy vs. Subdue MAXX
Untreated healthy control plant (left) versus Subdue MAXX-treated diseased plant (right).

Truban and Terrazole are often used by growers when their particular Pythium is resistant to Subdue MAXX. Other growers successfully rotate either Truban or Terrazole with Subdue MAXX in a program to control root rot and delay potential problems with fungicide resistance. Since Truban and Terrazole have the same active ingredient, rotating between these fungicides is not typically recommended by Michigan State University Extension. Aliette is a tool for Pythium control that can be helpful if used early and if the disease is not severe. For the best control, the time between fungicide applications should not be stretched beyond the minimum interval listed on the label.

Pythium ‘A’ Team

Truban WP/Terrazole WP


Subdue MAXX EC



Pythium ‘B/B-’ Team

Adorn SC


Aliette WDG


Captan WDG


Heritage WDG


FenStop SC


Segway SC


Acknowledgments. This material is based upon work supported by Specific Cooperative Agreement 58-1907-0-096 with USDA-ARS under the Floriculture and Nursery Research Initiative.

Dr. Hausbeck’s work is funded in part by MSU’s AgBioResearch.

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