3 Quick Tips: Managing Root Nibblers

Untreated, diseased

Pythium is a soil mold that is well known among greenhouse growers. This pest is not new nor one of the dreaded "invasive" pests that we seem to hear more about these days. Perhaps we've even grown a bit too comfortable with this pest and have become a bit lax in the needed preventive measures. After all, Pythium is just a root "nibbler." How much damage can it really cause? On poinsettias, a lack of vigor, uneven growth, and even stunted plants can result from a Pythium invasion. When the cultural aspects of the crop are a bit off balance, such as high salts, the negative effects of Pythium can become magnified resulting in plant death. Overwatering the crop will also tip the balance in favor of Pythium root rot.

The "seeds" of Pythium are microscopic and can be found on plant benches, greenhouse floors and hitchhike their way from greenhouse to greenhouse via soil and/or roots in plant plugs or prefinished plants. Even growers that start their material from seed run into trouble if Pythium takes up residence in the greenhouse where it survives from season to season. Reusing plug trays is one way that young, tender seedlings become exposed to Pythium root rot.

Root rot must be detected and identified early when fungicide drenches can be most effective. If Pythium has had a significant head start, the root system of some plants will be too rotted and a fungicide drench won't be able to rescue them. An early symptom of root rot often includes uneven plant growth. If looking across a crop in your greenhouse makes you seasick because the height is uneven and looks like "waves," you may need to look at the roots to get to the problem.
 

Effective fungicides
Some products that are effective in controlling Pythium root rot have been available for a long time. A fungicide such as Banrot 40WP is a mixture of two different active ingredients and targets Pythium along with Rhizoctonia and black root rot. Banrot can be a good choice when you need to make a treatment quickly and don't have time to determine which pathogen is the culprit. If Pythium is diagnosed as the problem, fungicides that are specific for Pythium can be used. These include Subdue MAXX (also available as Mefenoxam 2), Banol, Truban and Terrazole. 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. If Pythium is sensitive to Subdue MAXX, then this fungicide will be effective in halting root rot. Pythium should be tested each year that root rot is a problem to determine whether Subdue MAXX can be part of an effective fungicide program.

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, Terrazole, and Banrot have the same active ingredient, rotating among these fungicides is not recommended. The Aliette fungicide has provided mixed results in our trials at Michigan State University. In a recent study that was sponsored by IR-4, it did not protect poinsettias from Pythium. In contrast, FenStop, Adorn, and Heritage provided a good level of Pythium protection in the same poinsettia test. FenStop, Adorn, and Heritage are products that have not historically been part of our standard control measures for Pythium and we continue to learn more about their range of activity and how to best use them. Regardless of the fungicide, the time between fungicide applications should not be stretched beyond the minimum interval listed on the label.

Never become wholly reliant on one fungicide. Too much of one thing is not good. One fungicide should always be used in alternation with another fungicide that has a different mode of action. This type of approach ensures that each fungicide application attacks the Pythium in a unique way.

This research was funded in part by the IR-4 Project and Cooperative Agreement #59-1907-5-553 with the ARS under the Floriculture and Nursery Research Initiative.

Have a question? You can write Mary at hausbec1@msu.edu.