 Mary Hausbeck |
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 a growing medium 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 such as geraniums and poinsettias.
Limiting root rot
Sanitation is especially important in limiting root rot. First, use a pressure washer with soap and water. 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 plants less vulnerable to attack by root rot pathogens.
Early detection critical
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 a 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 pathogen 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.
Effective fungicides
The products proven to be especially effective in controlling Pythium root rot are those that have been around for a long time. For the most part, 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 rot pathogens. 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.
The problem with Pythium and fungicide resistance may also extend to Banol. Penn State University plant pathologist Gary Moorman has reported that some Pythium strains that are resistant to Subdue MAXX may also be resistant to Banol.
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. Aliette, FenStop and Segway are tools 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.
Avoiding resistance
One way to keep currently available fungicides working at their best is not to become wholly reliant on one fungicide. Fungicides should always be used in alternation, so that each application attacks the Pythium through a different mode of action.
It is critical to know the active ingredient of each fungicide and whether particular products should be alternated in a program. For instance, based on recent research, using Subdue MAXX in alternation with Banol probably isn’t the best approach because of the potential of Pythium to develop resistance to both of these products. Since Banrot, Terrazole, and Truban all have the same active ingredient (etridiazole), alternating among these products is not recommended.
This research was funded in part by Cooperative Agreement #59-1907-5-553 with the ARS under the Floriculture and Nursery Research Initiative.
Mary Hausbeck is professor at Michigan State University, Department of Plant Pathology, www.plantpathology.msu.edu.
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