Pythium and Rhizoctonia: A root rot primer

1. Why is Pythium such a troubling disease for many growers?
   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 the growing medium does not drain quickly or when weather doesn’t allow rapid drying.
    
2. How does Pythium get into the greenhouse?
   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 activate by the right plant and weather conditions. Although Pythium can be a problem on many annuals and perennials, it seems to favor certain crops (i.e., geraniums and poinsettias) more than others.
    
3. What are some of the symptoms of Rhizoctonia infection?
   Rhizoctonia fungi typically cause a dull brown to dark brown rot on cuttings or lower plant stems.
    

Pythium on geraniumSanitation is especially important in limiting Pythium root rot. Use a pressure washer with soap and water when cleaning walkways, benches, etc. Then treat cleaned surfaces with a disinfestant to remove any remaining “unseen” problem pathogens such as Pythium. Conditions that favor good plant growth and minimize stress make the plants less vulnerable to attack by a root rot.

Root rot must be detected and identified early while fungicide drenches can be most effective. Generally speaking, fungicides that control Pythium do not control Rhizoctonia and Thielaviopsis root rots. If Pythium is diagnosed as the disease pathogen, fungicides that are specific for the disease include mefenoxam (Subdue MAXX, Mefenoxam 2), propamocarb (Banol), etridizole (Truban, Terrazole) and fosetyl-al (Aliette). 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 is a tool for Pythium control that can be helpful if used early and if the disease is not severe. Other chemical controls include azoxystrobin (Heritage WDG), captan, fenamidone (Fenstop) and cyazofamid (Segway). For the best control, the time between fungicide applications should not be stretched beyond the minimum interval listed on the label.

Fungicides applied as drenches have been an important tool in preventing Rhizoctonia and halting its spread. The fungus Rhizoctonia can be controlled by pentachloronitrobenzene (Terraclor), triflumizole (Terraguard), polyoxin D zinc salt (Veranda O), fludioxonil (Medallion) and thiophanate-methyl (Cleary’s 3336, OHP 6672). The fungicide, Banrot, contains a combination of fungicides (etridiazole + thiophanate-methyl) and provides broad spectrum control against both Pythium and Rhizoctonia fungi. Other chemical controls include azoxystrobin (Heritage WDG) and captan.

Biocontrol agents are becoming more widely available for use in controlling damping-off fungi such as Rhizoctonia. Information on how to use these products most effectively is still being gathered.

There are commercial growing mixes available that are naturally suppressive to damping-off soilborne fungi. A “suppressive” medium is one that represses, restrains, or checks the growth of damping-off fungi. Initially, growers may want to test a suppressive medium in their production system by planting a small portion of their crop in the medium.

This research was funded in part by Floriculture Nursery and Research Initiative of the Agricultural Research Service under Cooperative Agreement #59-1907-5-553.

Mary hausbeck You can write Mary at hausbec1@msu.edu.

Quick Control Tips:
Sanitation is especially important in limiting Pythium root rot. Use a pressure washer with soap and water when cleaning walkways, benches, etc. Then treat cleaned surfaces with a disinfestant to remove any remaining “unseen” problem pathogens such as Pythium. Conditions that favor good plant growth and minimize stress make the plants less vulnerable to attack by a root rot.

Root rot must be detected and identified early while fungicide drenches can be most effective. Generally speaking, fungicides that control Pythium do not control Rhizoctonia and Thielaviopsis root rots. If Pythium is diagnosed as the disease pathogen, fungicides that are specific for the disease include mefenoxam (Subdue MAXX, Mefenoxam 2), propamocarb (Banol), etridizole (Truban, Terrazole) and fosetyl-al (Aliette). 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 is a tool for Pythium control that can be helpful if used early and if the disease is not severe. Other chemical controls include azoxystrobin (Heritage WDG), captan, fenamidone (Fenstop) and cyazofamid (Segway). For the best control, the time between fungicide applications should not be stretched beyond the minimum interval listed on the label.

Fungicides applied as drenches have been an important tool in preventing Rhizoctonia and halting its spread. The fungus Rhizoctonia can be controlled by pentachloronitrobenzene (Terraclor), triflumizole (Terraguard), polyoxin D zinc salt (Veranda O), fludioxonil (Medallion) and thiophanate-methyl (Cleary’s 3336, OHP 6672). The fungicide, Banrot, contains a combination of fungicides (etridiazole + thiophanate-methyl) and provides broad spectrum control against both Pythium and Rhizoctonia fungi. Other chemical controls include azoxystrobin (Heritage WDG) and captan.

Biocontrol agents are becoming more widely available for use in controlling damping-off fungi such as Rhizoctonia. Information on how to use these products most effectively is still being gathered.

There are commercial growing mixes available that are naturally suppressive to damping-off soilborne fungi. A “suppressive” medium is one that represses, restrains, or checks the growth of damping-off fungi. Initially, growers may want to test a suppressive medium in their production system by planting a small portion of their crop in the medium. 

This research was funded in part by Floriculture Nursery and Research Initiative of the Agricultural Research Service under Cooperative Agreement #59-1907-5-553.