Rhizoctonia rot on ornamentals

The Rhizoctonia solani fungus typically causes a dull brown to dark brown rot on lower plant stems (Photo 1). In severe cases it can also destroy the root system of an infected plant, resulting in wilting and plant death (Photos 2-6). This pathogen can thrive in wet/dry or warm/cool conditions. Sanitation is an important method of limiting the pathogen as it is most likely to spread with contaminated soil and used flats or pots.

by Mary Hausbeck

Photo. 1. Lesion on the stem of salvia at the soil line caused by Rhizoctonia. All images courtesy of Mary Hausbeck, MSU.

Photo. 2. Wilt and plant death of osteospermum infected with Rhizoctonia.

Growing mixes that are naturally suppressive to damping-off soilborne fungi are also available to growers. A suppressive medium is one that represses, restrains or checks the growth of damping-off fungi. Initially, interested growers may want to test the usefulness of suppressive media in their growing system by planting a small portion of their crop in suppressive media. Communication with sales representatives and extension personnel may be helpful to evaluate and perhaps modify growing systems using suppressive medium to achieve the best results.

Photo. 3. Wilt of calibrachoa infected with Rhizoctonia.

Photo. 4. Wilt and plant death of vinca infected with Rhizoctonia.

The “A” and “B+” Team tables of fungicides recommended for Rhizoctonia on ornamentals are the result of multiple trials conducted at Michigan State University. The FRAC code is an alphanumeric code assigned by the Fungicide Resistance Action Committee and is based on the mode of action of the active ingredient. When treating for Rhizoctonia rot, rotate among products with different FRAC codes to reduce the possibility of resistance developing in the Rhizoctonia fungus. Ornamental products listed in the A Team table consistently provide effective control. B Team products are a good rotational choice when disease pressure is not severe.

Photo. 5. Wilt of schizanthus infected with Rhizoctonia.

Photo. 6. Wilt and plant death of salvia infected with Rhizoctonia.

Terraclor, Terraguard, Cleary’s 3336/OHP 6672 and Medallion applied as a drench have been important tools in preventing Rhizoctonia on ornamentals and halting its spread. Biocontrol agents are becoming more widely available for use in controlling damping-off fungi such as Rhizoctonia, and repeated studies with the biopesticide active ingredient polyoxin D zinc salt have shown it to be a very effective product.

Source: MSU Extension