R is for Rhizoctonia rot on ornamentals

Mary Hausbeck<hausbec1@msu.edu>, Blair Harlan and Sheila Linderman, Michigan State University, Department of Plant, Soil and Microbial Sciences - May 04, 2017

Know the ABCs of ornamental greenhouse diseases, including Rhizoctonia rot.

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

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.

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.

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.

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.

*Rhizoctonia* A Team (ornamental recommendations only)
Product	Active ingredient	FRAC code
Affirm WDG	polyoxin D zinc salt	19
Emblem/Medallion WG	fludioxonil	12
Empress Intrinsic	pyraclostrobin	11
Orkestra	fluxapyroxad/pyraclostrobin	7/11
Pageant Intrinsic	pyraclostrobin/boscalid	11/7
Terraclor 400	PCNB	14
Tourney	metconazole	3

*Rhizoctonia* B+ Team (ornamental recommendations only)
Product	Active ingredient	FRAC code
Captan	captan	M04
Cleary’s 3336 WP/OHP 6672 FL	thiophanate-methyl	1
Heritage WG	azoxystrobin	11

Wilt and plant death of salvia

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

Wilt of calibrachoa

Photo. 3. Wilt of calibrachoa infected with Rhizoctonia.

Wilt and plant death of vinco

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

Wilt of schizanthus

Photo. 5. Wilt of schizanthus infected with Rhizoctonia.

Wilt and plant death of osteospermum

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

Learn more about the ABCs of ornamental greenhouse diseases

Acknowledgments. This material is based upon work supported by Cooperative Agreement 58-8062-5-036 with USDA ARS under the Floriculture and Nursery Research Initiative.

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

This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).