How do you know that herbaceous plants have a root rot problem? It likely will be fairly obvious by poor top growth, wilting, stunting, or off-color or nutrient-stressed foliage. Look at suspect plants more closely for mechanical injuries to the stem, insect feeding, stem or foliar lesions, evidence of drought stress, etc. Some root rots also discolor the stem a few inches above the ground. Others affect only roots. Dig an infected plant including the root system. Wash the roots by soaking them in a bucket of water. Then examine them for symptoms. Healthy roots should be firm, have white tips, and be white internally. Rotted roots are brown or black, often soft or shriveled, and do not have nice, white root tips.
Annuals and perennials may host many species of root rot fungi, including Pythium, Phytophthora, Rhizoctonia, Sclerotinia, and Sclerotium. All are soil-borne and have fairly wide host ranges, meaning they can infect many different plant species. Rhizoctonia is the one most common now. A disease of midsummer, it thrives in hot weather and does not require moisture to infect. Ordinarily, it infects at the soil line and may move up or down the stem. A dry rot may be seen at the soil line, making this disease easy to spot. The stem at the ground line appears dry and wiry.
Control of root rots should be aimed at prevention. Use only healthy transplants. Don't try to save by buying weak plants. They may be diseased, and you certainly won't save in the end. You would have diseased plants and have inoculated your soil with the pathogen. Proper site preparation to provide good water drainage away from roots is imperative. This fall, dig the soil in the entire planting bed to a depth of about 10 inches and work in organic matter if drainage needs to be improved. Use a balanced fertilizer if desired, but keep rates low on new transplants. Rotate plantings in the garden every 2 or 3 years with unre-lated plants to help prevent the buildup of pathogens. This practice is extremely helpful in preventing Rhizoctonia. Remove crop residue at the end of the season to reduce pathogen survival.
Even if these practices are followed, root rot may occur. Fungicides are available to control the major groups of fungi. The fungicides protect plant stems and roots not yet affected. Their use seems most significant in production areas or in cases where a root rot is discovered in a flower bed and the goal is to preserve remaining healthy plants to season's end. Fungicide options are too numerous to list here. Specific chemicals are listed by host crop in the Illinois Commercial Landscape & Turfgrass Pest Management Handbook or the Illinois Homeowners' Guide to Pest Management. After determining the chemicals registered on your crop, consider this information obtained from Diseases of Woody Ornamentals and Trees in Nurseries: Fungicides that have good efficacy against Rhizoctonia include thiophanate-methyl, iprodione, triflumizole, azoxystrobin, trifloxystrobin, fludioxonil, and PCNB. Consult Report on Plant Disease, no. 615, "Damping-off and Root Rots of House Plants and Garden Flowers."