Earlier in the season, we were concerned about drought conditions. Now it seems that heavy rains are more common. As they say, if you don’t like the weather, just wait and it will change. Excessively wet soils have led to many cases of rotted roots on annuals, perennials, and nonwoody plants in the landscape. We’ve had more problems with established plants because roots were initially injured by drought. Plants may be stunted or low in vigor, may grow slowly, or may wilt easily on a warm day. These problems always become more visible in hot weather because the lack of healthy roots causes a more rapid decline of plants, which is very noticeable to the homeowner, client, or groundskeeper. The foliage may turn yellow to brown and drop prematurely, usually starting with the older leaves and moving up the plant. The severity of the root rot depends on the fungal pathogen, the susceptibility of the host plant, and the soil and moisture conditions.
If a root rot is suspected, remove the plant from the ground carefully, place it in a bucket of water, and carefully move the plant up and down in the water to dislodge the soil. Then examine the roots for indications of rotting. If roots are washed too vigorously, all of the rotted tissue will be washed off, often leaving a white root interior that appears healthy, but close examination will show that such roots are much thinner than healthy white roots. A healthy plant has numerous white roots that appear fibrous. Roots of a diseased plant show various degrees of water soaking and usually are some shade of brown or black, both externally and internally. The discolored roots are often soft and mushy, while healthy roots are firm.
There are many root-rot pathogens, but the major root-rot fungi that are encountered in Illinois landscapes are Rhizoctonia, Fusarium, Pythium, and Phytophthora. In a very simplified scheme, we can group the first two fungi as those causing a dry rot, often with a reddish pink cast, to affected roots. Pythium and Phytophthora can be grouped as the types causing a soft, brown-to-black rot of roots. We can work with these in the lab to confirm the exact pathogen involved, but symptoms give you an idea of the disease involved.
We try to prevent root rots from becoming a problem in our gardens by using sound horticultural practices. This includes use of healthy transplants, proper site preparation to provide good water drainage away from roots, use of balanced fertilizer, and rotation in the garden plantings for 2 or 3 years with unrelated plants to help prevent the buildup of pathogens in one area. It is also important to remove crop residue at the end of the season to help reduce pathogen survival. Once a pathogen is identified, try to find and use resistant varieties when available.
Even if all of the above practices are followed, root rot may still occur. Fungicides are available to control the major groups of fungi discussed here. The fungicides protect plant stems and roots not yet affected but do not “cure” infected plants. Their use seems most significant in cases where a root rot is discovered in a flower bed and the goal is to preserve remaining healthy plants to the end of the season. Specific chemicals are listed by host crop in the Illinois Commercial Landscape and Turfgrass Pest Management Handbook 2000 or the Illinois Homeowners’ Guide to Pest Management. Consult RPD No. 615 for more details on root rots of garden plants.