Root rots of herbaceous plants are caused by fungi that live in the soil. Sometimes, the fungi are brought to your garden on plants or soil you place there. In other cases, the fungus may have been present but inactive as long as plants were vigorously growing. Root rots are generally more likely to occur in wet areas of the garden and early in the season when tissues are tender. In many areas of Illinois, rains have set the stage for problems with root rots. Some of the fungal pathogens we see as root rots in Illinois include Phytophthora, Pythium, Rhizoctonia, Fusarium, and Thielaviopsis.
Plants that are stunted, low in vigor, or slow growing--or those that wilt easily on a warm day--may be infected with a root rot. Diseased roots cannot absorb water and nutrients needed for growth. Many factors that affect root growth could cause the same symptoms as root rots. These might include factors such as flooding, drought, fertilizer injury, cool temperatures, too much shade, chemical injury, etc. Root rots may also cause the foliage to 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. In fact, dry conditions following infection by a root rot pathogen causes a more rapid decline of plants.
To confirm a root rot problem, carefully remove an affected plant from the soil, place it in a bucket of water and gently move the plant around in the water to wash off the soil. If roots are washed too vigorously, all of the rotted tissue will be washed off, often leaving a white root interior that appears healthy. A healthy plant has numerous white roots that appear fibrous. It even has visible white root hairs. Roots of a diseased plant appear water-soaked and usually are some shade of brown or black. 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 encountered in Illinois landscapes are Rhizoctonia, Fusarium, Pythium, and Phytophthora. In a simplified scheme, we can group the first two fungi as those causing a dry rot, often with a reddish pink cast to affected roots, as either Rhizoctonia or Fusarium. Pythium and Phytophthora can be grouped as the types causing a soft, brown-to-black rot of roots.
Control of root rots should be aimed at prevention. Use only healthy transplants. Weak plants may be diseased, and you won’t save time or money if you use weak plants. As poor drainage usually goes with root rot, proper site preparation to provide good water drainage away from roots is imperative. Pythium and Phytophthora are problems on wet sites, requiring moisture to infect. Use a balanced fertilizer if desired, but keep rates low on new transplants. Rotate plantings in the garden every 2 or 3 years with unrelated plants to help prevent the buildup of pathogens in one area. This is extremely helpful in preventing Fusarium and Rhizoctonia. Remove crop residue at season’s end to help reduce pathogen survival.
Even with these practices, root rot may still occur. Fungicides are available to help control the major groups of fungi discussed here. The fungicides protect plant stems and roots not yet affected but cannot magically revive dead plants. Fungicides are most useful 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. Affected plants are removed, and nearby plants treated with the appropriate fungicide. Many fungicides are specific to particular pathogens, so treatment relies on accurate diagnosis. Specific chemicals are listed by host crop in the Illinois Commercial Landscape and Turfgrass Pest Management Handbook or the Home, Yard, and Garden Pest Guide. Consult Report on Plant Disease, no. 615, “Damping-off and Root Rots of House Plants and Garden Flowers,” for more details on root rots. This publication is available in Illinois extension offices are on the Internet at http://www.ag.uiuc.edu/%7Evista/horticul.htm.