Issue 12, July 17, 2009
Root Rot of Herbaceous Plants
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.
Plants that have small, off-color leaves, are stunted, low in vigor, 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, excess 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 will depend 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 will cause 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 away, often leaving the remaining white interior that appears healthy. A healthy plant has numerous fibrous white roots. It will even have visible white root hairs. Roots of a diseased plant appear water-soaked and usually brown or black. The discolored roots are often soft and mushy, while healthy roots are firm. The image shows rotted roots of my Gypsophila (baby's breath) that I planted this spring in a soil with high clay content. The disease is Pythium root rot.
There are many root rot pathogens, but the major root rot fungi that will be encountered on herbaceous plants in Illinois landscapes are Rhizoctonia, Fusarium, Pythium, and Phytophthora. In a very simplified scheme we can group the first two fungi as those causing 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.
Control of root rots should be aimed at prevention. Use only healthy transplants. Weak plants may be diseased and you certainly won't save time or money in the end if you use weak plants. Since poor drainage usually goes hand in hand 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 annual 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 the end of the season to help reduce pathogen survival.
Even if all of the above practices are followed, root rot may still occur. In the case of my rotting Gypsophila, the problems were soil that did not drain as well as needed and about 5" too much rainfall. Fungicides are available to help control the pathogens discussed here. Fungicides will protect plant stems and roots not yet affected but will not 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 the end of the season. 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 of the root rot pathogen. Often root rot pathogens are cultured on agar in the lab. ELISA (enzyme linked immunosorbant assays) offers quicker and more accurate testing. The second image shows Rhizoctonia root rot on vinca alongside the blue ELISA test wells showing a positive reaction for Rhizoctonia. 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 RPD No. 615, Damping-off and Root Rots of House Plants and Garden Flowers for more details on root rots.--Nancy Pataky