All garden flowers may be affected by one or more root rots, and symptoms vary, making field identification difficult. Often there is not time to wait for a laboratory culture to identify the exact problem, so what do you do?
First look closely at the symptoms to establish that a root rot is involved. Plants may be stunted or low in vigor, may grow slowly, or may wilt easily on a warm day. 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 will depend on the fungal pathogen, the susceptibility of the host plant, and the soil conditions.
If a root rot is suspected, carefully remove the plant from the soil, gently wash off the roots, and examine them for indications of rotting. (If roots are washed too vigorously, the rotted tissue will be washed off, often leaving a white root interior that appears healthy. Wash the roots by gently moving the plant up and down in a bucket of water until soil is removed.) 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. 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 flowers 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.
Control of root rots should be aimed at prevention: Plant resistant varieties, use healthy transplants, prepare the site properly to provide good water drainage away from roots, use balanced fertilizer, and rotate garden plantings for two or three years with unrelated annual flowers to help prevent the buildup of pathogens in one area. 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. Fungicides are available to control the major groups of fungi discussed here. The fungicides will protect plant stems and roots not yet affected. Fungicide use is indicated 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, 1998-1988 and the Illinois Homeowners' Guide to Pest Management. Report on Plant Diseases No. 615 contains more details on root rots of garden plants.