Excessively wet soils early in the season have led to many cases of rotted roots on annuals, perennials, and nonwoody plants in the landscape. Plants may be stunted or low in vigor, may grow slowly, or may wilt easily on a warm day. Dry conditions following infection by a root rot pathogen cause plants to decline more rapidly. The foliage may turn yellow to brown and drop prematurely, usually starting with the older leaves and moving up the plant. Severity of the root rot depends on the fungal pathogen, the host’s susceptibility, and the soil and moisture conditions.
If a root rot is suspected, the plant should be carefully removed from the soil, placed in a bucket of water, and the roots carefully washed of soil so they can be examined for indications of rotting. If roots are washed too vigorously, all of the rotted tissue is washed off, often leaving a white root interior that appears healthy. Close examination shows that such roots are much thinner than healthy white roots. 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 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.
Control of root rots should be aimed at prevention: use of resistant varieties when root rots are known to be a problem, 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 two or three years with unrelated plants 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 these 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. 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 or the Illinois Homeowners’ Guide to Pest Management. Consult Report on Plant Disease No. 615 for more details on root rots of garden plants.