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Root Rots of Annuals and Perennials

May 22, 2002

Root rots are more likely to occur in wet areas of the garden and early in the season when tissues are tender. This spring, conditions have been ideal for root rots. For that reason, it may actually be to your benefit if you have not yet been able to put plants out in the garden.

If plants are stunted, low in vigor, or slow growing or wilt easily on a warm day, they may be infected with a root rot. Dying roots cannot absorb water and nutrients needed for growth. Obviously, other factors that affect root growth could cause the same symptoms. Such factors include drought, fertilizer injury, cool temperatures, too much shade, chemical injury, etc. Root rots may also cause the foliage to turn yel-low 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 plants to decline more rapidly.

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 will even have visible white root hairs. Roots of a diseased plant appear water-soaked and usually are some shade of brown or black. The dis colored 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 are types that cause a soft, brown tn˘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. Because poor drain-age 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 particularly likely problems on wet sites because they require 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 unre-lated 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 occur. Fungicides are available to help control the major groups of fungi discussed here. The fungicides will protect plant stems and roots not yet af-fected. They are most useful if 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 appro-priate fungicide. Many fungicides are specific to particular pathogens, so treatment depends on accurate diagnosis of the root rot pathogen. 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 RPD no. 615, “Damping-off and Root Rots of House Plants and Garden Flowers,” for more details on root rots. This publication is available on the University of Illinois Vista Web site or in Extension offices.

Author: Nancy Pataky


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