Many root rot pathogens exist, Rhizoctonia are a diverse group of plant pathogens, which cause not only root rot but stem and leaf diseases. Rhizoctonia root rot will attack herbaceous and woody ornamentals at the soil line and cause root loss, stem constriction and death of the tops of the plants (as stated by the Western Farm Service). There are several ways to control root rot: buying healthy plants, planting in well-drained soil and applying a fungicide.
Root rots are caused by fungi that are soil-borne. The fungi can find their way into your garden through plants that you purchase at the nursery or through soil that you add to your garden beds. It can also be present in your soil but inactive until the right conditions take place that activate the fungus. Most root rot fungi become active in early spring in wet soil that is not well-drained.
Signs of Root Rot
There are many signs of root rot: stunted plants, off-color leaves, wilting and slow growth. All of these symptoms are understandable when you consider that when root rot attacks a plant the roots are no longer capable of absorbing nutrients and water. According to the University of Illinois, "The severity of the root rot will depend on the fungal pathogen, the susceptibility of the host plant, and the soil and moisture conditions."
The best defense against rhizoctonia root rot is to plant only healthy plants in your garden. Do not purchase plants that appear weak or sick. Sometimes nurseries mark down weak plants to move them from the nursery. What may appear to be a bargain many not be in the long run if you are purchasing diseased plants. Also, plant in well-drained soil. The University of Illinois also suggests rotating your annual plantings every two to three years to avoid pathogen buildup.
The only way to determine if your plants have root rot is to dig up one and examine the roots. To treat the problem, you must be able to identify the root rot fungi. Rhizoctonia and fusarium root rot cause a dry rot that in most cases will give the roots a pink cast. Pythium and phytophthora root rot cause a soft rot that gives the roots a brown or black appearance. (Healthy roots will be firm and white.)
Once a plant is infected with rhizoctonia root rot it should be removed from the garden bed. Fungicides are an ineffective treatment for plants that have already been infected with the pathogen. Fungicides will be effective only to protect the remaining plants. But you will first have to identify the pathogen or fungus to get the correct fungicide. Western Farm Service suggests using Cleary's 3336 fungicide against rhizoctonia root rot. You may not always be able to tell by appearances; you may have to ask a lab to identify the pathogen for you.