Rhizoctonia root rot is cause by the fungus Rhizoctonia solani, which attacks a wide range of plant materials. It survives in the soil in one of two forms. The first form is a brown-to-black colored object that roughly resembles the droppings of some small animal. These objects, termed sclerotia, will send out hair-like structures in a favorable environment. The second means of survival is the hair-like growth itself. These mycellium reside on dead plant tissue, and will grow in favorable environments attacking more dead or decaying material near the soil surface. Occasionally, the growth will expand its feeding to young seeds or seedlings resulting in the disease that we refer to as rhizoctonia root rot or “damping off.”
In soybeans, red to red-brown colored regions of sunken tissue will appear on the root or stem near the soil surface. This damage may be limited in scope and merely speckle the hypocotyls of the bean, or the entire root from the soil surface down may be killed. Seedlings suffering such infection may become stunted and wilt. The cotyledons may also yellow. At first glance, the root and stem infection may appear similar to insect feeding (insect feeding may have played a role), but that “tell-all” red color distinguishes this disease from other forms of root damage. Vegetable crop damping off may begin with brown, water-soaked regions of plant tissue near the soil line. Infected seedlings eventually fall over.
Damaged areas usually occur in patches measuring a yard to three yards in width in heavy, poorly drained soils; in soils with lacking fertility; and in soils subject to rainfall followed by cool and then warm weather. The mycellium need some amount of moisture to grow and infect seeds or seedlings, and when this infection does occur, it may occur as early as when the seed just beings to germinate. In other cases in vegetables, a symptom called wirestem may develop. Stem tissue becomes woody both above and below the soil line resulting in death or severe stunting. Cabbage can develop a head rot stemming from this pathogen.
To ward off problems with this disease, producers can use a few simple options. First, numerous seed treatments are available in soybean and may ward off the disease long enough to establish a healthy root system. Secondly, warm soils of at least 55° can decrease problems with rhizoctonia. The optimum temperature for rhizoctonia actually occurs at soil temperatures in the high 70s to low 80s, but encouraging rapid germination and emergence by planting into a warm soil bed decreases the chances for infection in beans. The emerging seedling is more rapidly pulled above the soil surface by the hypocotyls arch and an adequate root system is more rapidly established. In other words, growth may outpace the capacity of the disease and may remove delicate portions of the plant from the zone of infection. Third, maintaining good drainage plays a significant role, and using the best quality seed available provides yet another management tool. In vegetable production, hot water treatments, discarding discolored transplants, long-term/multi-season rotations, etc. may all decrease the likelihood of encountering rhizoctonia.