While outbreaks of Rhizoctonia occur early in the season during canopy closure, symptoms are not generally visible until late summer or early fall. At times, a farmer will not be aware of the infection until harvest.
Rhizoctonia root rot infestation always appears as limited patches that spread along the rows. These small infected patches grow gradually. In extreme cases, entire rows or the whole field can be affected. What starts as a sudden wilting of the foliage gradually evolves into a chlorosis or complete necrosis of the leaves. The dead foliage remains attached to the crown of the sugarbeet, forming a brown rosette in the middle. New leaves may appear, however this is a sign the plant is near death.
Damage in the roots and crown is defined by a dark, black or brown dry rot. It can be observed on the surface and/or below the crown depending on the severity of the outbreak. In some cases, the entire sugarbeet will disappear entirely.
Diagnosing root rot is fairly easy due to the stark discoloration. However, Rhizoctonia may be confused with other root rots such as Pythium or Aphanomyces. Even lightning damage may resemble a Rhizoctonia outbreak. If uncertain of the particular affliction, growers should consult their Seedex dealer, an agriculturalist or other specialist.
Rhizoctonia is present across all areas of the United States where sugarbeets are grown. The severity of an outbreak may vary considerably depending on a number of environmental conditions.
In highly infected patches, the economic impact of an outbreak can be devastating. The consequences of a severe infection are:
- Major losses in yield (losses range from 25% to 100%)
- Reduction in sugar content
- Increased soil tare. Soil sticks to the fungus’ mycelium
- Poor industrial quality due to increased levels of sodium, potassium and nitrogen.
- Storage problems of sugarbeets in piles