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Managing of Rhizoctonia Damping-off and Root Rot of Sugarbeet (06/18/20)

Rhizoctonia damping-off and root rot, caused by Rhizoctonia solani, are common soil borne diseases of sugarbeet and a major problem for growers in Minnesota and North Dakota.

Rhizoctonia damping-off and root rot, caused by Rhizoctonia solani, are common soil borne diseases of sugarbeet and a major problem for growers in Minnesota and North Dakota. The pathogen may cause death of seedlings and older plants. Growers at some cooperatives have had to destroy fields with more than 50% incidence of Rhizoctonia root rot since infected roots do not store well in long term storage. Crop rotation with non-hosts such as wheat and barley, use of resistant varieties, and fungicides used on seed and applied so that they target the pathogen in the soil are strategies used to manage the disease. Disease symptoms: In infected seedlings, there is wilting followed by death because of infection of the hypocotyl at the soil line (Figure 1). In older plants, the most common symptom is wilting of the leaves, starting with the oldest leaves (Figure 2). Leaves may or may not become yellow. Severely infected plants typically become desiccated and withered before harvest (Figure 3). Some infected plants may survive but damaged roots will be of lower quality. Root rot infections typically starts at or just below the soil line (Fig. 4), and sometimes lower on the tap root. Disease management: Rotations with crops, such as wheat and barley, which are not host of R. solani AG 2-2 IIIB or AG 2-2 IV, will help to reduce the inoculum pressure. Since the pathogen is more severe in wet conditions, draining and leveling of fields with help in disease management. Fields with a history of Rhizoctonia crown and root rot should be planted to a variety with good disease resistance. The fungicides azoxystrobin (Quadris) at 9.2 to 16 fluid ounces per acre, or Priaxor at 6.7 fluid ounces per acre, applied in a 7-inch band before infection takes place provide effective disease control. It is best to apply fungicides when plants are at the 4-6 leaf stage and before they close rows so that the fungicide gets into the soil and close to the roots to prevent infection. Timing application just before a rain event, or before a scheduled irrigation where this is available, will help to get the fungicide in a position to protect the roots.

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Mohamed Khan

Extension Sugarbeet Specialist

NDSU & U of MN

701-231-8596

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