By O. Neher and C. M. Ocamb
Cause Rhizoctonia solani is a fungus that survives primarily as mycelium or sclerotia in plant debris. Rhizoctonia solani becomes active when soil is 77°F to 91°F, attacking any part of the root or crown area. The fungus is favored by poor soil structure and high soil moisture. Moisture or nutritional stress or injuries can predispose the crop to infection by R. solani. This pathogen can also cause post-emergence damping-off, resulting in reduced stands. Root and crown rot will result in smaller beets, and increased tare dockage from rotted beets in tare samples.
Symptoms Leaves of individual or groups of plants wilt during the heat of the day but recover partially in cool periods. Leaves may retain their color for a time but turn yellow and die. After severe daytime wilting, often the leaf petiole base darkens and the crown rots. Petioles remain attached to the crown and their base will have dark lesions. Symptoms may be observed as early as mid-June. Symptoms in July, August, and September are darkened petioles, crown rot, rotted or discolored roots, and dying plants. Severely infected roots may be a russeted brown or a dark brown to black. In some instances, rot may be restricted to dark circular areas on the root. The root may be cracked or intact; it appears to be rotted, but inside considerable tissue can appear firm and healthy.
Cultural control The most effective management is to promote good crop growth and avoid plant stress.
- Do not plant sugar beet crops consecutively. A 3-year rotation is the minimum; lengthen to 4 to 5 years if root rot is a problem. Shortened rotations in affected fields are associated with increased losses and a build-up of pathogen populations that make future control more difficult.
- Plan a cropping sequence that maintains organic matter and nutrition and controls weeds and beet-cyst nematodes. Precede sugar beets with corn or cereals, and incorporate crop residue uniformly distributed through the soil profile, ensuring adequate decomposition. Alfalfa, beans, corn, or potatoes should not precede sugar beets; severe root rot has been observed following these crops.
- Supply adequate nutrition as needed, but avoid excess nitrogen.
- Fall bed when feasible.
- Provide moisture as needed, but avoid over-irrigating throughout the growing season. Optimum soil moisture for sugar beet growth is between -40 and -60 centibars (cbars) soil matric potential.
- Control soil compaction.
- When hilling, avoid pushing soil into the crowns because the petiole and crown are the point where most pathogens infect.
- Maintain a 6- to 8-inch plant spacing. This helps to keep soil cooler.
Chemical control
- Elatus (Group 11 + 7) at 0.3 to 0.6 oz/1,000 row feet applied in a 7-inches or less T-band in furrow at planting for root rot and/or applied at 7.1 oz/A at the 2- to 8-leaf stage as a directed spray for crown rot. Can be applied up to the BBCH 31 growth stage. 12-hr reentry.
- Excalia (Group 7) at 2 fl oz/A on a 21-day interval. Preharvest interval is 50 days. 12-hr reentry.
- Proline 480 SC (Group 3) at 5.7 fl oz/A in a 7-inch band at 4-leaf stage to row closure. Preharvest interval is 7 days. 12-hr reentry.
- Quadris Flowable (Group 11) at 0.4 to 0.8 fl oz/1000 row feet applied in a 7-inches or less T-band in furrow at planting for root rot and/or applied at the 4- to 8-leaf stage as a directed spray for crown rot. Preharvest interval is 0 days. 4-hr reentry.
- Vertisan (Group 7) at 0.7 to 1.6 fl oz/1000 row-ft at planting as an in-furrow application. Do not use Vertisan when penthiopyrad was used as a seed treatment. Preharvest interval is 7 days; 0 days for forage and hay. 12-hr reentry.
Biological control
- SoilGard at 0.5 to 10 lb/A at 4-week intervals will suppress Pythium and Rhizoctonia. See label for specific application types and timings. Can be applied on the day of harvest. 4-hr reentry. O
Reference Gallian, J. 2001. Management of Sugarbeet Root Rots. PNW538. 7 pp.