Carrot (Daucus carota)-Violet Root Rot

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Cause Rhizoctonia crocorum (teleomorph: Helicobasidium brebissonii), a soilborne fungus. In the Pacific Northwest, the disease has been a problem in some carrot fields. The fungus reportedly has a wide host range including asparagus, artichoke, alfalfa, bean, beet, cabbage, celery, clover, dandelion, fennel, mangel, parsley, potato, rape, rhubarb, sugar beet, sweet potato, and turnip as well as numerous weed species. It can survive as mycelium or sclerotia in soil or on susceptible crop and weed hosts. The fungus may be spread as a contaminant on seed, plant parts, soil, tools, floodwater, and equipment. The mycelium grows through the soil from plant to plant. Optimum temperature for infection is 59 to 61°F. The disease is most destructive on heavy, unlimed soils.

Symptoms The disease attacks the plants' underground parts, causing stunting, yellowing, and wilting of aboveground parts or the plant's death. The fungus produces a loose mat of reddish purple mycelium over the underground plant portions. Embedded in the mat are numerous small dark sclerotia. A dry rot occurs, after which secondary organisms may cause a wet rot. A pink to brown characteristic mycelial mat may extend along the plant above the soil.

Cultural control

  • Use long rotations (4 to 5 years). Include cereals and grasses.
  • Provide good weed control and drainage.
  • Maintain good fertility, and lime acid soils to increase pH.

Biological control

  • T-22 HC at 0.088 to 3.52 lb/100 lb seed (commercial seed treatment) or 4 to 8 oz/A (planter box) will suppress certain fungal path-ogens including Fusarium, Rhizoctonia, and Pythium spp. Do not use in alkaline soils or if soil is below 50°F. 4-hr reentry. O