The disease gets its name from symptoms that occur on the stem at the soil level. Hard, clearly defined, water-soaked brown to red lesions initially appear on diseased stems near the soil line. Later, stems become wiry and slender at the point of the lesion.
Diseased crucifer plants transplanted to the field grow poorly, are stunted, and may eventually die, especially if there is inadequate moisture shortly after transplanting. If infected plants remain alive, they grow slowly, producing a spindly, stunted yellow plant, and often fail to produce a marketable head.
Bottom rot is primarily a problem on cabbage, bok choy, and Chinese cabbage crops. Once head formation begins, lower leaves that are in contact with the soil might become infected with Rhizoctonia.
Survival And Spread
Warm wet soils and delayed seedling emergence are conducive to infection by R. solani. Wirestem develops when the fungus invades the cortical tissue and girdles young stems. The Rhizoctonia fungus is a common soil inhabitant that can survive almost indefinitely in the soil in the form of small, hard, dark-brown, kernel-like bodies called sclerotia. It also can survive pathogenically on alternate hosts and cruciferous weeds, and saprophytically on crop residues.
Encourage rapid seed germination and vigorous growth of seedling by planting high-quality, heat-treated seed in warm, firm, well-prepared seedbeds. Provide adequate, but not excessive fertilization to promote vigorous growth of seedlings. Treat seed with a registered seed-protectant fungicide. Some cultivars may be less susceptible to infection, consult your seed rep for advice.
If transplants are used, do not plant too deep if Rhizoctonia diseases have been a problem in the field. Avoid planting into cold soils, excessive irrigation, poor drainage, and compaction. Discard all seedlings with discolored stems and roots. Avoid fertilizing transplants as nitrogen can delay hardening of transplants and render them more susceptible to attack. Ensure crop residues from the previous crop are thoroughly decomposed before planting. Deep disking to turn over the top six inches of soil will help speed this process. Crop rotation with non-hosts may provide some reduction in wirestem. Control volunteer mustard and other cruciferous weeds in and around fields.
SoilGard (Gliocladium virens strain GL-21, Certis USA) and other biologicals at planting and at regular intervals after planting may help suppress Pythium and Rhizoctonia. See label for specific application types and timings.
Soil drenches with pentachloronitrobenzine-based fungicides such as Terraclor (OHP Inc.) or Blocker (AMVAC Chemical Corp.) at planting are most effective when used in combination with cultural control strategies. Consult UF/IFAS recommendations for currently labeled fungicides for Rhizoctonia control in cole crops.