Disease appears at pre-flowering stage in 40-50 days old plants but can also occur on younger plants. Symptoms develop on leaves, sheaths and stalks and can later spread to ears. On leaves and sheaths, a number of soaked, discolored concentric bands and rings are visible, often brown, tan or gray in color. Usually, the symptoms appear initially on the first and second leaf sheath above the ground. Over time, a conspicuous light brown cottony growth with small, round, black specks develops on infected tissues and can later spread to the ears. Developing cob is completely damaged and dried prematurely with cracking of husk leaves. Severity of disease is dependent on the stage of ear development at the time of infection. If seedlings are affected, growing points die and whole plants could be blighted within a week.
The symptoms are caused by the soil-borne fungus Rhizoctonia solani, which can survive in the soil, on infected crop debris or on grass weeds. In response to favorable humidity and temperatures (15 to 35°C, optimal 30°C) at the onset of the growing season, fungal growth resumes and targets freshly-planted host crops. At 70% relative humidity, disease development is negligible/absent, whereas at 90-100 % R.H, highest level of disease is induced. The fungi spread by irrigation water, flooding and transport of contaminated soils in equipment or clothes. The disease is more prevalent in humid and hot weather in the tropics and subtropics. It is extremely difficult to control with fungicides and therefore, a combination of management practices is often required.
To reduce incidence and severity of the disease, maize seeds can be sterilized for 10 minutes in 1% sodium hypochlorite solution and 5% ethanol, washed three times with water and dried. An added treatment with formulations containing Bacillus subtilis enhances this effect. Products containing the fungus Trichoderma harzianum or T. viridi have also been used to successfully limit the spread of the disease.
Always consider an integrated approach with both preventive measures together with biological treatments if available. Maize seeds can be treated in a preventive way with captan, thiram or metalaxyl, washed three times with sterile distilled water and air-dried. Fungicide sprays may be economically viable when susceptible varieties are grown and climatic conditions favor disease severity. Products containing propiconazole are effective to avoid the worst of the symptoms.