· Sorghum · Maize
First symptoms of the disease appear during later stages of plant growth, when the inflorescence and ears appear. Tassels may be partly or entirely covered with black, powdery fungal growth. Unusual leaf-like structures can appear on the inflorescences or the ears. Affected ears are rounder than their healthy counterparts and are completely filled with black powdery masses. A tangled mass of vascular strands that are the remains of hard plant tissues mingles between spore masses. Infected plants usually have no silk or kernels on the ears. Excessive branching has been described as a secondary symptom.
The fungus Sphacelotheca reiliana can survive as spores in the soil for a number of years and is transmitted uniquely via the roots. It sporadically infects some plants in the field, mostly during the seedling stage. The fungus later grows within all plant parts, including the inflorescence (tassel) and the ear. This is reflected as black smut growth (spore masses) that consumes the tassels and sometimes completely replaces the kernels. Contamination from field to field may occur through contaminated equipment. Low soil moisture, warm temperatures (21 to 27°C), and nutrient deficiencies favor the infection and the progression of the disease. Once the infection has occurred there is no effective treatment to reduce damage on infected plants.
Application of manure with low carbon to nitrogen ratios can reduce the incidence of the disease. Beetles that feed on the fungus (Phalacrus obscurus and Lystronychus coeruleus) can serve as biological control agents. Seed treatment with bacterial extracts of Bacillus megaterium could also decrease the incidence of the disease.
Seeds can be treated with a systemic fungicide (carboxin) to prevent the fungus from infecting the plants in the first place but this only provides limited control. In-furrow fungicide treatments during seedling stage can also be effective, but may not be economically feasible.