Effective disease management

Disease attack by fungi, bacteria, viruses and nematodes is one of the causes of low yields in sorghum production. The four economically most damaging diseases of sorghum include stalk rots, anthracnose, smuts and downy mildew.

Stalk rot (Fusarium spp.) causes the premature death of plants and rotting of roots. Infected stalk tissue turns dark red. This disease is favoured by physiological stress, abundant moisture and moderate temperatures during the grain-filling period. The stalk-rotting funguses quickly invade plants and digest the remaining stalk structure, which eventually results in lodging of the plants. Use of resistant cultivars, proper crop rotation and proper plant spacing are effective in controlling stalk rot.

Anthracnose (Colletotrichum graminicola) causes orange, red or blackish-purple discolorations on leaves, which are small, circular or elliptical in shape. The disease is common in hot and humid climates and is prevalent in areas where high humidity alternates with relatively dry periods. The disease can be controlled using resistant varieties (hybrids), growing sorghum in rotation with non-cereals, preferably pulses, and by encouraging rapid decomposition of crop residues after harvest.

Smut (Sporisorium sorghi): Diseased plants show individual grains that are replaced by whitish to grey or brown smut sori. Heavily contaminated seed may turn greyish-black, especially in white-seeded sorghums. The fungus rarely survives in the soil between cropping seasons, therefore proper rotation can effectively control smut. Sorghum can only be infested by smut when the seeds are infested by airborne spores at harvest or in the soil before the seedlings emerge. Treatment of seeds with hot water successfully reduces the disease. Tolerant sorghum cultivars are less susceptible. Cultural control measures include proper crop rotation and removal of infected panicles.

Downy mildew (Peronosclerospora sorghi): The disease can cause serious yield losses in sorghum (and maize). It affects the plant at nearly all stages. Infection results in vivid green and white stripes on the leaves and heads that are partially or completely sterile. The major sources for infection are spores that survive in the soil and airborne spores coming from infected plants. High plant density and rainfall after planting encourages development of the disease. The disease is not transmitted by seeds, provided that they are properly dried and stored. Proper crop rotation is an effective cultural measure, but requires a break of at least 3 years between cultivating two sorghum or maize crops to prevent new spores being added to the soil. Resistant cultivars exist. Natural fungicide applied as seed treatment or preventive foliar spray provides further effective control.