Symptoms will vary slightly depending on the strength of the pathogen, the plant variety and the environmental conditions. Tan, diamond-shaped to elongated lesions with brownish margins appear first on lower leaves and then slowly move up to younger foliage. Lesions are of different sizes and they extend beyond the leaf veins. In susceptible plants, lesions may coalesce which results in complete blight of large parts of the leaves. Cobs can also show grayish covering and malformations at later stages of the disease. The loss of productivity due to leaf damage can lead to withered plants with broken stalks. Lodging can ensue.
The disease is caused by the fungus Cochliobolus heterostrophu (also known as Bipolaris maydis). The fungus survives in plant residues in the soil. When the conditions are favorable, it produces spores that are distributed to new plants by wind and rain splashes. It germinates on leaves and can complete its life cycle (from infection to new spore production) within 72 hours. The development of the fungus and the infection process are favored by moist weather, leaf wetness and temperatures ranging from 22 to 30°C. The damage on leaves reduces plant productivity and can lower the yield if the infection occurs early in the season.
Biocontrol with the competitive fungus Trichoderma atroviride SG3403 was used successfully against pathogen infection. However, field trials still need to be carried out to show the efficacy of this treatment in farms.
Always consider an integrated approach with preventive measures together with biological control measures if available. Fungicide application can effectively control the disease when applied at the right time. Consider an application only after weighting the development of the disease against the potential yield loss, the weather forecast and the growth stage of the plant. Any fast-acting, broad spectrum product is recommended, for example mancozeb (2.5 g/l) at 8-10 days interval.