Maize Ascochyta blight

Maize Ascochyta blight is caused by the fungus Ascochyta maydis, which belongs to the Sphaeropsidales order. It is a known foliar pathogen that overwinters as pycnidia in infested crop residues or infected seeds.

The fungus is identified by the formation of pycnidia, which are small, flask-shaped, dark-colored fruiting bodies embedded in host tissue. These structures release conidia, which serve as the primary inoculum for disease development.

Under laboratory conditions, microscopic analysis reveals the presence of hyaline, typically two-celled conidia. These spores are disseminated by rain splash and wind, facilitating the rapid spread of the disease during the growing season.

The biological cycle starts when favorable conditions (high humidity and moderate temperatures) trigger the release of conidia. These spores penetrate the maize tissue through stomata or wounds, initiating the infection process.

Correct identification is crucial as it helps differentiate Ascochyta blight from other common leaf spots. Diagnostic focus should remain on the presence of pycnidia in the necrotic tissue.

The primary host for Ascochyta maydis is maize. The pathogen targets leaves, causing significant damage to the plant's photosynthetic capacity, which is essential for successful grain filling and overall development.

Infection leads to premature leaf senescence and death. If the weather remains humid, the pathogen may spread to the leaf sheaths and occasionally the husks, compromising the health of the entire plant.

The impact of the disease is most severe when infection occurs early in the season. Reduced leaf area leads to lower dry matter accumulation, resulting in smaller, lighter kernels and reduced crop yield.

Crop losses are highly variable and dependent on weather conditions and the susceptibility of the hybrid. Severe epidemics can significantly reduce biomass production in both grain and silage corn varieties.

Beyond yield loss, the disease weakens the plant's structural integrity, potentially making it more susceptible to secondary stalk rot pathogens, which further complicate the harvest process.

The first symptoms appear as small, light-brown or yellowish spots on the leaf surface. These spots are often oval or elongated and expand as the disease progresses under favorable conditions.

Over time, the centers of the lesions turn greyish or white, while the edges remain dark. The most distinctive feature is the appearance of tiny black dots (pycnidia) arranged within the center of the necrotic spots.

Lesions often coalesce to cover large areas of the leaf blade, resulting in extensive necrosis. Affected leaves may wither, curl, and die prematurely, starting from the lower canopy and moving upward.

High relative humidity and frequent rainfall encourage the production of spores on the surface of the lesions, often presenting as a greyish, dusty layer on the necrotic tissue.

• Light-brown elongated leaf spots;
• Greyish necrotic centers with black pycnidia;
• Lesion coalescing;
• Premature leaf death and drying.

Integrated pest management starts with cultural practices. Crop rotation is highly effective, as it prevents the pathogen from persisting in the field and breaking the cycle of infection.

Deep tillage helps bury infected crop debris, which is the primary source of the fungus. Incorporating the residues into the soil speeds up their decomposition and eliminates the primary inoculum source.

The use of resistant or tolerant maize hybrids remains the most sustainable approach to control. Selecting varieties adapted to the local environment can significantly mitigate the impact of fungal diseases.

Seed treatments with systemic fungicides are recommended to protect the germinating seedlings. This ensures early protection and helps the plants establish a strong, healthy root and shoot system.

Foliar fungicides may be applied during the growing season if weather conditions favor disease development. Timely application, specifically at the first sign of symptoms, is critical to prevent widespread infection.