Zonate leaf spot

Zonate leaf spot is a fungal disease caused by the pathogen Gloeocercospora sorghi. It is a significant foliar pathogen known for creating distinct zonate patterns on the foliage of its hosts. The disease primarily affects plants in the Poaceae family, posing a challenge to crop health in tropical and temperate agricultural zones.

The primary host for this fungus is sorghum, including grain sorghum, forage sorghum, and sudangrass. While the disease is most commonly identified on leaves and leaf sheaths, severe infections can extend to the stalks and, in some cases, the panicles. This leads to substantial physiological stress for the crop throughout its developmental stages.

Symptoms are identified by circular to irregular lesions that exhibit a characteristic "zonate" appearance, consisting of alternating bands of reddish-purple, tan, or brown tissue. Under high humidity conditions, the centers of these lesions may develop a distinct pinkish mass of conidia. These lesions eventually enlarge and merge, resulting in large, necrotic patches on the leaves.

Environmental conditions play a critical role in the disease cycle, as the fungus thrives in warm temperatures (22°C to 28°C) and high moisture levels, such as those provided by heavy rainfall or prolonged dew. The fungus overwinters in soil as sclerotia or in crop debris. During the growing season, conidia are dispersed by wind-driven rain and splashing water to healthy tissue.

The economic impact of the disease is significant, as extensive leaf necrosis impairs the plant's photosynthetic capacity, resulting in reduced grain filling and poor biomass quality. Effective management strategies involve:

• implementing strict crop rotation cycles;
• thorough incorporation of infested residue into the soil;
• utilizing resistant or tolerant hybrids;
• timely application of registered foliar fungicides if disease pressure is high.