Pseudocercospora leaf spot of Combretum

Pseudocercospora leaf spot is a fungal disease caused by the pathogen Pseudocercospora combretacearum. This microorganism is a specialized plant pathogen that primarily attacks members of the Combretaceae family, particularly species within the Combretum genus. It is classified as an ascomycete fungus and is recognized for causing significant aesthetic and physiological damage to host plants in tropical and temperate landscapes.

The primary hosts for this pathogen are various Combretum species, which are widely valued for their ornamental properties. The fungus infects the leaves, leading to chlorosis, necrosis, and, in severe cases, defoliation. The disease is particularly problematic in nurseries where environmental conditions are often manipulated, creating an ideal microclimate for fungal development and spread.

The visual symptoms of the infection appear as distinct leaf spots. Initially, these spots are small and chlorotic, eventually turning brown or dark grey with a well-defined margin. Under conditions of high relative humidity, a greyish or olive-colored mycelial growth, representing the conidiophores and conidia of the fungus, can be observed on the undersides of the affected leaves. This sporulation serves as the primary source of secondary infection.

Disease development is heavily influenced by environmental factors. High humidity, frequent rainfall, and moderate temperatures are the primary drivers of Pseudocercospora combretacearum proliferation. The spores are easily disseminated by wind currents and water splashes from rain or overhead irrigation, allowing the pathogen to move rapidly through a planting site if proper sanitation is not maintained.

The economic and aesthetic impact of this disease is significant, as severe leaf drop can compromise the plant's health and lead to stunted growth. Effective disease management requires a multifaceted approach to break the infection cycle:

• Strict sanitation protocols including the removal and destruction of infected leaf litter.
• Pruning for improved air circulation to keep the canopy dry.
• Application of protective fungicides, such as copper-based sprays, during the early stages of the growing season.
• Avoidance of overhead irrigation to minimize leaf wetness duration.