Pecan scab

Pecan scab, caused by the ascomycete fungus Venturia effusa, is the most serious and economically damaging disease of pecans worldwide. This pathogen infects all green parts of the tree, including leaves, twigs, and nuts, making it a critical limiting factor for commercial pecan production in humid climates.

The fungus survives the winter as mycelium in infected shucks from the previous season and on dead twigs. In the spring, as conditions become favorable, the fungus produces conidia, which are disseminated by wind and splashing rain to susceptible new growth. Primary infection typically occurs during the early stages of bud break and leaf expansion.

Symptoms appear as small, circular, olive-to-brown lesions on the foliage and fruit. As the infection progresses, these spots enlarge and coalesce, eventually causing tissue necrosis. Severe infections lead to premature defoliation, while affected nuts may remain underdeveloped, turn black, and drop from the tree well before the harvest season.

The disease thrives in warm, humid weather. Frequent rainfall and prolonged periods of high humidity create an optimal environment for conidial production and infection. Poor orchard management, such as lack of canopy thinning or planting in high-density configurations, significantly increases the pressure of the disease by limiting airflow and promoting humidity.

Effective management requires an integrated approach. The most sustainable strategy is planting scab-resistant cultivars that minimize the need for chemical intervention. Cultural practices, including sanitation (removal of infected debris) and proper tree pruning to improve air circulation and light penetration, are essential. Fungicide applications remain the primary chemical defense and must be applied on a preventative schedule during the critical fruit-growing season.