Gnomonia leaf spot

Gnomonia leaf spot is a fungal disease caused by the pathogen Gnomonia caryae. It is a parasitic ascomycete that specifically targets trees within the Carya genus, with pecan trees being the most frequent hosts. The fungus survives the winter primarily in fallen leaves, acting as the primary source of infection in the following spring.

This disease primarily affects species of hickory and pecan. While it may appear in mature orchards, it is particularly problematic in nursery settings where higher tree density and reduced air circulation create a more favorable environment for the pathogen to establish itself and spread across the plantation.

Symptoms are manifested as brown to gray irregular spots on the leaflets. As the infection progresses, these necrotic spots enlarge and often develop a darker margin. A distinguishing feature is the appearance of small, black, pinhead-like structures known as pycnidia within the centers of these necrotic lesions.

The development and spread of Gnomonia caryae are heavily dependent on climatic factors, particularly prolonged periods of leaf wetness and moderate to warm temperatures. Spores are disseminated primarily through water splashes and wind currents, which transport them from the leaf litter to the lower canopy, starting the cycle of infection early in the season.

The economic impact of the disease is associated with premature defoliation, which hampers the tree's ability to photosynthesize effectively. This leads to reduced vigor, poor nut quality, and weakened resistance to environmental stress. Effective management relies on cultural practices, such as removing debris, alongside the judicious use of protective fungicides to manage early-season outbreaks.