Walnut anthracnose

Gnomonia leptostyla (syn. Ophiognomonia leptostyla) is a destructive fungal pathogen primarily affecting Juglans regia (Persian walnut). In its asexual state, it is known as Marssonina juglandis. It is responsible for walnut anthracnose, a disease that significantly impacts nut production worldwide by damaging foliage and fruit quality.

The characteristic symptoms include small, irregular dark-brown spots on leaves, which may enlarge and merge, leading to premature leaf drop. On the fruit, the lesions appear as depressed, dark, circular spots on the husk. If the infection occurs early in the season, it can result in shriveled kernels and complete loss of the commercial crop.

The life cycle of the pathogen begins in the spring when ascospores are released from overwintered infected fallen leaves. These spores are dispersed by wind and rain splashes to the newly emerging shoots and leaves. The fungus completes its cycle by producing conidia throughout the growing season, which facilitate secondary infections across the canopy.

Environmental conditions play a critical role in disease spread. The fungus thrives in warm and humid climates, with rainfall being the most important factor for spore dissemination and germination. Prolonged leaf wetness periods are essential for the infection process, making rainy seasons particularly favorable for severe outbreaks of anthracnose.

Management strategies focus on reducing the primary inoculum and protecting the trees during sensitive growth stages:

• Sanitation, involving the removal or destruction of infected fallen leaves to reduce overwintering sites.
• Pruning to increase airflow and sunlight penetration within the tree canopy.
• Timely application of protective fungicides, such as copper-based products or systemic fungicides applied before major rainfall events.
• Selecting resistant or less susceptible walnut cultivars suitable for the local climate.