Phyllosticta leaf spot of hop

Phyllosticta leaf spot is a fungal disease affecting hops (Humulus lupulus), caused by the pathogen Phyllosticta humuli. This fungus belongs to the order Sphaeropsidales and acts as a parasitic agent, primarily targeting the foliage, leading to necrosis and reduced vigor of the hop vines during the critical growth stages.

The infection manifests as small, angular or circular spots on the leaf blades. These lesions typically appear as light brown or greyish centers surrounded by a dark, distinct margin. As the infection progresses, the necrotic tissue may crack, causing the leaves to look shredded. Tiny, black pycnidia, which are the asexual fruiting bodies of the fungus, can be observed within the centers of these necrotic spots.

The life cycle of Phyllosticta humuli is characterized by overwintering as pycnidia or mycelium in dead plant debris on the ground. When environmental conditions become favorable in the spring, the fungus releases pycnidiospores. These spores are disseminated primarily by splashing rain, wind currents, and mechanical contact, facilitating the primary infection of new foliage.

The disease thrives in warm, humid conditions, particularly when leaf wetness persists for extended periods. Dense hop canopies that limit airflow and maintain high humidity create an ideal microclimate for the fungus to multiply. Consequently, severe outbreaks are more common in seasons with frequent rainfall and insufficient canopy management.

Effective management strategies for controlling Phyllosticta leaf spot include the following practices:

• Sanitation, including the removal and destruction of infected leaf litter to reduce the primary inoculum load.
• Optimizing hop training and pruning to ensure adequate sunlight penetration and airflow within the trellis system.
• Application of protective fungicides during periods of high humidity if disease pressure warrants chemical intervention.