Cylindrosporium leaf spot of hops

Cylindrosporium leaf spot is a fungal disease affecting hops (Humulus lupulus), caused by the pathogen Cylindrosporium humuli. As a specialized pathogen, it primarily targets the leaf tissues of hop plants, leading to severe defoliation if left uncontrolled. This disease is a significant concern for hop growers in regions with favorable environmental conditions for fungal development.

The symptoms typically manifest as small, circular, or angular spots on the leaves, which are light brown with a darker border. As the infection progresses, these spots may coalesce and the dead leaf tissue often dries out and falls away, creating a shothole effect. This premature loss of foliage directly impacts the overall vitality of the hop bine during the critical growth stages.

The development and spread of Cylindrosporium humuli are strongly linked to high humidity, frequent rainfall, and moderate temperatures. The fungus overwinters on infected plant debris left on the ground. During the spring, spores are released and dispersed by wind and rain splashes, initiating the infection cycle on the newly emerging foliage of the host plants.

The economic impact of this disease is considerable due to the reduction in photosynthesis caused by foliage loss. This decline in plant health results in reduced yields and lower quality of hop cones, including decreased alpha-acid content, which is essential for brewing quality. Severe infestations can weaken plants, making them more susceptible to other stressors or pests.

Effective management strategies include both cultural and chemical control measures. Key practices are:

• Removal and destruction of crop residues at the end of the season.
• Deep plowing to bury infested plant material.
• Proper vine training to improve canopy airflow and reduce humidity around the leaves.
• Timely application of registered fungicides during periods of peak risk.