Downy mildew of hop

Downy mildew, caused by the oomycete Pseudoperonospora humuli, is arguably the most destructive disease affecting hop (Humulus lupulus) production worldwide. It is a highly specialized pathogen that thrives in humid climates, capable of causing devastating yield losses if left uncontrolled.

The primary symptom, often seen in early spring, is the emergence of 'spikes' — brittle, stunted, and distorted shoots covered with gray-black fungal spores. As the season progresses, the infection manifests as angular, chlorotic lesions on the leaves, which eventually turn necrotic and brown. Flower clusters and developing cones can also become infected, leading to total necrosis.

The disease cycle is heavily influenced by environmental conditions, particularly rainfall and temperature. Moisture is essential for the production and dispersal of zoospores, which spread the disease across the hop yard. The fungus survives the winter primarily as mycelium within the crowns (rhizomes) of the hop plants, providing a recurring source of infection each year.

The impact of this disease on hop quality is severe. Beyond direct yield reduction, the infection interferes with the physiological development of the plant, leading to poor cone formation and lower alpha-acid content. Severe outbreaks can weaken the rhizomes so significantly that the plants die during the winter, necessitating costly replanting.

Effective management requires an integrated approach to maintain plant health and suppress pathogen populations:

• Strict sanitation protocols, including the removal and destruction of all infected basal spikes.
• Selection of resistant or tolerant hop varieties suitable for the local climate.
• Cultural practices that promote canopy airflow, such as appropriate spacing and stringing techniques.
• A preventative chemical control program utilizing copper-based protectants and systemic fungicides timed according to weather forecasts.