Downy mildew of hops

Downy mildew of hops, caused by the oomycete Pseudoperonospora humuli, is a devastating disease affecting the hop industry globally. It primarily targets the hop plant (Humulus lupulus), leading to stunted growth, reduced cone quality, and in severe cases, the complete death of the plant.

The pathogen survives the winter as mycelium within the crowns and rhizomes of infected plants. As the weather warms in spring, the fungus produces spores that are spread by rain splashes and wind, initiating primary infections on newly emerged shoots, which often appear stunted and brittle.

Key symptoms include angular, chlorotic lesions on the leaves, which eventually turn necrotic and brown. A characteristic grey or purple-tinged fuzzy growth of spores is visible on the underside of the leaves in humid conditions. When the cones are affected, they become discolored and fail to develop properly, significantly reducing the yield.

The development of the disease is highly dependent on environmental conditions, specifically high humidity, rainfall, and moderate temperatures. Free moisture on the foliage is essential for spore germination and infection, making wet springs particularly hazardous for hop yards with poor drainage or restricted airflow.

Control strategies involve a combination of cultural and chemical practices. Growers should select resistant cultivars, ensure proper pruning of basal foliage to improve airflow, and sanitize equipment to prevent spore spread. Fungicide applications, including copper-based products and systemic chemistry, are crucial when environmental conditions favor the disease.