Hop Downy Mildew

Description

Hops are dioecious flowering plants in the Cannabaceae family and are indigenous to the temperate regions of the Northern Hemisphere. The primary use of hop cones is as a flavoring and preservative of beer.

Downy mildew is a serious disease of hop caused by the fungus-like organism Pseudoperonospora humuli. Damage is dependent upon cultivar susceptibility and weather conditions during the growing season, but severe infection can lead to a complete loss in marketable yield.

Symptoms and effects

Symptoms of downy mildew first appear in the spring as infected shoots, often called “basal spikes”. These spikes are stunted and show chlorotic, yellow-green leaves that are brittle and curl downward.

Lesions on the leaves appear angular and water soaked and are confined within leaf veins. Reproductive structures may form as a purple-gray to black downy, fuzzy growth. Infected cones become brown, shriveled, and dried-up.

Disease cycle

This pathogen overwinters within hop buds and crowns and moves up the plant during the growing season, infecting shoots, leaves, and cones.

Management

Hop downy mildew can be managed by integrating cultivar selection, sanitation practices, appropriate harvest timing, and timely applications of fungicides. Reducing the amount of time foliage is wet can help minimize the disease. Overhead irrigation should be avoided.

There are no hop cultivars that are completely immune to downy mildew. Moderately resistant cultivars include ‘Cascade’, ‘Fuggle’, ‘Perle’, ‘Tettnanger’, and ‘Willamette’. More susceptible cultivars include ‘Custer’, ‘Galena’, and ‘Nugget’.

Selecting disease-free rhizome for planting is extremely important for reducing downy mildew within the hop yard. Heavily diseased plants should be removed early and the primary basal spikes eliminated. Stripping the foliage from the bottom of the plant after training can reduce inoculum density and prevent disease spread into the canopy. Fungicides should be applied prior or soon after infection, and forecasting systems are in development to aid in fungicide timing decisions.