Hop Powdery Mildew

Description

Hops are dioecious flowering plants in the Cannabaceae family and are indigenous to the temperature regions of the Northern Hemisphere. Hop cones are primarily used as a flavoring agent and preservative of beer.

Powdery mildew is caused by the fungus Podosphaera macularis and occurs in most commercial production regions around the world. The level of cone infection is directly correlated with the level of leaf infection, though even severe leaf infections do not appear to impact bine growth. Losses are associated with cone infections and the cost of controlling the disease.

Symptoms and effects

Signs of powdery mildew, as the name suggests, are powdery and glistening white colonies that may be discrete (affecting small, circular areas) or indiscrete (covering large areas of the plant).

Expression of signs and symptoms is dependent on the cultivar, weather conditions, and fertility. Infected cones and burrs show only the white fungal structures initially. Symptoms such as brown necrotic tissue develop later in the disease cycle. Cones infected early in their development are often distorted or reduced in size.

Disease cycle

The pathogen overwinters in the soil in hardy fungal survival structures or as mycelia (fungal threads) in plant buds.

Management

The most effective way to manage powdery mildew on hop is through prevention. The use of resistant cultivars and prophylactic fungicide programs, supplemented by cultural practices, is best. Early and continuous intervention throughout the growing season is critical.

Varieties resistant to Pacific-Northwestern powdery mildew include ‘Nugget’, ‘Cascade’, and ‘Mt. Hood’. Moderately resistant varieties include ‘Fuggle’, ‘Perle’, ‘Tettnang’, and ‘Hallertau’.

There are several fungicides that are effective against powdery mildew. When choosing reduced-risk, single-site mode of action fungicides, carefully follow resistance management practices, as per the label. Make few or no fungicide applications during burr development, because the burr is highly susceptible to damage. To reduce disease development during this time, remove basal growth just prior to flowering and apply a protectant fungicide with long residual actions.

Cultural practices that help prevent downy mildew include managing nutrients and water, reducing initial inoculum, and removing basal growth. Fertilizer and water inputs should be sufficient but not excessive.

Methods of removing inoculum vary depending on the overwintering method of the pathogen. Where chasmothecia (hardy survival structures) develop, removing diseased tissue from the field by cultivating the soil or burying the plant residue can be effective. If the pathogen overwinters in buds, removing buds 5-7 cm below the soil prior to shoot emergence can significantly reduce the number of flag shoots in the spring. Spring pruning that completely removes green tissue can be nearly as effective as crowning.

Applying a band of fungicide onto hills before closure can help reduce disease spread prior to training or pruning. Once the bines have grown 2.5-3.0 m up the string, remove basal growth periodically to prevent damage to the growing tip.