August 2015 Plant Disease Advisory

new growth chlorosis on hop shoot
Chlorosis of new growth resulting from systemic infection of hop shoot with downy mildew (photo courtesy J. Havill).

With an increasing number of microbreweries cropping up in Virginia, growing hops is becoming an increasingly popular endeavor. Hop yards, as they are known, are becoming a more common sight at breweries and on farms across the Commonwealth and, as a result, hop diseases are also becoming more prevalent. One of the most common diseases of hops in other hop growing areas is downy mildew, caused by the pathogen Pseudoperonospora humuli. Because this pathogen is favored by conditions of high moisture and humidity, it is likely to be an important disease in Virginia as well.

Already several samples sent to the Virginia Tech Plant Disease Clinic this year have been confirmed with hop downy mildew. A variety of symptoms are associated with downy mildew on hops. If plants are systemically infected, shoots that emerge from crowns may be yellow, with shortened internodes and downward-curling leaves. These symptoms could easily be confused with nutrient deficiency. Aerial spikes may also develop chlorosis and later they may desiccate and fall away from the string to which they are trained. Leaf infections result in localized chlorotic or necrotic lesions delimited by veins. Sporangia of the pathogen may develop on lower surfaces of the spots, forming a dark, downy growth. 

Once symptoms develop, the bacterial disease is typically difficult to control. Cultural controls that minimize periods of leaf wetness should be practiced to avoid conditions favorable for development of bacterial leaf blight: increase plant spacing and avoid overhead irrigation or at least water early in the day so that foliage dries relatively quickly. It is also important to adequately ventilate and heat the greenhouse to promote foliar drying. The bacteria are spread by splashing water and driving rain, so minimize water splash when possible. Chemical control with appropriately labeled copper or other bactericidal products can be used to help manage the disease, but will not be effective on plants severely affected and/or under conditions very favorable for spread and infection by the bacteria.

angular spots on hop leaf
Angular spots on hop leaf caused by downy mildew (photo courtesy T.M. Likins).

The pathogen also infects the cones and can cause browning of individual bracts or entire cones later in the season. Crown infection results in reddish brown to black flecks or streaks under the bark. Cultivars vary in their susceptibility to downy mildew. Downy mildew-resistant cultivars that should be considered for new plantings include Cascade, Fuggle, Magnum, Newport, and Perle. The cultivars Cluster, East Kent Golding, Tolhurst, and Vanguard are susceptible not only to downy mildew, but also to powdery mildew, and should be avoided. 

To reduce chances of planting infected rhizomes, it is best to purchase certified disease-free rhizomes for new plantings. Avoiding overhead irrigation and spacing plants adequately will reduce leaf wetness and humidity in the canopy, which favor the disease. Following recommended pruning practices is also important for downy mildew control. In addition to these cultural practices, a variety of fungicides are registered for downy mildew control in hops; however, care must be taken to rotate fungicides with different chemistries to avoid development of fungicide resistance in the pathogen. Detailed recommendations on cultural and chemical controls for hops downy mildew are available in the Virginia Pest Management Guide for Horticultural and Forest Crops.