Disease Affecting Hop Crops: Downy Mildew
In the world of hop cultivation, downy mildew is a common disease that can cause significant damage. This article provides a comprehensive guide on how to manage downy mildew, focusing on both conventional and organic methods.
The primary method to manage downy mildew in susceptible and some moderately resistant cultivars is the season-long application of fungicides. For conventional growers, effective chemical control options include fungicides containing Group 4 active ingredients, such as Vireo WDG. It's essential to follow resistance management strategies, like rotating fungicide groups and tank mixing with fungicides of different modes of action, to avoid resistance buildup. PFS KP 46 is another chemical that targets downy mildew and related diseases, suitable for hop disease management.
However, for backyard and organic growers, synthetic fungicides may be limited due to restrictions. In such cases, biological and cultural control methods become crucial. Selecting hop varieties with natural resistance to downy mildew, such as Hall Gold, Hall Magnum, Hall Tradition, New Port, Perle, Spalter, and some others, can significantly reduce disease incidence. Cultural practices like improving air circulation, avoiding excessive moisture, and removing infected plant material also help reduce mildew development.
Organic growers can use approved biological fungicides or biopesticides derived from natural microbial agents or plant extracts. However, specific names and efficacy depend on local organic certification guidelines and are not detailed in this article. Consulting local cooperative extension services or organic growers' networks can provide tailored recommendations.
The disease can move systemically into the basal spikes in the spring. Spores on the underside of leaves are visible during the growing season, as shown in Figures 2B and 2C. Aerial spikes with downy mildew spores on the underside of the leaves are visible in Figure 3. Spores are wind dispersed and can initiate new infections on new leaves, aerial spikes, flowers, and cones.
Downy mildew symptoms include angular chlorotic (yellow) spots on the upper side of the leaves, which eventually turn brown and have a crusty appearance. Infected cones begin to turn brownish grey at the base of the bracteoles. Diseased spikes are stunted, pale green, and brittle, and infected flower clusters shrivel, turn brown, and dry out.
To prevent the spread of downy mildew, bines should be trained early to prevent them from coming into contact with the soil, and diseased aerial spikes should be removed and destroyed. In the spring, basal foliage should be cut off of the crown, removed from the hop yard, and destroyed.
For backyard and organic growers, it's advisable to contact local extension services knowledgeable about organic disease management in hops for more tailored advice. When purchasing rhizomes or starter plants, it's essential to choose a reputable propagator or nursery that uses best propagation, sanitation, and other integrated disease management strategies.
In summary, the ideal method for controlling hops downy mildew is selecting cultivars with resistance to downy mildew. Employ cultural controls such as good spacing, pruning, and sanitation. Apply biological fungicides or biopesticides approved for organic use (seek local guidance). For conventional chemical control, employ Group 4 fungicides like Vireo WDG, with appropriate resistance management. With these practices, hop growers can effectively manage downy mildew and maintain a healthy hop yard.
