Hops growers are re-emerging throughout the country, helping to regenerate small craft beverage production, focusing on local ingredients, short supply chains and regionally-adapted varieties. Properly identifying both powdery and downy mildew, learning how to treat them, and most importantly taking preventative measures, is key to the success of local and regional hop supply chains in many of these historic growing regions.

Hop downy mildew, caused by the pathogen Pseudoperonospora humuli, is prevalent in moist, humid conditions. It’s an oomycete and once it has infected the plant, it is difficult to control. Oomycetes, often called water molds, are not related to fungi, as once believed. They are a separate classification of organism and differ from fungi by having flagella to propel them through water, and cell walls made of cellulose, not chitin. They also have differences in life cycles. Emergent symptoms of hop downy mildew can resemble nutrient deficiencies, so recognizing what you see early in the game is paramount in order to gain control over this disease, which can be devastating to hops, causing both local infections in various parts of the plant as well as systemic crown infections which reduce vigor over time, eventually killing the hop plant.

Hop powdery mildew, an infection caused by the fungus Podosphaera macularis, typically emerges in early spring, along with the new plant growth, in infected fields. The fungus, which can overwinter in the field as well as in the plants as bud infections or as sexually reproduced fungal structures, known as chasmothecia, emerges with new spring growth. Proliferation is via spores, which are released into the hop yard and land on young, highly susceptible leaves, causing secondary infections. Active from about 55 to 85 degrees F, powdery mildew does not require water. It affects cones and flowers too, and can cause significant yield and quality loss if not mitigated.

Bill Weldon, a Ph.D. student at Cornell University, recently presented some of his research on Cornell’s School of Integrative Plant Science’s YouTube channel. His presentation, “Differences in Managing the Mildews,” is geared to helping hop growers control these common diseases.


“These diseases are capable of causing some pretty drastic yield losses in your yard,” Weldon said.

Knowing what to look for is the key to effective control. The fungicides that target powdery mildew and those sprays that control downy mildew aren’t always cross-effective, and aren’t the only answer to disease control.

Basal spikes of downy mildew are chlorotic, downward curling and stunted, with short distances between leaves. The undersides of the leaves have purplish spore masses, a characteristic trait of downy mildew. The spores are only found on the leaf undersides, unlike with other pathogens, and are a “defining characteristic” of the disease, Weldon said. If downy mildew has overwintered, these basal spikes will emerge in the spring. They can be mixed in with normal shoots, and if left to mature the pathogen can progress and form on lateral branches. Downy mildew-infected lateral spikes grow higher up in the canopy and cause cone damage and yield loss.

In powdery mildew, flag shoots (the equivalent of the basal spike in downy mildew) are the first sign of overwintered infection each spring. These flag shoots are stunted and white in appearance, and the shoot “sticks out like a flag,” Weldon said. Powdery mildew spores are transparent, and these clear spore masses can be found on any green above-ground tissue. Masses of these spores appear white under the microscope, and look white and fluffy to the naked eye (or gray if they are older colonies).

Necrotic brown on the hop leaves is a response to downy mildew colonies forming on the underside of leaves. Powdery mildew, in contrast, will appear white on the leaves. While the mildews can both impact cones, it’s easier to scout for and differentiate between them on the leaves.

“The easiest way to identify these pathogens is not by differentiating how your cone looks,” Weldon said, as leaf damage is easier to use to determine which pathogen you have.

Preventing Loss

Varietal resistance for hop downy mildew is complicated. The pathogen can affect the cones, decreasing yield, or it can cause a systemic crown infection. Some hop cultivars may be resistant to either the foliar and/or systemic infection, or susceptible or resistant to both types of downy mildew infection.

“There are so many factors at play,” Weldon said, and the result is “more of a spectrum (of resistance) than a clear-cut definition.”

Varieties once thought to be resistant to powdery mildew have recently succumbed in the Pacific Northwest, where the pathogen was first found on hops about 20 years ago. Nugget and Cascade are two of these cultivars. There are different strains of powdery mildew, some of which have adapted to the climate of that region.

Weather conditions do favor the emergence of one disease over the other. Downy mildew is favored in wet, humid, rainy conditions and periods where dew points are high. Downy mildew requires leaf wetness, so keeping leaves dry is an important management tool.

Powdery mildew doesn’t require water. Overcast temperate days are most conducive to growth, as direct sunlight can damage the translucent spores.

Weather conditions, as well as varietal susceptibility, should be used to guide hop yard scouting programs, Weldon said. Scouting needs to begin as soon as shoots emerge and continue regularly throughout the growing season. At shoot emergence, the prompt identification and removal of basal spikes or flag shoots limits spore release and decreases disease prevalence.

Shoot training is another key time period for scouting, to remove growing tips infected with downy mildew. When lateral branches emerge, aerial spikes caused by downy mildew can be identified and removed, as they will not form proper cones. Cones are most susceptible to downy and powdery mildew infection from one week prior to bloom to four weeks post bloom. Infection at this time results in the most fluctuation to normal alpha acid levels, and occurs in both diseases.

Game Changer

Scouting doesn’t have to be all work and no play, however. GameTheory, a Vermont company which partners with organizations to create games for research and education, has developed a virtual reality program that teaches farmers how to scout for pests and diseases in hop yards. The idea came from University of Vermont Extension agronomist Heather Darby. Across the Fence, a TV series produced by the University of Vermont Extension, recently explored the process behind the game’s development.

Darby hoped that virtual reality could provide farmers with simulated hands-on experience in identifying pest and disease concerns so they’d be ready to do so in the hop yard. The game allows farmers to closely examine any suspected signs of virtual diseases or pests and identify what they see. The program keeps score and lets players know what they’ve missed, what they’ve misidentified and what they did correctly. From Japanese beetles to mildew, hop farmers can hone their scouting skills, and better protect their crop, by playing the game.

“Farmers work best by doing, and we know that,” Darby said. “Can we teach farmers how to scout for insect pests through virtual reality…so that they really feel like they’re doing?”

The game, still in its prototype form, is expected to be available on multiple platforms. Similar games are in the works for farmers raising other crops.