This disease is caused by the fungus Pseudoperonospora humuli. The fungus infects only the hop plant and will not affect other garden plants. The number of infected shoots may vary from none to all in any given hill. Infected shoots are stunted, brittle, and lighter in color than healthy shoots. The leaves are often deformed and curled.
Infected shoots are unable to climb.
Gray or black masses of fungal spores are often present on the underside of infected leaves. Spores are dispersed by wind and rain. Spiked shoots should be removed promptly and buried. Flowers often become infected when blooming occurs during wet weather. Young cones that are infected stop growing and turn brown.