Description and life history
Hop downy mildew is caused by the organism Pseudoperonospora humuli, a fungus related to the causal agents of hop black root and potato late blight. Like the powdery mildew fungus, hop downy mildew is an obligate parasite and can live and reproduce only in living host tissue. Cultivated hop, Humulus lupulus is its only host. Even the closely related annual or Japanese hop, H. japonicus, appears immune.

The fungus overwinters either as bud infections or as a systemically infected crown. In the spring, infected shoots, called primary spikes, emerge from the crown. Primary spikes are stunted, pale-green to yellow, upright, and brittle with downward cupped leaves.

Under cool, moist, conditions, the stem and lower leaf surfaces may bear masses of gray to black spores. These spores (zoosporangia) detach in response to mechanical agitation, raindrop splashes, and changes in relative humidity, and may be borne by the wind or by water droplets to healthy hop tissues. In the presence of free water, zoosporangia germinate releasing zoospores that swim until they reach a leaf pore (stomata), where they encyst and develop a germ tube that grows within the leaf and initiates an infection. The infection process, from zoosporangial germination to infection of the internal leaf cells, requires 2 to 3 hours at 70F. Zoosprangia will die within two days if environmental conditions do not favor germination.

Sporangia landing on buds may initiate systemic infection of axillary or terminal buds. These infected buds may in turn produce secondary spikes, which look much like primary spikes but, arise from healthy vines instead of infected crowns and usually are borne higher on the plant. Secondary infections are uncommon in the normally hot, dry climate in southern Idaho. (back to top)

Damage to Hop
Losses due to downy mildew occur at several points in the disease cycle. Crown infections can result in crown rot and plant death. Bud infections do not cause plant death, but do contribute to poor plant vigor. Vine infections reduce vine vigor and may spike the growing point necessitating retraining and increasing labor costs. Flower and cone infections directly reduce marketable yield, but are uncommon in the hop growing area of southern Idaho. (back to top)

Downy Mildew Management
Downy mildew thrives in environments with moderate temperatures, high humidity, and frequent precipitation. Whenever possible, resistant varieties should be planted in fields known to have conditions favoring disease development. Cultural practices that increase air movement, decreases relative humidity, and increases summer temperatures will also help control downy mildew. When conditions favoring disease development prevail, cultural practices and plant resistance may fail to provide adequate control. Under these conditions chemical fungicides are available for downy mildew control.