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Powdery mildew of hops
Podosphaera macularis (= Sphaerotheca humuli)
Synonyms
 
None

Plant hosts
 
Humulus species, including H. lupulus (commercially grown hop) 

Symptoms
 
In the spring, new shoots infected with P. macularis may appear entirely white. These shoots produce conidia (fungal spores) that initiate secondary infections on susceptible leaves. The secondary infections appear as whitish, powdery spots on either the upper or lower leaf surface. The spots appear within 5- to 10-days of initial infection when conidia begin to form and may cover the entire leaf surface. Young leaves are more susceptible to infection than mature leaves. Flowers and cones may also be infected; growth stops at the point of infection. Because of this, infected hop cones may appear stunted or malformed. These cones also tend to mature rapidly, eventually leading to uneven crop maturity and cone shatter.
 
Symptomatic hop leaf
 
Symptomatic hop leaf 

Transmission
 
Conidia are spread naturally by wind currents. The disease may also be spread through the movement of infected plant material.

Geographic distribution
 
USA (California, Connecticut, Idaho, Minnesota, New Mexico, New York, Oregon, Washington, and Wisconsin), Canada (Ontario), Asia, Europe, South Africa, Argentina, and Brazil. 

Applicable regulations
 
OAR 603-052-1020, Quarantine against powdery mildew of hops. 
 
Return to plant pathogen list.