From Alison Grant, OMAFRA/University of Guelph Undergraduate Student Experiential Learning Program Student and Melanie Filotas, OMAFRA IPM Specialist:
While downy mildew is likely the most problematic disease of hops in Ontario, growers should be aware that another well known pathogen of hops, powdery mildew, can also occur here. Powdery mildew was observed in the last few weeks on hops in the Simcoe area. Symptoms were quite isolated and harvest was almost complete, thus it was not likely to have a significant impact on cones this year. However, it is still important for Ontario growers to be aware that this disease can occur in their crops, and to be able to distinguish between it and downy mildew, as they have different management strategies.
Powdery mildew of hops is caused by the fungus Podosphaera macularis. It first appears on the tops or undersides of the leaves, and can spread to the bines and any other green tissue present. It produces white, fuzzy, ‘powdery’-like lesions, often roughly circular in shape.
Around 1-3 days prior to the white colonies appearing on the surface, the young leaves may possess raised cankers or blisters.
Powdery mildew is favoured by high fertility (excessive nitrogen), as well as moist environments and shade. Unlike downy mildew, the powdery mildew pathogen does not need free water to infect the plants and spores can germinate under high humidity alone, which is why powdery mildew can be a bigger problem in dry climates. Once the white fungal structures have subsided, the infected tissue then becomes brown and necrotic. If the cones are affected by powdery mildew, the growth will likely be stunted and cones may not develop fully.
Powdery mildew mycelia overwinter either in the buds, soil, or in nearby plant residue. In order to sporulate, the temperature must exceed 10oC and humidity high. The optimal infection temperature is 18-21oC. If the plant is affected in the spring, a flag shoot with stunted leaves is produced and the white sporulation and growth of mycelia occurs immediately. Like downy mildew, infected plants may also produce uninfected shoots. The spores are dispersed through the air by the wind and are transported to neighboring plant material. There can be up to 40 generations produced over one growing season in areas where powdery mildew is more prominent, mainly in the Pacific North West.
Powdery mildew may be confused with downy mildew, as powdery mildew can also infect the undersides of the leaves, causing yellow (chlorotic) spots on the tops of the leaves, resembling the symptoms of downy mildew. However, powdery mildew will not be exclusively located on the underside of the leaf and the typical white growth should be present on the upper surface of some of the leaves as well. The important difference to note is that powdery mildew produces white colonies and downy mildew possesses purplish-black sporulation under the right conditions (moist and dark environments).
One way to try to distinguish between the two is to leave the suspected leaves (or any other plant material, such as the cones) in a plastic bag with a moist paper towel at room temperature overnight. If black sporulation appears on the lower surface, the plant is likely infected with downy mildew, and if the growth is white it may be powdery mildew.
Powdery mildew has arrived so late in the season it is unlikely to have a significant impact on yields or cone quality. However it is worth inspecting late harvested varieties for symptoms of this disease.