Powdery mildew

While downy mildew is still the predominant disease affecting hops in Ontario, it’s not the only disease growers have to worry about.  Over the last week, we have had several reports of powdery mildew in commercial hop yards across the province.   Powdery mildew was first reported in Ontario in 2014 (see this blog post for that report https://onspecialtycrops.ca/2014/09/05/powdery-mildew-detected-in-ontario-hops/), however it has not been commonly reported from commercial yards until this year.  Powdery mildew was also reported earlier this season affecting commercial hop yards in Michigan.

Conditions this year in many areas of Ontario have been ideal for the spread of powdery mildew.  Optimal conditions for powdery mildew infection in hops include high humidity, reduced light levels/cloudy conditions and temperatures between 18 and 21°C (although infection can occur between 8 and 28°C).  Unlike downy mildew, powdery mildew does not require free water (wet leaves) to infect and the spores can germinate under high humidity alone.

Despite the similarity in the names of the two diseases, powdery mildew (Podosphaera macularis)  is caused by a completely different pathogen than the one causing downy mildew (Pseudoperonospora humuli), and generally requires different fungicides to control it.  See this earlier blog post for the differences between downy and powdery  mildews (https://onspecialtycrops.ca/2014/09/05/not-all-mildews-are-created-equal/).

At this time, all Ontario growers should be actively scouting for symptoms of powdery mildew and consider preventative sprays to control the disease Powdery mildew may initially appear on leaves as pale yellow spots which develop into colonies of powdery white spores.

Powdery mildew initially appears as pale yellow spots (upper leaf) that develop into colonies of white spores (lower leaf)

In contrast, downy mildew lesions produce dark purplish spores. Downy mildew lesions also sporulate only on the leaf underside, while powdery mildew will sporulate on both sides of the leaf.

Downy mildew (left) versus powdery mildew (right) spores. Powdery mildew spores are white instead of dark, may appear more “powder-like” and can be found on both the upper and lower leaf surface.

Powdery mildew will also affect the cones, which may also produce powdery white spores.

Severe powdery mildew infection of hops cones

Sporulation on cones is not always obvious. As infected cone tissue dies, it may become reddish-brown, and symptoms may be difficult to distinguish from downy mildew or Alternaria cone disorder, which is common in hops damaged by wind or other mechanical injury.  At the Simcoe station research yard, we have found that Alternaria is often restricted to the tips of the bracts, giving the cones a striped appearance, while powdery mildew is more widespread throughout the cone.

Hops cones infected with powdery mildew (left) and alternaria cone disorder (right). Note the striped appearance and lack of powdery white spores on the cones infected with Alternaria.

In Ontario, several fungicides are registered for control or suppression of powdery mildew, however some have long pre-harvest intervals and cannot be applied this close to harvest.  Pristine (boscalid + pyraclostrobin) is registered for control of powdery mildew, however in Canada this product has a 46 day pre-harvest interval, so at this time can only be applied to late season varieties that will not be harvested until September 13 or later (if the product was applied today), or to plants that are not being harvested due to downy mildew or other damage.  Quintec (quinoxyfen) and Vivando (metrafenone) are registered for suppression of powdery mildew on hops, and have shorter pre-harvest intervals (21 days for Quintec and 14 days for Vivando).  For organic growers, MilStop and Sirocco are labelled for suppression of powdery mildew and have a 0 day pre-harvest interval, however these products should be applied very early in the disease cycle.  Crop oils (Vegol and Purespray Green Oil) are not options at this time as they cannot be used after burr development.