Hop downy mildew

Paraperonospora minor is the primary pathogen responsible for hop downy mildew, a devastating disease that affects commercial hop production globally. As an obligate parasite and member of the oomycetes, it thrives by colonizing the tissues of the host plant, often causing irreversible structural damage if left untreated.

The host range is specific to the hop plant (Humulus lupulus). The pathogen attacks all aerial parts of the vine, including leaves, stems, flowers, and cones. Economic damage is severe, as infection results in distorted growth, stunted vines, and poor-quality cones that are unsuitable for brewing, leading to substantial financial losses for farmers.

Symptoms typically begin with angular, yellow-to-brown lesions on leaves, often bounded by leaf veins. Under humid conditions, a grayish-purple fungal-like growth appears on the underside of the leaves. Severely infected shoots, known as "spikes," become brittle, curled, and stunted, failing to climb the trellis properly and reducing the plant's overall photosynthetic capacity.

Environmental conditions are the driving force behind the spread of this disease. High moisture, cool to moderate temperatures, and periods of leaf wetness are ideal for the production and dispersal of zoospores. The pathogen overwinters primarily as mycelium in the rootstocks, allowing the infection to re-emerge in new spring growth year after year.

Management strategies focus on reducing inoculum pressure and protecting the crop through a multi-faceted approach:

• Selecting disease-resistant hop varieties.
• Removing and destroying basal "spikes" early in the growing season.
• Applying fungicides regularly, particularly during periods of high precipitation.
• Implementing proper canopy management to ensure air circulation and reduce humidity.
• Maintaining field hygiene by clearing debris and preventing cross-contamination from old yards.