Hop powdery mildew

The fungus Podosphaera macularis is the primary pathogen responsible for powdery mildew in hops. As an obligate biotroph, it requires living host tissue to grow and reproduce, targeting the common hop (Humulus lupulus). This disease is globally recognized as one of the most destructive threats to commercial hop production due to its rapid spread and direct impact on quality.

The fungus overwinters primarily as mycelium within hop crowns or as dormant cleistothecia on infected leaf debris. When spring temperatures rise, primary inoculum is released. Secondary spread occurs throughout the growing season via conidia, which are easily disseminated by wind currents, allowing the disease to progress quickly across large fields under favorable conditions.

Symptoms are characterized by the appearance of white, powdery colonies of fungal mycelium and spores on leaf surfaces, petioles, stems, and lateral branches. If the infection reaches the hop cones, they become stunted, distorted, and develop a dull, brown appearance. This results in severe downgrading of the product due to the loss of essential oils and the development of off-flavors.

Environmental factors such as moderate temperatures between 18°C and 25°C are ideal for the pathogen. High humidity promotes colony development, although, unlike many other downy mildews, this pathogen does not strictly require free water to infect plants. Dense canopy structures and poor airflow contribute significantly to the severity of outbreaks within the hop yard.

Effective management requires an integrated approach to minimize fungal pressure. Strategies include:

• Planting resistant or tolerant hop cultivars.
• Managing crop residue and crown sanitation to reduce primary inoculum.
• Ensuring proper spacing and trellising to enhance air circulation.
• Applying prophylactic or curative fungicides at critical growth stages, rotating active ingredients to prevent resistance.