Hop powdery mildew

Taxonomic classification and pathogen type: Podosphaera macularis is classified within the kingdom Fungi, phylum Ascomycota, class Leotiomycetes, order Erysiphales, and family Erysiphaceae. It is an obligate biotrophic fungus, meaning it requires living host tissue to complete its life cycle. It is a highly specialized pathogen that primarily targets species within the Cannabaceae family, most notably common hop (Humulus lupulus).

Diseases and crops: The fungus is the causative agent of powdery mildew of hops. It affects virtually all aerial portions of the plant, including leaves, petioles, stems, and specifically hop cones. The infection is characterized by the appearance of white, powdery spots composed of fungal mycelium and asexual conidia. As the disease progresses, it can cause severe chlorosis, tissue distortion, and stunted development of the cones.

Biology and life cycle: The pathogen's life cycle involves both asexual and sexual stages. Podosphaera macularis overwinters primarily in the form of cleistothecia in plant debris or as mycelium in dormant buds. In the spring, spores are released to initiate primary infections. Throughout the growing season, the fungus produces massive amounts of conidia, which are dispersed by wind, allowing for rapid secondary infections across the hop yard.

Development conditions and impact: Powdery mildew thrives in moderate temperatures ranging between 18°C and 25°C, coupled with high humidity levels. The economic impact of this fungus is significant; infection can lead to substantial yield losses and render hop cones commercially worthless due to off-flavors and poor quality attributes. Heavy infestations can weaken the plants, affecting their longevity and future productivity.

Control and management measures: Effective management of hop powdery mildew requires an integrated pest management (IPM) approach:

• Selection of resistant or tolerant hop cultivars.
• Strict sanitation practices, including the removal and destruction of crop residues after harvest.
• Improving air circulation within the canopy through proper spacing and canopy management.
• Timely application of protective and systemic fungicides, including sulfur-based products, strobilurins, and DMIs.
• Frequent field scouting to identify infection centers before they become widespread.