Pseudocercospora leaf spot of hop

Pathogen and disease type. The leaf spot disease in hops is caused by the fungus Pseudocercospora humuli. This pathogen is a leaf-infecting fungus that belongs to the group of cercospora-like species, which are known to cause significant damage to the photosynthetic capacity of host plants.

Affected crops. The disease primarily affects the common hop (Humulus lupulus). It is a specialized pathogen that targets wild and cultivated hop yards, posing a risk to crop yields and the overall vitality of the plant biomass during the growing season.

Symptoms and signs. The infection starts as small, angular spots on the leaves, which often turn brown or greyish-brown as the tissue becomes necrotic. Under high humidity, a velvety dark olive-colored growth—consisting of the fungus's conidiophores and conidia—appears on the underside of the infected leaves.

Conditions for development. The pathogen thrives in warm and humid environments. Conidia are spread by wind, rain splashes, and insects, facilitating rapid secondary infections. The fungus overwinters in plant debris, serving as a primary source of inoculum for the next season’s hop growth.

Damage and control measures. The disease leads to premature defoliation, which stunts the plant's growth and reduces the quality and volume of hop cones. To manage and prevent the spread of the pathogen, growers should:

• Remove and destroy all infected plant residues at the end of the season to reduce inoculum.
• Maintain proper spacing and training to ensure good airflow within the hop yard.
• Apply appropriate fungicides during the early stages of disease development.
• Monitor the crop regularly to detect early signs of leaf spot before it spreads extensively.