Cercospora leaf spot of viburnum

The causal agent of Cercospora leaf spot on viburnum is the phytopathogenic fungus Pseudocercospora viburnigena. This fungal disease is a common problem for various Viburnum species, primarily manifesting as characteristic leaf lesions that negatively impact the health and aesthetic value of the ornamental shrubs.

The disease primarily targets the foliage of viburnum plants. While it most commonly affects the leaves, under favorable conditions, the infection can also involve petioles and young, non-lignified stems. Severe infestations can lead to partial or total defoliation, significantly stressing the plant during the growing season.

Symptoms typically present as distinct circular or angular spots on the leaves. These spots are initially small and brownish but can enlarge and merge, developing a darker border. Under high humidity, the centers of these spots may produce fungal spores, appearing as tiny dark specks. Eventually, the affected leaf tissue turns yellow and dries out, causing premature leaf drop.

The development of Pseudocercospora viburnigena is heavily dependent on moisture and moderate temperatures. The fungus overwinters in fallen foliage and infected plant debris. During the spring and summer, spores are spread through rain splash and wind, facilitating the rapid colonization of new leaves in humid or overcrowded conditions.

The damage caused by this disease is mainly related to reduced photosynthesis and the resulting weakening of the plant's overall vigor. Implementing an integrated protection strategy is crucial for maintaining plant health:

• Sanitation: removing and disposing of fallen leaves to reduce the primary inoculum load.
• Pruning: maintaining proper spacing and thinning the crown to improve airflow and reduce leaf wetness duration.
• Chemical control: applying copper-based fungicides or systemic treatments during the spring if the history of the disease exists in the area.