Powdery mildew of viburnum

The causative agent of the disease is an obligate parasite, the fungus Erysiphe hedwigii. This pathogen belongs to the Erysiphaceae family and colonizes the surface of the host plant, absorbing nutrients through specialized feeding structures known as haustoria, which penetrate the epidermal cell walls.

The primary host for this fungus is the Viburnum species, particularly Viburnum opulus. The infection typically targets young, succulent leaves, developing shoots, and flower buds, significantly hindering the plant's growth and reducing its overall aesthetic appeal in landscape plantings.

Early symptoms appear as small patches of white, powdery fungal mycelium on the leaf surfaces. As the infection progresses, the mycelial mat thickens and can cover the entire leaf area, eventually turning greyish-brown and developing small black dots, which are the fungal fruiting bodies (cleistothecia).

Optimal conditions for disease development include high humidity, moderate temperatures (ranging from +18°C to +25°C), and poor air circulation. Densely planted shrubs and excessive shade are major contributing factors that facilitate the rapid spread of conidia by wind and water splashes during the growing season.

The damage caused by the disease includes inhibited photosynthesis, stunted shoot growth, and premature leaf senescence. If left untreated, the plant becomes significantly weakened, losing its vigor and becoming more susceptible to winter frost damage, which can lead to dieback in severe cases.

• Pruning and thinning the shrub crown to enhance air movement and light penetration.
• Strict sanitation, including the removal and destruction of fallen leaves and infected plant debris.
• Application of approved fungicides, such as sulfur-based products or systemic triazole sprays.
• Avoiding excessive nitrogen fertilization, which promotes soft, vulnerable new growth.