Powdery mildew of spirea

Pathogen and disease type. The disease is caused by the obligate biotrophic fungus Podosphaera spiraeae. As a powdery mildew pathogen, it primarily colonizes the host plant's surface, extracting nutrients through specialized structures called haustoria, which penetrate the epidermal cells of leaves and tender stems.

Host range. The primary hosts are various species and cultivars of Spiraea. While the fungus is specialized, it can be problematic in nurseries and landscape environments where susceptible varieties are grown in close proximity, leading to rapid cross-contamination within the genus.

Symptoms and signs. The characteristic sign of the infection is the appearance of white, mycelial fungal mats on the foliage. These patches expand and develop a dusty, flour-like appearance. Common symptoms include curling or distorted leaves, yellowing of foliage, stunted growth of shoots, and early defoliation if the infestation remains unchecked.

Environmental conditions. Podosphaera spiraeae thrives in warm, humid environments with poor air circulation. High humidity during summer days followed by cool nights promotes spore dispersal and infection. Dense foliage or crowded plantings create microclimates that are ideal for the rapid proliferation of the fungal pathogen.

Impact and control measures. The disease significantly reduces the ornamental value of the shrubs and compromises plant vigor. Effective management involves cultural practices such as pruning for air circulation and avoiding overhead irrigation. Chemical control using systemic fungicides, such as myclobutanil or propiconazole, is highly effective when applied preventively or upon early detection of the white mildew signs.