Ash powdery mildew

The disease is caused by the fungus Phyllactinia fraxini, an obligate biotrophic parasite that belongs to the Erysiphaceae family. Unlike many other powdery mildews, it typically colonizes the abaxial (underside) surface of the leaves, where it develops its mycelial network. The survival of this pathogen during winter is guaranteed by cleistothecia, which form on the leaf surface and are adapted to withstand harsh environmental conditions.

The primary host for this pathogen is the European ash (Fraxinus excelsior) and several related species. The disease is predominantly observed in nursery settings, urban parks, and woodland areas where high plant density and specific microclimatic conditions facilitate the persistence of the fungal inoculum from year to year.

Diagnostic signs begin with a white, powder-like fungal growth appearing on the underside of leaves. As the infection progresses, the foliage may exhibit chlorosis, curling, and early senescence. Close inspection reveals small, dark, spherical bodies known as cleistothecia embedded within the mycelial mat, which are the reproductive structures of the fungus.

Epidemiological development is heavily influenced by high humidity and moderate temperatures during the summer months. The pathogen disperses its conidia via wind currents, allowing for rapid colonization of susceptible host tissues. Poor air circulation caused by overcrowded plantings is a significant environmental risk factor that promotes the rapid spread of the disease.

Management of Phyllactinia fraxini involves both preventative and curative strategies. Sanitation is the most critical step, requiring the removal and destruction of fallen leaves to eliminate primary inoculum sources. In highly valuable collections or nursery stock, the application of systemic fungicides or sulfur-based treatments during the early stages of infestation can effectively mitigate the impact on tree vitality and growth.