Hazelnut powdery mildew

The disease is caused by the fungus Erysiphe corylacearum, a specialized pathogen responsible for powdery mildew on hazelnut and filbert trees. As an obligate biotroph, this fungus requires living plant tissue to complete its life cycle. It colonizes the surface of leaves, shoots, and sometimes fruits, depriving the host plant of essential nutrients and disrupting its physiological functions.

The primary host plants are species of the genus Corylus, with commercial hazelnut cultivars being particularly susceptible. The disease is widespread across hazelnut-growing regions globally. It can affect plants at any stage of development, but its impact is most significant in dense orchards where environmental conditions facilitate rapid fungal colonization during the active growth period.

Early symptoms include the appearance of white, dusty, felt-like mycelium patches on the foliage. As the infection progresses, these patches expand and cover larger areas of the leaves, stems, and buds. Heavily infected leaves may yellow, curl, or become distorted. By late summer, small, dark, spherical structures called cleistothecia develop on the white fungal mats, allowing the pathogen to survive the winter on fallen debris.

The development of Erysiphe corylacearum is favored by warm, humid conditions. Ideal temperatures for mycelial growth range from 18°C to 25°C. High humidity and poor air circulation within the canopy promote the germination of conidia and the spread of the disease. The windborne spores can travel long distances, allowing the fungus to infect previously healthy neighboring orchards or sections of the plantation.

The economic impact of powdery mildew is substantial, as it leads to defoliation and reduced photosynthesis, which directly affects nut yield and quality. Infected trees also show reduced vigor and susceptibility to cold damage in winter. Effective management requires an integrated approach including chemical control (sulfur-based fungicides, triazoles, or strobilurins), maintaining orchard hygiene by removing infected plant material, and implementing proper pruning practices to improve ventilation.