Powdery mildew, along with downy mildew is the most common and vital pathogen of grapevines worldwide. E.necator is an obligate biotrophic fungus, which means that it needs healthy tissues to complete its life cycle. It is an ectoparasitic fungus, which means that the fungus grows only on the leaf surface. Pathogen spores can be dispersed via wind and rain water. If it’s not treated efficiently, it can destroy the entire crop. The pathogen can survive in volunteer plants in the field or healthy parts of the host as mycelium and conidia. It can survive, also, in dead plant debris. In warm environments, it can survive as mycelium and conidia in green shoots remaining on the grapevine. The symptoms of the disease can progressively reduce the quality and quantity of crop production. It infects all the green parts of grapevine. Lateral shoots are very susceptible. Pathogen’s spores (conidia) can germinate even without humid conditions on the leaf surface. Primary infections take place in spring as the infected nodes sprouting. The European Vitis varieties are more susceptible to powdery mildew than the American ones.
Leaves: Yellowish irregular oil spots/lesions on the upper leaf surface that eventually turn brown due to the necrosis of the infected plant tissue. In optimal conditions, a whitish, powdery fungal sporulation growth evolves on the lesions. The spots gradually expand, and the whole leaf may die. The leaf margins may curl. Older, basal leaves are infected first.
Stem: Shoots are covered with the typical powdery growth that mainly turns to brown discoloration in matured vines.
Fruits: More susceptible to powdery mildew infection after bloom. When berries are <2mm, they become necrotic and fall off, while the bigger ones stop growing and crack open on the infection area, because of the necrosis of epidermal cells. The ripped berries will dry up after secondary infection from another pathogen. Moreover, if the infection occurs after the veraison, berries don’t crack open and covered with brown netlike areas.
Flowers: if the infection occurs before bloom, blossoms may fall off and fail to set fruits.
Diseases of fruit trees and grapevine- Panagopoulos G.C
The American Phytopathological Society