any of various fungi of the genus Erysiphe producing powdery conidia that appear as a white. fuzzy coating on the upper leaf surfaces. often causing distortion of the leaf.

Merriam-Webster Online Dictionary
powdery mildew (noun)
an ascomycetous fungus (family Erysiphaceae) producing abundant powdery conidia on the host
a plant disease caused by a powdery mildew
powdery mildew (Wikipedia)
Powdery mildew
Downy and Powdery mildew on grape leaf.JPG
Example of powdery mildew (right) along with Downy mildew on a grape leaf
Causal agentsSpecies of fungi in the orders Erysiphales

Powdery mildew is a fungal disease that affects a wide range of plants. Powdery mildew diseases are caused by many different species of fungi in the order Erysiphales, with Podosphaera xanthii (a.k.a. Sphaerotheca fuliginea) being the most commonly reported cause.Erysiphe cichoracearum was formerly reported to be the primary causal organism throughout most of the world. Powdery mildew is one of the easier plant diseases to identify, as its symptoms are quite distinctive. Infected plants display white powdery spots on the leaves and stems. The lower leaves are the most affected, but the mildew can appear on any above-ground part of the plant. As the disease progresses, the spots get larger and denser as large numbers of asexual spores are formed, and the mildew may spread up and down the length of the plant.

Powdery mildew grows well in environments with low humidity and moderate temperatures. Greenhouses provide an ideal moist, temperate environment for the spread of the disease. This causes harm to agricultural and horticultural practices where powdery mildew may thrive in a greenhouse setting. In an agricultural or horticultural setting, the pathogen can be controlled using chemical methods, bio organic methods, and genetic resistance. It is important to be aware of powdery mildew and its management as the resulting disease can significantly reduce important crop yields.

Powdery mildew growing on a leaf (magnified)
Powdery mildew growing on a leaf (high magnification)
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