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 high humidity and moderate temperatures.  In an agricultural setting, the pathogen can be controlled using chemical methods, genetic resistance, and careful farming methods. It is important to be aware of powdery mildew and its management as the resulting disease can significantly reduce crop yields.  Greenhouses provide an ideal moist, temperate environment for the spread of the disease.
Powdery mildew fungi reproduce both sexually and asexually. Sexual reproduction is via chasmothecia (formerly cleistothecium), a type of ascocarp. Within each ascocarp are several asci. Over time, ascospores mature and are released to initiate new infections. Conditions necessary for spore maturation differ among species.
Vectors of transmission
Wooly aphids ( Eriosomatinae) and other sucking insects are often vectors of transmission for powdery mildew, and other infectious diseases. Typically wooly aphids in sub temperate climates precede and are an indicator of various infections, including Powdery mildew. Aphids penetrate plant surfaces where they often reside and provide a host of potential inoculants through physical, digestive or fecal secretions. Aphids are often an indicator of other potential plant problems.
In an agricultural setting, the pathogen can be controlled using chemical methods, genetic resistance, and careful farming methods.
Another chemical treatment involves treating with a silicon solution or calcium silicate slag. Silicon helps the plant cells defend against fungal attack by degrading haustoria and by producing callose and papilla. With silicon treatment, epidermal cells are less susceptible to powdery mildew of wheat. 
Milk has long been popular with home gardeners and small-scale organic growers as a treatment for powdery mildew. Milk is diluted with water (typically 1:10) and sprayed on susceptible plants at the first sign of infection, or as a preventative measure, with repeated weekly application often controlling or eliminating the disease. Studies have shown milk's effectiveness as comparable to some conventional fungicides,  and better than benomyl and fenarimol at higher concentrations.  Milk has proven effective in treating powdery mildew of summer squash,  pumpkins,  grapes,  and roses.  The exact mechanism of action is unknown, but one known effect is that ferroglobulin, a protein in whey, produces oxygen radicals when exposed to sunlight, and contact with these radicals is damaging to the fungus. 
Dilute sprays containing sodium bicarbonate (baking soda) and vegetable or mineral oils in water are often recommended for controlling powdery mildew, but such mixtures have limited and inconsistent efficacy.  While sodium bicarbonate has been shown to reduce to growth of mildews in lab tests, sprays containing only baking soda and water are not effective in controlling fungal diseases on infected plants, and high concentrations of sodium are harmful to plants. 
Powdery mildews of various plants
Wheat, barley and other cereals
Apples and pears
Gourds and melons
Since 1925, commercial Cucumis melo (cantaloup and muskmelon) production has been engaged in a biological "arms race" against cucurbit powdery mildew ( CPM ) caused by the fungus Podosphaera xanthii , with new cultivars of melons being developed for resistance to successively arising races of the fungus, identified simply as race 1, race 2, etc. (seven in total by 2004), for races found around the world, and race N1 through N4 for some divergent races native to Japan. Various subraces have been identified, and given names such as race 2U.S., race 3.5, and race 4.5.  A new race S was discovered in 2003, and a specific melon cultivar ( C. melo var. acidulus 'PI 313970') found resistant to it, then used for backcrossing to increase resistance in other cultivars.  Such modern selective breeding of plants for phytopathological resistance to particular fungal races involves a great deal of genetic research ; this PI 313970 versus race S case involved multi-stage hybridization to propagate a recessive gene, pm-S in successive generations, and how this may affect other recessive and codominant genes for resistance to other races of P. xanthii "remains to be determined". 
A 2004 literature review regarding powdery mildew races that parasitize various cucurbit plants concluded that "race identification is important for basic research and is especially important for the commercial seed industry, which requires accuracy in declaring the type and level of resistance ... in its products". However, identifying specific races was seen as having little utility in horticulture for choosing specific cultivars, because of the rapidity with which the local pathogen population can change geographically, seasonally, and by host plant.
At least three other Erysiphaceae fungi can cause powdery mildew in cucurbits: The most frequent, after P. xanthii , is Erysiphe cichoracearum , the former primary causal organism throughout most of the world.   Podosphaera fusca is another, sometimes considered synonymous with P. xanthii .  Cucumbers in greenhouse environments have also been reported to be susceptible to Leveillula taurica . 
Sawadaea tulasnei is a fungus that causes powdery mildew on tree leaves. This fungus attacks the leaves of the Acer platanoides (Norway maple) in North America, Great Britain, and Ireland, Acer palmatum (also known as the Japanese maple or smooth Japanese maple). 
Golovinomyces orontii causes powdery mildew on Arabidopsis (rockcress) leaves.
Hyperparasites of powdery mildew
Ampelomyces quisqualis is an anamorphic fungus that is a hyperparasite of powdery mildews. This parasitism reduces growth and may eventually kill the mildew, so investigations on biological control of powdery mildews (especially in high-value crops such as grapes) have taken place for over 50 years, resulting in the development of products such as 'AQ10'.