Mildew
Photo Credit: Georg Slickers, http://en.wikipedia.org/wiki/Image:Halyzia_sedecimguttata_20050831_716_part.jpg, used under GNU Free Documentation License.
Mildew refers to certain kinds of mold or fungus that affect garden plants. The name mildew, in Old English, meant honeydew, that is to say, the substance secreted by aphids on leaves. These were thought to come from the air like dew.
When referred in horticultural and gardening, mildew refers to powdery mildew, which is caused by different species of fungi in the order Erysiphales. Most species of mildew are specific to specific hosts, and are all parasites of flowering plants.
Another type of mildew is downy mildew. Downy mildews are caused by fungus-like organisms in the family Peronosporaceae (Oomycota). There are many species, and are each parasitic on a narrow range of hosts. Downy mildews are a problem for growers of potatoes, grapes, tobacco and cucurbits.
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