Oidium is variously referred to as a fungal spore, which is the offspring body of a fungus, or as an actual fungi itself of the Ascomycota order. It is more commonly known as powdery mildew due to its parasitic nature of existing as a soft film on the surface of host plants such as grape vines. Mildew odium can have a devastating effect on wine crops, and is known to have contributed to the near collapse of the wine industry in Europe in the mid-19th century.
Within the Ascomycota order for fungi, there are a great variety of forms, but they share the common feature of being spore shooters that distribute their offspring by rapidly dispersing them into the surrounding air. The Oidium group is a subdivision within this order known as a genus that contains dozens of species. Almost all Oidium species are known to be plant pathogens that exist and act as powdery mildew agents on the surface of the green parts of vines. They attack the vines and turn them black, as well as yellowing foliage in the process, causing the plants to wilt. While an Oidium fungus does not always kill the host plant, it will reduce its growth rate and, in the case of grape vines, affect the skin color of grapes, which ultimately degrades the final wine product produced from them.
Fungi have a propensity to spread quickly in damp, cool environments once established, such as in vineyards, but the cause of the ongoing wine crop devastation in Europe in the 19th century was partially manmade. A worldwide scientific interest in botanical specimens led European horticulturalists to import wild vine samples from the US for study. At the same time, Henri Marès, a Frenchman, had perfected a method of sulfuring vines to protect them from Oidium infections. The American vines carried Oidium, as well as an infestation of tiny yellow-green aphids of the genus Phylloxera, to which they were naturally resistant. The European vines had no resistance to the aphids and they rapidly spread throughout European vineyards over the next 11 years, causing additional crop loss from plants that had not already succumbed to Oidium.
From 1854 to the 1880s, vines died off across a broad region of western Europe centered on France, primarily from Oidium and Phylloxera attacks, as well as from downy mildew and black rot which also were carried in on imported species. It wasn't until European vines were grafted into American strains to build in resistance to these pests at the end of the 19th century that the crops began to recover. Other species of Oidium still present problems with crop growth as of 2011. These include the Oidium lycopersicum species that attacks tomato vines and is found throughout the US state of Connecticut, and the Oidium mangiferae species that attacks mango trees in the Far East countries of China, India, and Pakistan, as well as other regions of the globe such as Mexico.