_History and Distribution._--The powdery mildew or Oidium was observed for the first time in the year 1845 in hothouses in England. It immediately began to spread, and in a few years infested all the vine districts of the Old World. Before any remedy had been discovered, many vine districts were so injured that they have not since been able to recover. Thus in 1850 and 1851 France suffered greatly from this mildew, and the Island of Madeira, which for three hundred years had produced the finest wines, had its grapevines so injured that they up to this time have not again produced as good a quality of grapes as before the advent of the disease. The Grecian Islands as well as Morea were also visited by the powdery mildew, and though the latter is now kept in control, the general opinion is that the quality of the currants is not as high as it was before the mildew appeared. Now there is probably no place in the Old World where grapes are not attacked by this mildew, although some places are injured much more than others. Adjoining vineyards are often differently attacked, some being even entirely free, while others are visited yearly. Young vines are less attacked than old ones, and in favorable places the mildew seldom infests vines before they are two or three years old. Elevated places and localities exposed to winds and cold are generally attacked by the first stages of this oidium, while its second or last stage prefers low, damp places exposed to dew or fog. The American form of the powdery mildew or _Uncinula spiralis_ differs in some respects from the European Oidium, not as to its effects, but as to its microscopical characteristics. The Oidium occurs in Europe only with certain generative organs called gonidia, while the American _Uncinula_ also develops so-called _peritheca_. It is more than probable that both fungi belong to the same species, but until these perithecal organs have been found on the European Oidium, the proper name for our mildew must be _Uncinula_, and not Oidium. It is also probable that the _Uncinula_ fungus is a native of this continent, and that it from here has spread to Europe, where the natural conditions are such that only the gonidial form of the fungus has been able to develop. In general appearance and in their effects the _Uncinula_ and Oidium are identical.