Oidium
A name given to many of the minute Fungi included under the name Mildew, at a time when they were regarded as distinct and good species, without regard to the forms which, it is now known, they assume when fully mature. See Mildew. In the Oidium stage, the Fungus consists of a white coating over all parts of the infested plants. Under the microscope, this is seen to consist of a tangled network of nearly transparent filaments, made up of cells joined end to end. Most of the filaments creep on the surface of leaves, or stems, of the host-plants, pushing fine-lobed branches into the cells of the latter, to suck food from them; while erect filaments stand up over the surface of the Fungus. Each filament is made up of a row of cells, and those at its tip, after a time, break off, and fall away from the lower part, as reproductive bodies (conidia); these in some Fungi are globular, in others barrel-shaped, or nearly cylindrical. But many of the Fungi formerly placed in the group called Oidium, when traced to their full development, are found to form reproductive bodies (spores) in asci, which are inclosed in a case, or perithecium, formed of brown cells, closely joined side by side. Frequently, many perithecia are formed on a patch of the Fungus resembling grains of gunpowder strewn over the white or pale grey surface. The perithecia usually bear, on their outer surface, simple or forked hairs or prickles, the differences in which help in distinguishing the species and genera of the group. The number of asci in each perithecium, and the number of spores in each ascus, differ considerably in the various genera. There are many different kinds of Oidium, but all have a characteristic appearance, that enables an expert to recognise a member of the group, on seeing the affected plant. Several of the species are met with on uncultivated plants, others on cultivated ones; frequently, they injure the latter very much, e.g., Erysiphe graminis, on grass-leaves, as whitish spots on leaves and stems of grasses; E. Martii, on Peas; and so forth. In some species, e.g., E. graminis, the perithecia are formed only under very favourable circumstances; hence they are much better known as Oidium than in the perfect state. This leads to a few species, the Oidium of which is, at times, very plentiful and harmful, while their perithecia remain unknown. In this latter group may be noticed that found on Turnips, in various parts of England, and also in Scotland, caused by an Oidium(O. Balsamii); and the widely-known disease of the Grape Vine, characterised by the formation of white patches on leaves and Grapes, these patches belonging to a Fungus only too famous as Oidium Tuckeri. The various Oidia prove hurtful by absorbing the protoplasm from the cells of their hosts, by means of the small lobed branchlets pushed into the tissues of the latter. The result of this is to cause the parts attacked to become yellow and faded, and, after a time, death of the part, or of the whole plant, ensues. The Vines in many Vine-producing countries were exceedingly injured by O. Tuckeri; the Hops in England, and elsewhere, are often seriously damaged by Sphaerotheca Castagnei; and many other plants suffer, in greater or less degree, from attacks of other species of this group. Remedy. The superficial position of the Fungi renders direct treatment easy. A thorough application to the diseased parts of the host-plants of flowers of sulphur, or of potassium sulphide in weak solution, is found sufficient to put an end to the growth of the Fungus, with out injuring the host. The solution has been recently found most useful in the removal of Green Fly and Red Spider, as well as of Mildew.