Mildew (Ang Sax. mildedw; Ger. Mehl-thau, meal dew), a name applied to various minute fungi, especially by agriculturists and horticulturist to those which are found upon and are injurious to their crops. The name was originally applied to the white moulds; in common use it is not restricted to these, hut designates also dark-colored fungi and those of different genera and sub-orders. (See Fungi.) One of the most widely disseminated mildews is that which attacks the grape vine appearing as grayish spots upon the under surface of the leaves, the young shoots, and the stems of the fruit; it often destroys the foliage, and consequently the fruit fails to ripen. It has produced incalculable damage to the vineyards of Europe as well as of this country, and though some varieties are more susceptible to its attacks than others, almost all in certain seasons are affected. An English gardener, Mr. Tucker, gave special attention to the subject, and the fungus, in acknowledgment of his services, was called oidium Tuckeri, a name by which it is generally known in horticultural works; but Dr. Berkeley, a high authority in cryptogam-ic botany, considers it not an oidium, but a form of an erysiphe, a very polymorphous genus, in which there are five different kinds of fructification.
Whether this view be correct or not, the plant is now quite well understood, as are the means of combating it. With grapes grown under glass, where the cultivator can control the humidity of the atmosphere, mildew is easily managed; but in the open vineyard it demands constant vigilance, and the vineyardist should daily examine the vines most liable to its attacks, and at the first indication of its presence apply sulphur. In some of the wine-growing districts of Europe sulphuring is practised systematically, whether mildew appears or not. With a view to destroy the spores, the vines before the buds swell and the trellises are sprinkled with a solution of 8 1/2oz. common salt and 4 oz. saltpetre in 36 oz. of water, and 10 drops each of oil of rosemary and lavender are added; one part of this is mixed with 100 parts of water and thoroughly applied by means of a syringe. As soon as the leaves expand they are well dusted with flowers of sulphur, for the application of which a bellows has been especially contrived which blows the sulphur as a cloud of dust, and when the bellows is properly handled every part of the vine will be powdered with it. A similar application is made when the vines are in blossom, another when the grapes are as large as a pea, and a fourth when they begin to color.
In this country the grape growers generally content themselves with using sulphur at the first appearance of the trouble. Its efficacy is well established, provided it be applied in time. Mildew usually appears upon the grape in prolonged warm and damp weather, and it often follows a sudden change of temperature. - Rose growers are sometimes great losers by mildew; the leaves become parched and blistered, and the young stems and unexpanded buds are misshapen and covered with a gray mould; this is attributed to a different plant from that upon the grape, spherotheca pannosa. A similar blight comes upon hop vines, often seriously affecting the crop. Cucumbers, lettuce, and other succulent vegetables are injured in a similar manner in unfavorable seasons; and in this country a late crop of peas is almost impossible by reason of an erysiphe which covers the foliage in such abundance that the plants appear as if dusted with a white powder; the European pea mildew or blight is E. Martii, but we are not aware that our species has been identified as the same. The pea is also attacked by another fungus, peronospora mcice.
Near large cities immense quantities of lettuce are forced under glass, to supply the demand during winter; were there no difficulties to contend with, this would be an exceedingly profitable culture, but often the grower finds his crop, just as it is nearly ready for market, rendered almost worthless by the advent of a mildew or mould. Peronospora ganglifor-tnis is one of the destructive lettuce fungi, but it is probably not the only one. As with other plants under glass, lettuce is usually attacked by mildew after a sudden change of temperature, and all the grower can do is to preserve the proper conditions of heat and moisture as preventives, for when it is established there is no remedy. - The most important of these minute fungi is the wheat mildew, or rust as it is more generally called in this country, puc-cinia graminis, of which figures are given in the article Fungi. This obstacle to successful wheat growing has been known from very early times, but its real nature was only discovered early in the present century.
With a view to destroy any spores that may be with the grain, it is common to treat the seed wheat with a solution of sulphate of copper. - There is scarcely a cultivated or wild plant which is not in some seasons the host of these fungi, which are so minute that their structure can only be seen by the aid of strong magnifiers; in one sense they are among the most important plants to the cultivator, and often determine his success or failure; the minute mildew of the grape in the wine regions of Europe has brought ruin to whole neighborhoods and driven families to emigration. - Another set of fungi attacks dead vegetable matter. When linen or cotton fabrics are kept in a damp place or laid away before they are perfectly dry, they become covered with dark spots which the housekeeper knows as mildew; this is a species of cladosporium, which in some of its forms attacks the leaves of the apple and pear, and also produces the dark blotches sometimes found on otherwise fair specimens of the fruit. Paper, whether upon damp walls or stored in a damp place, is attacked by a chcetomium, an ascotricha, or some other form of mildew, and similar fungi appear upon damp plastered Avails. (See Fungi).
Pea Mildew (Leaflet natural size, Fungus magnified).
Paper Mildew (magnified).