Mildew
A name sometimes applied to several kinds of microscopic fungi that live as parasites on various flowering plants, cultivated and uncultivated; it is, of course, on the former that their occurrence is of interest to the horticulturist. The name is believed to be connected with the German Mehl-thau, or Meal-dew, because of the appearance some Mildews give the affected parts of being sprinkled with flour or meal. Besides living on plants, the Mildews are also found on cloth, on paper, on leather, and even on glass; but the forms growing on these need not be further adverted to here. Nor is it necessary to dwell upon the Mildew of Wheat, Barley, and other grasses, which, if forming dark brown spots, is caused by a species of Puccinia (usually P. graminis); or, if the spots are orange, by the uredospores of the same fungi. The Mildews of most consequence in gardens are white, and form a coat all over the leaves and young shoots of the plants attacked by them. On microscopic inspection, this is found to consist of slender, creeping threads, which branch freely, and give off branches that rise erect into the air as slender filaments, made up of a single row of cells, of which the terminal ones are larger, oval, and break away from the tip backwards, to form spores, which serve for the reproduction of the fungi. Very many plants are liable to be attacked by this white coating, and there is reason to believe that there are many kinds of it, though all much alike. The various kinds are included, as species, under the genus Oidium; but they are known to be merely a stage in development of more highly developed fungi, belonging to Erisypheae. If some kinds of Oidium, e.g., those on Peas, on Hops, or on Roses, are watched, it will be found that, after a time, the surface becomes studded with small, yellowish objects, which soon grow darker in colour, and at length become quite black, and resemble grains of gunpowder, scattered abundantly over the Oidium. With the microscope these are seen to be fruits of the fungus. Each is a slightly-flattened sphere, with a thin, black wall, formed of cells (perithecium) surrounding several small transparent bladders (asci), each of which incloses two, four, or eight (rarely six) spores. These are believed to be the result of sexual reproduction. They are very small, and are easily carried about by the wind, or by other means. If they fall on a suitable host-plant, they push out a fine thread, which reproduces the Oidium stage once more. The threads of the Oidium lie on the outer surface of the plants (not penetrating among the cells, like the Potato Disease fungus, and many others), where they form a web, nourished by means of small branches (haustoria, or suckers), that penetrate into the outer cells (epiderm) of the plant, and absorb food from the cell contents. Some kinds of Oidium have not yet been traced to the sexually mature form, e.g., Oidium Tuckeri, which is often very hurtful to Vines; O. Balsamii, on Turnip leaves, &c. Besides these two, the following White Mildews, of which the fully-developed state is known, are, at times, destructive to garden plants; Sphaerotheca pannosa, in dense, grey patches, on Peach and Rose twigs and leaves; S. Castagnei, very injurious to Hops, as well as to many weeds; Podosphaera Oxyacanthae on Hawthorn twigs and leaves; Erisyphe Martii on Peas, and other leguminous plants, and on many weeds; E. communis and E. Cichoracearum, on various flowers; Microsphaera Berberidis, on Barberry leaves; M. Grossulariae, on Gooseberry bushes; and Phyllactinia suffulta, on many trees and shrubs.
Remedies. White Mildews, caused by Oidium, being the only kinds that directly concern horticulturists to any extent, the present remarks are restricted to these. As the parasites are external to the host-plants, they can be attacked directly, and can be eradicated without much injury to the latter. Sulphur has proved to be the most reliable means of destroying the fungus. Flowers of sulphur may be dusted over the leaves and other diseased parts; or it may be applied with a syringe, if mixed in water. If applied to bunches of grapes, the sulphur must, of course, be washed off before they are sent to the table or the market. Another useful application is prepared by boiling 1lb. of flowers of sulphur and 1lb. of quicklime, in five pints of water, in an earthen pot, for ten minutes. It should be constantly stirred while it is boiling, then allowed to settle, and the clear liquid poured off for use. The plants should be syringed with a mixture of this preparation with 100 times its bulk of water. Washing the greenhouse flues with a mixture of 1/4lb. each of flowers of sulphur and of quicklime, in three gallons of water, is also recommended. The fumes emitted under this treatment kill the fungus. The door of the house should be kept closed for about an hour, to retain the fumes, and then the place should be well aired. Ewing's Mildew Composition, much diluted (1oz. to 1 gallon of tepid water), is used with the syringe, and is also very effective. Mildew is apt to prove most destructive in ill-ventilated situations, and it may often be checked, or even prevented by attention to ventilation.