In the late summer and autumn perithecia begin to appear among the hyphse. At first they are yellowish, but they soon turn black. They are numerous, more than one hundred thousand having been counted on the upper surface of a single leaf. Their further history has been discussed. In certain parts of California they are found in June. Their formation seems to depend upon a peculiar set of weather conditions: if the temperature suddenly falls to about 50° Fahr. just after a period of warm, moist weather, they are produced in great numbers. In other parts of the country where these conditions do not prevail in early summer they are more scarce.
The fungus is favored by sheltered, shaded conditions. Thus vines in the open are less likely to show Powdery Mildew. While the fungus requires less moisture than most fungi, it will not grow in an exceptionally dry atmosphere. This is noticeable in California, mildew being far more abundant along the coast than in the drier regions. Vines in low places or along streams are often diseased, whereas the remainder of the vineyard is free from Powdery Mildew. Rains or fogs, in the spring or early summer, accompanied by warm weather are highly favorable to the organism. It grows between 50° and 95° Fahr., but does so rather slowly except between 75° and 90° Fahr. It ceases growth below 50° and above 100° Fahr. These characteristics of the fungus result in its somewhat erratic appearance in different seasons and in different localities.
In handling Powdery Mildew the following points should be taken into account:
(1) The causal fungus may possibly live for one to one and one - half years in the soil. If the perithecia are buried, they are only preserved until plowed up again a year later.
(2) The fungus is favored by moisture in spite of the fact that it can withstand more drought than other grape fungi. Hence anything which permits sun and air to reach all parts of the vine may lessen the danger from mildew: (a) remove rows of trees which shade the vines on the south side; it is assumed that this will be done only in the case of trees of small worth; (b) plant vines at a reasonable distance apart so that they may dry off more quickly after dews, fogs and rains; (c) rows which extend north and south are said to evaporate their moisture most easily; this should be borne in mind when planting grapes in mildew-regions; (d) likewise, low - trellised vines are more easily dried by the sun and wind; (e) wet places should be drained; (f) vines should be pruned so that they will spread; this is of value in the moisture consideration.
The fungus may be eradicated from the vines by the use of sulfur dust (powdered sulfur, or flowers of sulfur); in some regions this may be done cheaply and effectively. Old and young vines alike should be dusted. The sulfur acts by killing the mycelium of the fungus which, it will be remembered, is superficial. Fortunately the sulfur acts at temperatures which are optimum for the fungus, the rate of the killing increasing as the temperature rises from 75° to 100° Fahr. So far as possible, dusting should be avoided when the vines are very wet. Applications of dust to the soil have been shown to have practically no effect on Powdery Mildew. In California injury from the use of sulfur may be expected only when the temperature is above 110° Fahr. and only on varieties like the Isabella, Othello and other American varieties. In the Chautauqua belt of New York sulfur dust causes considerable injury and should not be used except on vines of European origin. It has been noticed that sulfur - dusted grapes show less tendency to drop their blossoms, and also ripen their fruit seven to ten days earlier than vines not so treated.
One to six applications of sulfur are necessary, depending on the locality, weather, variety and exposure. Apply as follows:
(1) when the shoots are about six or eight inches long; (2) during or just before blossoming; (3) later applications are necessary only on very susceptible varieties or where irrigation is practiced when grapes are half-grown. Winter treatments are not generally commendable. The cost, including material and labor, varies from forty to fifty cents the acre for one treatment. Where it is necessary to treat vines for Downy Mildew and Black Rot, bordeaux mixture may be suitable for Powdery Mildew. Liquid fungicides, however, are not as effective as sulfur dust, and in vineyards where this disease is particularly troublesome along with other diseases, it is best to follow the bordeaux sprayings with dust.
The fungus shows preference for European vines; all American varieties are less susceptible. These points are worth remembering in planting the vineyard where Powdery Mildew is prevalent.
Bioletti, F. T. Oidium or powdery mildew of the vine. California Agr. Exp. Sta. Bul. 186: 315-350. 1907. Reddick, D., and Gladwin, F. E. Powdery mildew of grapes and its control in the United States. Int. Cong. Vit. Rept. 1915: 117125. 1915. Bioletti, F. T., and Flossfeder, F. C. H. Oidium or powdery mildew of the vine. California Agr. Exp. Sta. Circ. 144: 1 - 12. 1915. Scribner, F. L. Report on the fungus diseases of the grape vine.
II. The powdery mildew. U. S. Agr. Dept. Bot. Div. Plant Path. Sec. Bui. 2: 18-28. 1886. Jackson, H. S. Grape diseases. Powdery mildew. Oregon Crop Pest and Hort. Bienn. rept. 1911-1912: 269 - 270. 1913. Scribner, F. L. The powdery mildew. U. S. Agr. Comm. Rept. 1886:
105 - 109. 1887. Scribner, F. L. Distribution and severity of the grape mildews and black rot in the United States. U. S. Agr. Comm. Rept. 1886:115116. 1887.