It is chiefly the young leaves and the tips of branches of young cherries that suffer from the effects of Powdery Mildew. But the disease is commonly observed also on mature trees. Furthermore the mildew pathogene attacks other fruit-trees like the apple, plum, peach, crab - apple, quince and persimmon, as well as certain shrubs, including juneberry, hawthorn, spiraea and huckleberry.
Cherry Powdery Mildew was first described in France about 1800, on hawthorn. Subsequently it was discovered in Germany on the cherry. In America the disease is very common and widespread in the eastern and central states, and is reported from the Rocky Mountain states and California. It also occurs in Canada.
The pathogene shows preference for budded sour cherry stock, although sweet varieties and mazzards are known to suffer from its attacks. The mahaleb cherry and the variety Governor Wood, a sweet variety, have exhibited a certain amount of resistance, while the Hoy variety has been noted as free from mildew in cases where other varieties succumb.
Powdery Mildew is usually found on the young sprouts and at the tips of branches, where it affects both the foliage and the wood (Fig. 49). It may be observed as early as the first of June, but does not develop sufficiently to attract attention until July, and it increases in prominence throughout the late summer and early autumn. Affected parts show small, round, whitish blotches, having a radiating appearance. These blotches spread and coalesce so that a considerable portion of an affected leaf or twig becomes covered by a white felt. These mildewed spots soon exhibit black spherical bodies which are scattered over the affected area. The internodes of diseased twigs are shortened and thickened (Fig. 49, right). Both surfaces of the leaves are liable to attack, although as a rule only one side of a single leaf shows mildew at a given time. Diseased leaves are caused to curl inward and upward in a very marked fashion.
The pathogene is a fungus known as Podosphcera Oxyacanthce. It thrives best during warm, dry weather; therefore outbreaks right.
Fig. 49. - Powdery Mildew of cherry; healthy shoot on left, diseased on occur in seasons characterized by such conditions. A drought in the Mississippi Valley in 1887 and 1888 was accompanied by an epiphytotic of cherry mildew.
The fungus hibernates as mature perithecia. In the spring ascospores which develop within these bodies escape by the disintegration of the perithecia. Those ascospores which fall on susceptible parts germinate in the presence of moisture, the germtube developing into mycelium which grows over the surface of leaves and twigs. This growth becomes profuse and constitutes the mildew so noticeable as a symptom of the disease. None of the mycelial threads enters the tissues of the affected plant, but small suckers, called haustoria, arise as branches from the hyphae and penetrate the outer cells of the leaf or twig. As development of the fungus proceeds, conidiophores grow erect and bear chains of conidia. These spores in mass give the powdery aspect to the lesions. At maturity the conidia are carried by the wind to other leaves and twigs, where new infections are initiated. Later in the summer perithecia begin to develop. These are yellowish at first, later brownish, and finally black. When mature, each perithecium contains a single ascus, which contains eight ascospores. Control.
The application of some standard fungicide is an efficient remedy for Powdery Mildew of the cherry. It is recommended that the orchard be sprayed with bordeaux mixture, or sulfur dust, making the first application as soon as the disease appears. Repeat the application ten days later, if necessary. Sulfur dust may also be used with good success in the orchard. Sulfur is preferable in the nursery. Lime sulfur 1 to 50 applied as directed for the orchard has proved satisfactory. Add 3 pounds of iron-sulfate to each 50 gallons of the spray mixture; this procedure increases adhesiveness and decreases the caustic qualities of the fungicide. Sulfur dust 90 parts, and powdered lead 10 parts, has proved satisfactory and effective, and may be substituted for lime sulfur solution. Applications subsequent to the first may number two to four, depending on the severity of the disease. As a rule the schedule for the control of cherry Leaf Blight keeps mildew under control (see page 175).
Stewart, V. B. Some important leaf diseases of nursery stock.
Powdery mildew of cherry. Cornell Univ. Agr. Exp. Sta. Bul.
358:192 - 194. 1915. Galloway, B. T. The powdery mildew of the cherry. U. S. Agr.
Comm. Rept. 1888: 352 - 357. 1889. Hein, W. H. Two prevalent cherry diseases. Powdery mildew.
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