It was formerly believed that the peach and rose mildews were the same, but recently it has been shown that the two are not absolutely identical. The peach Powdery Mildew probably occurs also on the nectarine.
This disease is undoubtedly world-wide in its range, and has been found occasionally in the United States since 1886. It affected peach trees in that section of the country about Maryland and Delaware from 1886 to 1891, occurring annually during those years. In 1891 the disease was observed in Michigan and Georgia, and three years later, 1894, it was reported as causing serious damage in western New York. As a rule, however, this disease is of relatively little consequence in New York State. In 1905 peach mildew attracted attention in Colorado, and in 1907 it was discovered in Nebraska and Utah.
While peach mildew is not everywhere destructive, it does cause considerable damage at times. Twigs may be checked in their growth or even killed; the foliage may be greatly reduced in size and efficiency; and the future health of the tree may be impaired. Seedlings are often badly affected, and nursery stock suffers more than orchard trees. The last, however, are not uncommonly affected, which, in case of severe infection, may be entirely ruined for market. Fortunately, not many trees in a given orchard are extensively damaged.
The susceptibility of varieties is not marked; this is true at least for Colorado conditions. In Michigan, the popular opinion prevails that only those varieties with serrate leaves and lacking glands are affected with Powdery Mildew. It has been found by authorities, however, that this is not always true. In Ohio, it has been observed that the Early Crawford may be badly affected. At Geneva, New York, the disease was found on but eight out of three hundred and fifty varieties: Bailey, Conkling, Tillotson, Simmons No. 1, Wright, Morrell, Thomas Rivers and Illinois Peach. In western New York, the Crawford and especially the Denton are most susceptible.
Injury is done by the mildew-pathogene to the leaves (Fig. 83), twigs (Fig. 83) and fruits (Fig. 84). Perhaps the fruits are least affected of all. Along the lower surface of the foliage, particularly at the midrib, are to be found abundant, irregular, white blotches - the mildew (Fig. 83). It may also occur on the upper surface. As a result of the affection the foliar parts crumple and curl, the edges rolling toward and parallel to the midrib (Fig. 83). Young leaves, when affected, fall prematurely. Older leaves are dwarfed. Signs of the disease may first be observed in early summer.
Only young growing twigs are affected. Conspicuous white patches appear on the surface. The bark becomes dry and brown at infected points, and in severe cases the leaves fall as a result. Finally the bark shrivels and the young tips become curved; growth is thus checked and the twigs may die. The disease may involve the whole or, more commonly, only a portion of the fruit. It appears as a frostlike covering which assumes a pure white aspect due to the presence of the mildew-patho-gene (Fig. 84). The affected peach flesh hardens and the skin turns brown, and finally the peach cracks. Young fruits are often caused to fall prematurely. In Oregon two types of symptoms have been noted: (1) mildew scattered on the fruit and leaves, twigs less affected; (2) general on all parts. The former is thought to be the true peach mildew disease; the latter the cherry mildew (see page 177). Cause.
The name of this Powdery Mildew organism is Sphcerotheca pannosa var. Persicce. Its mycelium grows in dense superficial patches, giving to the lesions the Powdery Mildew aspect already described. Soon after the fungus appears, great numbers of conidia are developed. From the mycelium erect stalks are formed which bear conidia in chains at the tip. These stalks are the conidiophores; they, with their conidia, add to the felt-like, whitish growth, covering the diseased portion. The conidia are scattered throughout the summer, and those which fall on peaches will, under favorable conditions, germinate to start a new mildew spot. From the germtube a dense mat of mycelium develops. At intervals over the hyphal system, sucker - like bodies, called haustoria, are sent into the epidermal cells. These haustoria are the feeding organs of the fungus, and as a result of their activity the attacked portion becomes brown and cracked.
Fig. 83. - Peach mildew on shoots and leaves.
Fig. 84. - Peach mildew on fruits.
After midsummer perithecia may be developed. These bodies when formed carry the fungus through the winter, but as a rule they are extremely rare. When found they occur more on twigs than on leaves. In those cases where perithecia do not develop, it is thought that the fungus hibernates as mycelium in the buds.
The fungus thrives best in a warm, moist, shaded location. Trees that are closely planted are more likely to be affected; this is due to the increased moisture content of the air about such trees.
Wherever possible those conditions which favor the fungus should be avoided. Attention should be given to trees planted near one another, and to those with a dense foliage. A good circulation of air and plenty of sunlight is essential in this connection.
As a fungicide, copper salts have not been generally effective. On the other hand, sulfur has proved to be satisfactory. Sulfur applied either as flowers of sulfur (dust) or as lime sulfur solution is a success. Lime sulfur 1 to 50, with the addition of three pounds of iron-sulfate to fifty gallons of the mixture to increase the fungicidal properties and adhesiveness, is efficient. Sulfur dust is desirable in that less time is required for the application. This should be applied early in the morning while the foliage is still damp. If the weather subsequent to the dusting operation is warm and dry, the fungicide may be expected to be more efficient. Rainy periods, on the other hand, tend to decrease the efficiency by washing the sulfur away; however, this is not serious except in cases of rain periods of long duration. The fungicide used for Powdery Mildew should be applied as soon as the disease appears. The weather and consequent abundance of the disease determine the number of later applications. In some years two applications are sufficient; in others it is profitable to give the trees five or six treatments at intervals of about one or two weeks. In the far West the spraying schedule for the Coryneum Blight and Brown Rot is satisfactory for the scattered form of Powdery Mildew. When the generalized form, that is on all parts, is present (that caused by the cherry Powdery Mildew pathogene, Podosphaera Oxyacanthce), the control problem is more difficult. In any case it is best to remove badly diseased seedlings.
Jackson, H. S. Peach diseases. Powdery mildew. Oregon Crop Pest and Hort. Bienn. Rept. 1911-1912: 257 - 259. 1913.
Whipple, O. B. Peach mildew. Colorado Agr. Exp. Sta. Bul. 107: 3 - 7. 1906.
Stewart, V. B. Some important leaf diseases of nursery stock. Mildew of rose and peach. Cornell Univ. Agr. Exp. Sta. Bul. 358: 221 - 226. 1915.
Stewart, F. C. Notes on New York plant diseases, I. Peach. Powdery mildew, Sphaerotheca pannosa (Wallr.) Lev. New York (Geneva) Agr. Exp. Sta. Bul. 328: 370 - 371. 1910.
Selby, A. D. Preliminary report upon diseases of the peach. Peach mildew. Ohio Agr. Exp. Sta. Bul. 92: 225 - 226. 1898.
Smith, E. F. Field notes, 1891. Peach mildew. Journ. Myc. 7: 90 - 91. 1892.
Caesar, L. Peach diseases. Powdery mildew. Ontario Agr. Dept. Bul. 201:40 - 41. 1912.
In some sections of the country the peach among many other fruit-trees is injured by the Armillaria Root Rot. For a fuller discussion of the disease, see Apple, page 96.
The disease in question has been called rhizomorphic Root Rot. It occurs also on the apple and cherry. Facts concerning its range, symptoms, cause and control are discussed under Apple, page 102.