Certain varieties of apples show this very familiar disfiguration. The Rhode Island, Peck Pleasant, Rome, Baldwin and Northern Spy are regarded as most susceptible, although in general the disease is most noticeable on light - colored varieties. English growers find that the Newton Wonder is affected more than other varieties. This disease is also found on pears, particularly on the varieties Anjou and Lawrence.
The injury produced is not deep-seated, consequently the disease is not so serious from the standpoint of destructiveness as would first appear. The growth of the affected fruit is not checked, nor does the real quality of the fruit seem to be impaired. But the apple is blemished and thus it may be rendered unmarketable; at all events, fruit affected with Sooty blotch is less salable than clean fruit. While the chief loss lies in the lowering of the market value of the fruit, it should be noted that later ill effects are sometimes shown in the wilting of affected fruit and even in the decay induced by organisms which are always ready to take advantage of a weakened apple. Although no dollar estimates of losses incurred by this disease are at hand, its economic importance cannot be questioned.
For the past twenty years the prevalence of the trouble has been noticed commonly along the Atlantic seaboard south of the New England states. But the disease occurs elsewhere; it is found in Canada as far north as Quebec, and is well known throughout the eastern, middle and western states. It was unusually common in 1902 in Rhode Island and Connecticut. Again in 1906 an outbreak occurred in these and surrounding states, particularly in New York, Pennsylvania, Maryland, West Virginia and eastern Nebraska. Special notice of it was made in Ohio about 1890 and in the subsequent years 1896 and 1909. The disease was very prevalent in New York in 1915. It was first recorded from the state of Washington in 1907. In 1910, authorities claim, Sooty blotch was first recorded in England.
The names Sooty blotch and Fly Speck adequately describe the appearance and effects of this disease. The blotches (Fig. 28) are abundant in the months of July and August. They are irregular in outline, tending to be circular. At first the color is pale, but later, as the name suggests, the color is a sooty-brown or black. On account of the appearance of affected fruit, dealers often call it the cloud or clouded fruit (Fig. 28). Single spots measure from one-fourth to one-half of an inch in diameter; often several lesions coalesce, covering the apple as if with soot. Spots exhibit a radiating structure composed of a thin felt of dark-brown interwoven threads which are seen with the naked eye or better with a hand-lens. Fly Speck (Fig. 29), while formerly regarded as distinct from Sooty blotch, is now considered simply as a different symptom of the same disease. The two are found in the same situations and under similar conditions except that Fly Speck develops later than Sooty blotch. Fly Speck needs little description (Fig. 29). Groups of a half dozen to one hundred black, shiny dots appear on the surface of an apple; the specks resemble closely those made by the fly (Fig. 29). Both Sooty blotch and Fly Speck are very superficial in nature, and there is no hardening or cracking of the apple skin as in scab. Cause.
Fig. 28. - Sooty blotch.
The pathogene causing the disease was described on Newtown Pippins from the state of Pennsylvania in 1831. More attention was paid to the fungus than to its effects on the apple. It is suggested that, since it was discovered in America nearly a century ago, the pathogene was carried across the ocean from the United States to England. It has frequently been observed, for many years, in English markets on American-grown apples. On its discovery in the latter country, in 1910, its history with particular reference to its origin aroused great curiosity. English growers of fruit had not forgotten their experience with introduced parasites, especially those of such alarming habits as the gooseberry mildew fungus and others.
Fig. 29. - Fly Speck.
The radiating threads which compose the blotches are hyphse of the fungus Leptothyrium pomi. Likewise the black specks belong to this fungus; they are sclerotial bodies. It is unknown just how the causal fungus passes the winter. It has been suggested that it hibernates on the apple-twigs, presumably as specks or sclerotial bodies. In the late spring each sclerotial body undergoes certain developmental changes which result in a pycnidium. Conidia, developed within the pycnidia, serve to bring about the first infections in the summer. On the surface of the apple a conidium germinates in the presence of moisture with the result that a radiating growth of mycelium is initiated. Apparently inoculations do not occur prior to the month of July. The threads extend themselves superficially, or at most penetrating the cuticle but to a slight degree. As the hyphae grow, they branch uninterruptedly until there is a prostrate, soot-colored mat of fungus threads; these compose the blotches, whence the name Sooty blotch. The cells of the radiating hyphae become enlarged and function in propagation by breaking away and causing new infections. This probably composes the inoculum during midsummer; for at this season no fruiting structures and no spores are to be found. When the fungus develops the Fly Speck stage, the sclerotial bodies may possibly break away from the skin of the apple and may subsequently be washed to new quarters, where secondary infections may occur. This point, however, needs confirmation. In all cases the fungus is highly favored by damp situations and it is most abundant in seasons of considerable late summer rainfall. Little difficulty is experienced in dry seasons; in some years Sooty blotch and Fly Speck are practically unknown.
Fortunately the disease may be kept under control by the methods of treatment employed for other more serious diseases like apple scab. If the schedule advised for the control of scab (see page 12) is rigidly followed, satisfactory results so far as Sooty blotch and Fly Speck are concerned will be obtained. Particular attention should be given to the treatments in July and August.
Sturgis, W. C. On the cause and prevention of a fungous disease of the apple. Connecticut Agr. Exp. Sta. Rept. 21: 171 - 175. 1898.
Clinton, G. P. Fly speck, Leptothyrium pomi (Mont, and Fr.) Sacc. Sooty blotch, Phyllachora pomigena (Schw.) Sacc. Connecticut Agr. Exp. Sta. Rept. 1903: 299 - 300; 302. 1903.
Selby, A. D. Some diseases of orchard and garden fruits. Sooty fungus and Fly Speck fungus - an old enemy in a wet season. Ohio Agr. Exp. Sta. Bul. 79: 133 - 134. 1897.
Salmon, E. S. Sooty blotch: a new fungous disease of apples. Gard. Chron. 3: 48: 443. 1910.
Salmon, E. S. Sooty blotch: a new fungous disease of apples. Southeastern Agr. Col. (Wye) Rept. on Econ. Myc. 1909-1910:29 - 32. 1910.
Macoun, W. T. Report of the horticulturist. Sooty or Fly Speck fungus. Canada Exp. Farms Rept. 1906: 123 - 124. 1907.
Quaintance, A. L., and Scott, W. M. The more important insect and fungous enemies of the fruit and foliage of the apple. Sooty fungus and flyspeck. U. S. Agr. Dept. Farmers' bul. 492:36 - 37. 1912.