This disease, known as peach scab, freckles and Black Spot, was first described in lower Austria in 1876. Since then it has been known commonly in the United States, and it occurs to an injurious extent wherever peaches are grown east of the Rocky Mountains. It is known also in California and occurs to some extent in Canada.
The damage done by peach scab is apparently not realized by growers, some regarding the disease as a necessary evil, scarcely apprehending that their fruit is bringing in the market 25 per cent less price than clean fruit would command. Sometimes the disease is mistaken for a peculiarity of the affected variety. In spite of these misconceptions many growers do appreciate the importance of peach scab and that it is the cause of widespread injury to the peach-crop. There is no decay of the fruit, but its market value is lowered; the size is reduced and the fruit dwarfed; the fruit is sometimes cracked, allowing rot-producing organisms to enter the flesh to cause subsequent rapid decay. Affected fruits may drop prematurely, and those which are picked do not ship well. In some seasons the loss, in Indiana for example, has been estimated at ten per cent of the crop, while in the eastern United States the loss has been put at the same figure. The total annual loss has been placed at $1,000,000 in the United States. The growing of certain susceptible commercial varieties has been prohibited by this disease. Heavy losses occur in West Virginia and western Maryland. In Ohio, in 1896, cases are recorded where 20-50 per cent of the crop was lost, while in New Jersey as much as 75 per cent of the fruit has been known to be affected in certain localities. In central and southern New Jersey the trouble is considered by peach - growers as one of their worst foes, while in the hilly portions of the northern part of the state, the disease does not cause injury sufficient to make spraying for its control a profitable or necessary operation.
The fruits, leaves and twigs are affected. On the fruit (Fig. 78) small, round, olive-black spots begin to show about six weeks after the blossoms fall, or from June 15 until July 10, depending on the variety. Lesions most frequently occur on the upper side of the fruit. If the spots are numerous, they coalesce, forming a large, irregular diseased area covering a greater portion of the fruit's surface (Fig. 78) and preventing the normal expansion of the skin as well as interfering with the ripening and mellowing of the flesh. The fruit often becomes one-sided, due to a formation of a protective cork-layer under the diseased area. This layer is incapable of further growth, and hence an ill-formed fruit results. Often the cork-layer is ruptured, leaving deep cracks (Fig. 78) through which the Brown Rot pathogene enters (see page 275). Attacks are generally most noticeable on the late varieties, due, in part perhaps, to the fact that the fruit of such varieties is exposed to infection over a longer period. Of the commercial varieties, the Heath is said to be most susceptible. The Bilyen and Hill's Chili are also badly affected, and the Salway, Smock and Morris' White suffer severely. The Elberta is sometimes badly diseased, but is more resistant than those just mentioned. The Carman, Hiley, Champion and Belle are slightly affected. Trees grown in higher places are more free from scab than those in low areas.
Fig. 78. - Peach scab; types of lesions on the fruit.
The twigs have more or less circular blotches of a yellowish brown color, with a dark-gray or bluish border. Frequently the spots become confluent, masking the normal pink - brown color of the young bark. The cuticle is separated from the cells beneath, corky cells are developed under the lesion, and in some cases the cambium is killed, and as a result the twig dies.
That portion of the leaves of the peach lying midway between the main veins is especially affected. Brown, scattering spots are developed in which the tissue dries up and finally falls away, leaving circular holes.
Peach scab is caused by the fungus Cladosporium carpophilum. Its mycelium occupies the space left by the partial separation of the cuticle from the underlying cells. On the twigs the mycelium hibernates in the form of dark-brown spherical cells. It is possible that conidia lying about the lesions may also be capable of tiding the fungus over winter. From the resting-cells described above, conidia are produced in the spring, and the latter are carried to the leaves and fruits. Germination follows shortly and penetration is effected. Inoculation begins four to five weeks after the petals fall, but symptoms do not show to the naked eye for about three weeks subsequently. Inoculations and infections continue to take place until about one month before the fruit matures. As the fungus grows on the fruit the mycelium attaches itself closely to the surface between the hairs, forming a mat of short, plump cells which give rise to conidiophores and conidia. The flesh of the peach is not penetrated, but the close contact of the fungus with the outer cells allows absorption of nutrition from the fruit through the unbroken walls. Evidently there is some injury to the outer cells, for a cork - layer is developed just beneath the lesion, suggesting an attempt on the part of the affected cells to repair the injury. Some time during the growing season the fungus infects the twigs, developing lesions as previously described. In these diseased areas the pathogene passes the winter.
In the early history of the disease, peach scab was less amenable to treatment than it now is, because of severe injury of fungicides to the foliage. With the development of self-boiled lime sulfur it has become possible to control scab without injury to the fruit or foliage. Where scab alone is to be treated, use self-boiled lime sulfur 8-8-50 as follows: (1) four to five weeks after the petals drop; (2) about three to four weeks later. The second application is usually unnecessary if the first is thorough. Use about one-half to one gallon on trees three to four years old, depending upon the type of nozzle. The cost of one application on four-year - old trees has been estimated at four cents a tree. Spraying as directed above has been known to give from 92 1/2 to 99 per cent of the crop free from scab.
Most peach-orchards in eastern United States should be given treatment for Brown Rot as well as for scab. It should be remembered that preventing scab is an important step in the control of Brown Rot.
Whitmarsh, R. D. Black spot or scab (Cladosporium carpophyllum Thiim.). Massachusetts Agr. Exp. Sta. Rept. 23:165 - 167.
1911. Sturgis, W. C. Scab and mold of peaches. Connecticut Agr. Exp.
Sta. Rept. 18:138-139. 1895. Sturgis, W. C. On the probable winter-condition of the fungus of peach scab. Connecticut Agr. Exp. Sta. Rept. 20:269 - 271.
Chester, F. D. The spotting of peaches. Delaware Agr. Exp. Sta.
Rept. 8: 60 - 63. 1896.
Arthur, J. C. Spotting of peaches and cucumbers. Indiana Agr.
Exp. Sta. Bul. 19: 5-10. 1889. Scott, W. M., and Quaintance, A. L. Spraying peaches for the control of Brown Rot, scab, and curculio. U. S. Agr. Dept. Farmers' bul.
440: 6-40. 1911. Scott, W. M., and Ayers, T. W. The control of peach Brown Rot and scab. U. S. Agr. Dept. Plant Indus. Bur. Bul. 174: 7-26. 1910. Selby, A. D. Peach scab. Ohio Agr. Exp. Sta. Bul. 92: 220 - 222.
1898. Blake, M. A., and Farley, A. J. Peach scab. New Jersey Agr. Exp.
Sta. Bul. 236:4 - 30. 1911.