This disease, which has been called scald and Leaf Blight, is known in nearly all countries where the pear is grown. It is recorded from Germany, Sweden, Italy and France, and is especially common in eastern United States. In New York it is most abundant in the Hudson Valley and in the western part of the state, but perhaps even in these regions is less important than farther south and west. It is agreed that pear seedlings are most seriously affected; in some nurseries their cultivation has been greatly hindered, and in certain cases their growing has been abandoned. In Europe the Leaf Blight has been known for at least a century, while in the United States it has been the subject of horticultural writings and discussions for many years. Very recently it was found on pears in Australia. As already indicated, the disease is found more commonly in nurseries than in orchards, yet the latter are not exempt from attacks of the Leaf Blight pathogene. In the nursery the trouble affects the leaves and twigs of seedlings; in the orchard, the fruits (Fig. 98), in addition, are susceptible. It appears that all varieties show the disease, yet the Kieffer and Angouleme are more resistant than the Seckel, Wilder Early and Sheldon. It has also been observed that the Flemish, Lawrence, Bosc and Clapp Favorite are resistant. Symptoms.
The disease makes its appearance early in the spring soon after the leaves develop. There develops a small circular, carmine-red spot, first on the upper, and then penetrating to the lower, surface. The color soon changes from red to dark - brown, with a slightly elevated, minute black spot in the center. If the lesions are numerous, they may merge, and thus the tissue between them turns brown. Affected young leaves shrivel; older ones change only in color. Badly diseased leaves turn yellow and fall prematurely.
The fruit shows the same carmine - red spot which afterward assumes a darker color (Fig. 98). The skin is roughened, and the growth of the epidermis is hindered, causing a deep crack in the flesh.
Fig. 98. Leaf Blight lesions on pear - fruit.
The twigs, petioles and leaf - scales also exhibit signs of the disease similar to those on the foliage. The lesions on the twigs, however, are more elongated and become depressed, and finally girdling results.
The Leaf Blight disease is caused by a fungus, Fabrcea maculata. Throughout the summer its conidia, developed upon the leaves, fruits and twigs, are scattered to other leaves, where infection results. It may be that certain of these conidia pass the winter on diseased twigs, producing infections the following spring. The chief method of hibernation, however, is by means of apothecial bodies in fallen leaves. Infected leaves, either at maturity or prematurity, fall to the ground in the late summer or autumn, carrying the fungus with them. Further activities on the part of the pathogene in these old leaves result in the formation of apothecia. In the spring, ascospores are discharged from these fruiting structures, and are carried to the susceptible parts. About one week later, the effects of the work of the parasite within the tissues are visible to the naked eye; and within a month, or less, after the ascospores are discharged, a new crop of spores, conidia, is developed. These conidia are borne in acervuli which are to be found in the center of each spot. Conidia from this source may infect the quince; likewise, the conidia from the quince may infect the pear. This is an adaptation on the part of the fungus which renders its control more difficult. Furthermore, these conidia may infect the hawthorn, apple and other closely related plants, thus adding complications to the application of remedial measures. All these plants, the quince, the apple and others, must be regarded as a source of trouble to the pears.
For the orchard trees and pear stocks, spraying is profitable and effective. Lime sulfur 1 to 50 may be used with safety and with success. It is recommended that iron - sulfate be added to the above, at the rate of 3 pounds to 50 gallons, to increase the adhesiveness of the fungicide. The first application should be made soon after the first leaves develop. There should be four or five applications subsequent to the first, depending upon the season. In seasons with only moderate rainfall an application should be made once in every three weeks.
Galloway, B. T. Leaf Blight and cracking of the pear. U. S. Agr.
Cornell Univ. Agr. Exp. Sta. Bul. 145: 611-615. 1898. Barrus, M. F. Diseases of pear. Leaf blight. In The Fruit Industry in New York State. New York Agr. Dept. Bul. 79: 1050 - 1051.
1916. Kinney, L. F. The leaf blight of the pear. Rhode Island Agr. Exp.
Sta. Bul. 27: 3-7. 1894. Kinney, L. F. The leaf blight and cracking of the pear. Rhode Island Agr. Exp. Sta. Rept. 1894: 189-192. 1895. Waite, M. B. Treatment of pear Leaf Blight in the orchard. Journ.
Myc. 7: 333 - 338. 1894. McAlpine, D. Leaf scald or fruit spot. Victoria Agr. Dept. Journ. 9:
512 - 515. 1911. (See additional references under Quince, page 390.)