The pear leaf disease here discussed is better known as Leaf Spot, Septoria Leaf Spot and ashy Leaf Spot. It was first described in the United States in 1897. During that and the succeeding year it was a prominent and important pear trouble. In fact, the disease is usually common every year. The ravages of the disease have always been confined to the eastern portion of the United States, whereas in Europe it is widespread. Orchard trees and nursery-stock suffer alike. A few cases have been found on apple - foliage.
This leaf disease is capable of bringing about considerable damage to the pear. Leaves are frequently so badly infected that premature defoliation takes place in August or earlier. This results in checking the growth of the affected tree and prevents the proper ripening of the buds and shoots. This untimely defoliation may also cause the tree to put out new growth which cannot harden before winter. Thus the tree is made sensitive to winter injury.
Some varieties of pears are more affected than others. It z was observed several years ago that the Bosc, Sheldon, Seckel, Anjou, Bartlett and others are injured more than the Flemish, Clairgeau and Duchess. The Kieffer, Lawrence and Mount Vernon are relatively resistant to Leaf Spot. In the nursery, budded pear - stock, after the first year, may show occasional lesions, but budded stock of two or more years is often badly injured late in the summer. Symptoms.
Only the leaves are affected. The mature spots are recognized by their well-defined angular margins and their grayish white centers in which a few pyc-nidia develop (Fig. 93). They are found on the upper surface. The formation of a spot proceeds as follows. The affected tissue becomes brownish, which discoloration enlarges until the mature size of the spot is reached. These usually measure about one-sixth of an inch or less in diameter. The margin, while sharply defined, is angular and crenulate. With age the brown color at the center turns grayish white and becomes somewhat transparent. About this light-colored central area is a brown zone which merges into a purplish zone and finally into the healthy green portion of the leaf. Ordinarily lesions are not abundant, but under exceptionally favorable conditions numerous spots develop over the leaf-surface. Several may merge, involving large areas of leaf - tissue.
Fig. 93. - Mycosphaerella Leaf Spot on pear - leaves.
The causal organism is the fungus Mycospksrella sentina; it is better known as Septoria Pyricola, although the former name is now accepted.
The fungus passes the winter in fallen leaves which were diseased the previous summer. From these leaves ascospores are discharged into the air, and they are then carried to the new leaves. Within about fifteen days spots will become visible to the naked eye. During this period the fungus spores have germinated, the germtubes have penetrated the leaf, and the developing mycelium has established a food relation with the pear leaf-cells. The attacked cells are killed, and in a few days after the lesion becomes evident summer fruiting bodies begin to appear. They are formed beneath the upper surface of the leaf. They are composed of interwoven hyphal threads and break through the leaf-surface at maturity. When thus exposed they appear as black, flask-shaped bodies and are easily seen with a lens or the naked eye. Within two weeks after the effects of the fungus on the leaf begin to show, countless mature pycnospores are developed within the pycnidia. When the fruiting structure is moistened, these spores coil out through the mouth of the pycnidium and are readily disseminated by the rain. They fall on other leaves and new infections result. This repetition continues throughout the summer. In the autumn the leaves fall to the ground, and those affected by the Leaf Spot fungus carry the mycelium with them in their tissues. In the fallen leaves the fungus becomes saprophytic, living throughout the winter on the dead tissues. With the advent of spring perithecia are formed, within which ascospores are matured for the first infections. It is held by some authorities that the pycnospores may winter over. The evidence on this point is as yet incomplete.
Neglected trees are worst affected. It has been found that spraying the pear foliage with bordeaux mixture 4-5-50 or lime sulfur solution 1-50 will satisfactorily control Myco-sphserella Leaf Spot. Three applications are sufficient in the orchard; (1) just after the petals fall; (2) two weeks later; (3) two weeks after the second application. In the nursery it is recommended that two-year-old budded pears be sprayed shortly after the new leaves open. Subsequent sprayings should be made at two-week intervals; three sprayings in all will prove profitable for two-year-old budded stock. But since one-year - old budded stock is rarely attacked to a damaging extent, it is regarded as unprofitable to spray such trees for this disease.
Stewart, V. B. Some important leaf diseases of nursery stock. Septoria Leaf Spot of the pear. Cornell Univ. Agr. Exp. Sta. Bul. 358:216 - 219. 1915.
Barrus, M. F, Diseases of pears. Leaf spot. In The Fruit Industry in New York State. New York Agr. Dept. Bul. 79:1049 - 1050. 1916.
A roughening of the bark of both pears and apples is exceedingly common in eastern United States. (See Apple, page 130.)