Pear rusts occur generally throughout the world. In the United States two important rust diseases of pear exist, but fortunately for all concerned, the one is confined to the eastern part of the country, while the other (see page 345) occurs only on the Pacific Coast. They are accordingly named eastern rust and Pacific Coast rust.
The rust disease here considered is very similar to the apple rust (see page 63). As already indicated, this trouble has a wide range over the United States east of the Mississippi River. It has been recorded from Connecticut, Massachusetts, New Jersey, North Carolina, Alabama, Mississippi, Indiana, Wisconsin and Iowa. It is relatively rare in New York State.
Rust is destructive in that it interferes with the normal activities of the plant. In severe cases defoliation results. Affected fruits are rendered worthless, and are usually less than one - half normal size. Several Japanese varieties of pears are likely to be injured. The Worden, Bartlett, Bosc and Duchess are said to remain relatively free from the disease.
Leaves and fruits (Fig. 94) are rusted. In June, orange-colored or dark-brown spots with red borders appear on the upper surface of the foliage. These lesions measure from one-fourth to one-half of an inch in diameter. Toward fall, in August and September, the lower surfaces of the leaves exhibit at first dark spots, which lack a red border, and soon develop finger-like cups in clusters. This is known as the cluster-cup stage of the rust fungus. Fruits are affected from the time when they are no larger than a pea (Fig. 94). The cups already mentioned for leaves develop anywhere on the fruit, although they are more common near the stalk end (Fig. 94). They are orange-colored and possess fringed margins. As a result of the affection, fruits are dwarfed (Fig. 94). These symptoms may also show on the apple and hawthorn, but less commonly than on the pear. Another stage of the fungus occurs on the red cedar. Galls are produced; they are an inch or less in diameter, very irregular surfaced, being marked by brown depressed scars. In the late spring dark-brown, wedge - shaped horns, which later become yellowish, are developed.
The eastern pear rust is caused by the fungus Gymnospo-rangium globosum. Like other rust fungi it has two distinct host plants, the pear and the red cedar. If the spots of an affected pear leaf be examined, the upper surface will show small black fruiting bodies known as pycnia. These do not function in the life-history of the pathogene so far as is known, but are mentioned for their diagnostic value. They are found in the early summer. Shortly after their appearance another type of fruiting body develops on the lower surface opposite the pycnia. These structures are known as secia. Occasionally they are found on the upper side of a leaf and even on the petioles. Ordinarily they are grouped irregularly over the affected area, although sometimes they are arranged in two rows, one on either side of the midrib. Aecia may also develop on the apple, hawthorn and mountain-ash. These bodies are deep cup-shaped, with their walls split part way to the base, and are considerably larger and lighter colored than the pycnia. Within the aecia are developed seciospores which blow to and infect the red cedar. These spores may carry for a considerable distance. Cedar-apples are produced on the cedar as a result of seciospore infection. These cedar-apples are merely irregular, globoid galls measuring an inch or less in diameter. Their appearance has given rise to the popular term cedar-apples. The fungus mycelium developed from the seciospores grows within the tissues of the red cedar, stimulating them to over-growth, whence the gall. From the surface of the gall arise numerous short, beak-like horns which are composed of another kind of rust spores - the teliospores. In this rust fungus these spores may be produced year after year for several seasons from the same gall. When moist the beak - like horns become gelatinous and assume a bright orange color. In this gelatinous mass the teliospores grow out and produce a number of smaller spores called sporidia. This takes place in the spring. These sporidia are blown to the pear, apple or other host, where infection results.
Fig. 94. - Eastern rust on pear - fruits.
Spraying for pear rust is of little or doubtful value. Since the red cedar harbors the fungus, and since the fungus requires the presence of this plant in order to perpetuate itself, it is logical to destroy all red cedars within a reasonable distance. In the case of apple rust the distance is put at one mile (see page 70), within which radius all cedar trees should be eradicated.
Pammel, L. H. The cedar apple fungi and apple rust in Iowa. Gymnosporangium globosum. Iowa Agr. Exp. Sta. Bul. 84: 10 - 16.
1905. Thaxter, R. Miscellaneous notes. Rust of pears. Connecticut Agr. Exp. Sta. Rept. 1890: 98. 1891. Stewart, F. C. Notes on New York plant diseases, I. Pear. Rust, Gymnosporangium globosum Farl. New York (Geneva) Agr.
Exp. Sta. Bul. 328: 376 - 377. 1910.