Scab on the Pear

J. R. Kienholz.

Pear scab is a disease of world-wide importance. Fungicides have to be used every year to control it in all but a few and districts. During seasons favorable for scab infections, losses of 20 to 30 percent of the fruit are not uncommon, and if control measures are neglected, practically all of the fruit may be unmarketable.

The scab fungus (Venturia pyrina) may attack the flower parts, fruit, leaves, or young twigs. Wet weather, particularly early in the growing season, is probably the greatest factor in determining the increase or decline of pear scab from year to year. A closely related fungus (V. inaequalis) causes apple scab, but it cannot infect pears, and the pear scab fungus cannot infect apples. The observations reported here pertain mainly to the conditions in the Pacific Northwest.

ABOUT THE TIME the petals fall in the spring, dark velvety or sooty spots may appear on young fruits. Infections may occur on the leaves, the fruit stems, the calyx lobes, or even the flower petals. If the fruit stems are attacked, the young fruits are usually weakened and drop. If fruit growth continues, a badly misshapen pear results. The scab spots usually enlarge as the fruit grows until arrested either by hot, dry weather or by fungicidal sprays. Early infections eventually may involve the whole side of a fruit; later infections may result in numerous smaller spots. The central, or older part, of an early scab spot may die and leave a corky or russeted skin area. An olive-brown rim of active fungus tissue often remains alive at the edge of the spot. This tissue may produce new spores and the spores in turn cause numerous new infections during wet weather.

Leaf infections are like those on the fruit, but they usually appear slightly later. They are more common on the lower surface of the leaves than on the upper. Individual spots rarely become more than three-fourths inch in diameter and appear black, because of the compact growth of the fungus. Sometimes the fungus threads radiate in less compact masses to produce a fernlike, irregular scab patch. Single scab spots are common on the leaves. They often envelop the midrib or leaf veins and cause considerable puckering or twisting of the leaf. As the scab spots mature and dry out, they may crack or tear, leaving a jagged, black-rimmed hole in the leaf. Less food is manufactured for the tree by scabby leaves. Numerous infections may cause the leaves to weaken, dry up, and drop prematurely. Continued attacks of the disease will devitalize the trees.

New twig growth on pear trees may be infected at any time during the growing season. Most infections result when rains are frequent. During spring and summer new twig lesions are inconspicuous. They appear merely as small, blisterlike pustules, no larger than a pinhead. Sometimes the fungus remains invisible within the twigs as a latent infection. Occasionally a shallow, spore-producing layer is formed. A corky layer often forms beneath the twig infections and many are sloughed off before autumn, leaving small depressions on the twigs. If the twig infections are not sloughed, the fungus remains dormant in the twigs until the following spring, when renewal of tree growth causes the fungus to become active again. The fungus breaks through the epidermis of the twig, forming a scab pustule. Numerous scab spores mature in the pustules, usually before the blossom clusters separate. The overwintered twig lesions also are generally sloughed off during the growing season, but occasionally a few remain to form spores the next season. Old sloughed lesions may appear in 2-to 5-year-old branches as small crater-like depressions.

THE PEAR SCAB FUNGUS has two distinct stages in its life cycle. Throughout the growing season the fungus lives as a true parasite within the pear tissue. After primary infections occur in the spring the fungus pushes or breaks through the surface of infected fruit or leaves, and produces summer spores (conidia). These spores are formed in great numbers when weather conditions are suitable. They are dispersed by rain and wind to cause new infections. Each spore, if moisture and temperature are favorable, can develop a tiny germ tube, which may enter the host tissue and cause a new infection. These processes may occur many times during the season and numerous secondary infections may result.

The summer or parasitic stage may also overwinter in infected twigs. Because the spores are generally mature before the pear buds unfold, it is the most important source for primary infections the following spring.

The fungus also overwinters as a saprophyte on the infected leaves that drop in the fall. Thick-walled, flask-shaped, beaked structures, known as perithecia, are formed within the old leaf tissue during the winter. Numerous tiny, saclike organs, called asci, containing eight small, two-celled spores (ascospores), are formed within the perithecia. The ascospores become pale olive green as they approach maturity, about the time the pear buds are unfolding. When moisture and temperature conditions are favorable, they are discharged through the beaks of the perithecia into the air. Air currents carry the spores to the trees, where they may germinate and cause primary spring infections. Those infections soon produce the summer spores, and secondary infections result when conditions are favorable for the fungus.

Scab spores are disseminated only during moist periods. Continual moisture for 5 to 48 hours is necessary before the spores germinate and infect susceptible pear tissue. If the temperature during the rainy period is about 75 F., a spore may germinate and infect the plant within approximately 4 to 5 hours. At lower temperatures moisture must persist for a longer time, so that at 40 , a wet period of approximately 48 hours is required before infection occurs. After infection has taken place, 12 to 25 days are required before the fungus grows enough to become visible to the naked eye. When scab spots suddenly appear on fruit during dry weather, it must be remembered that the infections occurred during a previous rainy period.

MOST OF THE COMMERCIAL pear varieties commonly grown are susceptible to scab infections. Fruits of Anjou, Bartlett, Cornice, Winter Nelis, Easter, Forelle, Seckel, and Flemish Beauty often are severely affected. Bose fruits are very susceptible in the young stage, but about the time they shed their pubescence they become highly resistant. Frequently a variety only lightly attacked in one district may be the most seriously affected in another. The first spray is generally timed according to pear bud development. Since a pollinizer variety like Easter unfolds its buds earlier than Anjou, the early spray, if timed for the Anjou variety, may allow some infections on exposed Easter buds. If timed for the Easter variety, the spray is too early to be fully effective on the Anjou variety.

Twig infections have never been observed on Bartlett in the Hood River Valley of Oregon. Yet in districts with higher rainfall twig scab is common on the variety. Climatic conditions, fertilizers, pruning practices, soils, cover crops, and other factors all have a part in modifying growth characteristics of pear trees and their susceptibility or resistance to the scab fungus.

The fungus also varies its attack on pears. G. W. Keitt, at the Wisconsin Agricultural Experiment Station, determined in greenhouse tests that at least two strains of the scab fungus exist in Oregon. One strain attacks Bartlett but not Anjou pears. Another infects Anjou but not Bartlett. This condition has been observed under field conditions in Oregon. Probably other strains of the fungus exist in nature, but their relationships have not been definitely demonstrated.

Trees should be well spaced and pruned to give an open type of growth so that proper aeration occurs within the trees and to allow thorough spray coverages to be applied. Eliminating excessive water-sprout growth during the summer is often profitable, because it reduces the chance for additional twig infections. Early spring cultivation, to bury the old infected leaves, will lower the amount of the overwintering fungus, which may cause primary infections. Heavy cover crops often prevent rapid evaporation of moisture in an orchard, thereby extending the infection time for the scab fungus. On the other hand, sprinkling orchards during dry periods has not materially increased the scab problem.