Rust Diseases of Apple

D. H. Palmiter.

Many a fruit-growing district in the East is "a land of red apples and red cedars." In the Midwest where cedars did not occur naturally the pioneers often planted them for windbreaks near orchards. The combination would be felicitous were it not that some fungi require both apples and cedars for their development.

Before our cultivated apples, quince, and other pome fruits were introduced from Europe, three such fungi, natives, infected the leaves or fruit of native crab apples, hawthorn, serviceberry, and sorbus, all members of the rose family. They could not overwinter on those hosts, however, and used the cedar trees Juniperus virginianae, J. scopulorum, J. horizontalis, and J. communis as winter hosts.

Since three different fungi are responsible for three different rust diseases of apples, we must know something of their life history and symptoms on both apple and cedar before we can control them.

The best known of the three diseases is cedar-apple rust. It produces yellow lesions on the leaves and fruit of wild and cultivated apples and is caused by the fungus Gymnosporangium juniperi virginianae.

A similar disease is hawthorn rust. Its common native host is hawthorn. It also infests apple leaves but not the fruit. It is caused by the fungus G. globosum.

The third disease is quince rust. It is destructive on quince. Its native host plants include hawthorn, Amelanchier, and wild apples. It also infects the fruit of some varieties of apple. The fungus is G. clavipes.

The three fungi were studied early in the nineteenth century. At first the forms on the cedar and on the apple were considered to be different. By 1886, however, more study showed that the spores produced by the fungus on cedar trees would infect apples and that the spores produced on apples were responsible for new infections on the cedar. The rust diseases cannot spread from cedar to cedar nor from apple to apple but must alternate between the two hosts.

The fungus that causes cedar-apple rust spends almost 2 years of its life cycle on the cedar trees. Cedar leaves are infected between July and April by aeciospores blown from lesions that develop on apple leaves. Small brown galls, called cedar apples, appear during the summer but do not mature until the following spring, when they may be 2 inches in diameter. After a few warm spring rains, about the time apple blossoms are in the pink stage, the galls increase in size and extrude gelatinous tendrils from round depressions on all sides of the galls. The tendrils long, thin, and bright orange, form a mass several times that of the original brown galls. The tendrils, or horns, consist of many two-celled teliospores, which germinate in the tendrils by producing four sporidia from each of the two cells. All teliospores do not germinate at the same time. With each rain the horns push out farther and expose more spores. When the supply of teliospores is exhausted the galls dry up and may drop. If the twigs bearing the galls are killed, the dead galls often remain hanging on the cedars through the next year.

After the small sporidia are produced, a decrease in humidity causes them to be discharged forcibly into the air. Air currents carry them considerable distances. Viable spores have been collected by airplane traps several miles from red cedars at altitudes up to one-half mile. They may eventually settle on apple leaves or fruit. If a film of moisture is present and the temperature is between 56 and 61 F., the sporidia germinate quickly and penetrate the host tissue in 1 to 3 hours. Little germination occurs at temperatures below 47 or above 85 . Either leaf surface may be infected. Fruit lesions are usually near the blossom end.

The yellow rust spots appear on the upper surface of the apple leaves in 1 to 3 weeks, depending on the temperature and the susceptibility of the variety. As the spots increase in size, a sticky exudate containing pycniospores appears. The true function of this spore stage was not known until 1933, when investigations by J. C. Liu at the University of Wisconsin showed that insects are attracted by this exudate. They carry spores from one rust spot to another and thus fertilize the fungus. Thus it continues growth and completes its life cycle on the apple tree by producing the final spore stage.

After fertilization, the I fungus grows through the leaf and produces fruiting bodies, called aecia, on the lower surface of the leaf. Fruit of susceptible apple varieties also may be infected and spores may be produced. Aeciospores are produced in thin-walled tubes. In July and August the spores mature and are released by the splitting of the tube walls. They are carried about by the wind. Those that land on cedar leaves may germinate at once to start galls or may remain dormant until the following spring, when they germinate to infect the spring, growth. Now that better control methods are available, few spores are produced on orchard trees. Infection of cedar trees is largely from spores produced on wild or seedling apple trees growing near the cedars.

THE LIFE HISTORY of the hawthorn rust fungus is about the same as that Of the cedar-apple rust fungus. The galls produced on cedar trees are more irregular in Shape than the apple rust galls. The teliospores are extruded as dark-orange, wedge-shaped protrusions during spring rains. Sporidia may be discharged for 2 or 3 days while the galls are drying. Unlike the apple rust galls, they persist and produce spores again another year.

Upper left: Mosaic, a virus disease of the apple tree. Upper right: The scab fungus forms dark-brown, moldlike patches of fungus growth on apple leaves. Lower left: Spots, caused by rust infections, on apple leaves. Lower right: Rust galls on cedar trees expand in the spring, forming orange masses covered with rust spores.

The hawthorn rust fungus does not cause lesions on apple fruit. The lesions on the upper surface of apple leaves are similar to those of the apple rust fungus but smaller. On the lower leaf surface, the two fungi look quite different. With the hawthorn rust the aecia are relatively few in number and are at the center of the lesion and surrounded by a region of orange-colored leaf tissue. The peridial tubes, in which the spores are formed, are persistent and long and spread apart to release the aeciospores. They do not curl back, as do those of the apple rust fungus.

The aeciospores of the hawthorn rust are mature in the fall. The wind distributes them. They do not germinate well until cold weather approaches. Some spores perhaps do not germinate until the following spring, when new galls on the cedars are started. If no apple orchards are nearby or if orchards are well sprayed, aeciospores from infected hawthorn trees may serve to reinfect the cedars.