Fruits and Nuts

Scab of Apples

G. W. Keitt.

Apple scab occurs throughout the world where apples are grown, except in very dry or warm climates. It is most severe in localities with cool, moist springs and summers, as in parts of the north central and northeastern United States and southeastern Canada and northwestern Europe.

In most of the important apple-growing areas, efficient and economical control of scab is one of the chief essentials for successful growing of apples. If scab is severe, crops of the more susceptible varieties may be a total loss unless control measures are applied. Even the best of the control programs now most widely used may fail to give satisfactory results when the disease is very severe or conditions hinder the application of fungicides.

Apple scab is caused by the parasitic fungus, Venturia inaequalis. Little is known about the history of the disease before the modern study of fungi and the discovery of their ability to cause diseases of plants. The scab fungus was first reported and named in Sweden in 1819. It was first reported from the United States in 1834, Probably the scab fungus is an age-old parasite that has become an increasingly important agent of disease as the apple has been modified under culture and grown in large plantings.

The fungus attacks the cultivated varieties of apples and crab apples and numerous other species of the genus Malus, including the common wildcrab apples M. coronaria and M. iowensis. It is not known to attack plants outside the genus Malus. Other species of Venturia, however, cause similar scab diseases of plants of genera closely related to Malus, such as Pyrus (pears) and Crataegus (hawthorns).

In most parts of the United States and Canada the scab fungus attacks only the leaves, blossoms, and fruits of the apple plant, but in some cool, moist climates, the young twigs and bud scales also may be infected.

Upper left: Apple scab, a common disease rendering the fruit worthless for market. Upper right: Bitter rot can destroy the entire apple crop within a few days. Lower left: Apple blotch, a midsummer disease, makes characteristic spots on fruits. Lower right: Black rot, a secondary fungus, is common on the fruit at harvesttime.

SCAB MAY OCCUR on both surfaces of the leaf blade and on the midrib and petiole. It often appears first on the under surface, because it is first exposed to infection as the buds break and the leaves unfold.

The scab spots lesions usually appear first as small, olive-colored areas, which increase in size and may darken with age. They often take on a velvety appearance on account of the abundant production of spores on the ends of short, erect, threadlike branches of the fungus. The earliest lesions are largest, sometimes one-half inch or more in diameter. Later lesions tend to be smaller, because resistance to the disease increases with the age of the leaf.

Some of the spots are fully covered by the fungus growth and have a definite, round margin. Others show a radiating and netlike pattern of fungus growth with a less definite margin. As the leaf attains its full size, the upper surface usually becomes resistant to infection; the midrib remains susceptible somewhat longer than the leaf blade. The under surface usually remains susceptible throughout the season, but the development of the fungus in the mature leaf may be so restricted that the individual lesions are not easily recognized. These late, under-surface infections may be important for development of the fruiting bodies of the fungus that produce ascospores, which start infection in the following spring.

The larger scab spots, especially on the upper surface of the leaf, may become brown, except at the margins. That happens when the fungus in the middle portions of the spots dies. Sometimes the leaf tissues under the spots are killed. Sometimes the leaf tissues remain alive throughout the season. If infection is abundant, the spots may merge. Severely infected leaves may be shed. As will be seen later, leaf infection is of great importance to over-wintering of the fungus and to the development and the control of scab in the following season.