Blotch of Apples

John C. Dunegan.

The fungus Phyllosticta solitaria causes blotch, a serious disease of apples.

It has not been found in the far West, but east of the Rocky Mountains it has been observed in all apple sections, from New Jersey west to Nebraska and southward to the southern limits of apple growing. It causes most trouble in southern orchards.

The fungus attacks the leaves, fruit, and the current season's twig growth of the apple tree. The following spring the fruiting bodies pycnidia formed on the twig cankers ooze many small, colorless, one-celled spores. Raindrops wash therm the new leaves, fruit, and shoots, where they germinate and start anew the cycle of infection. The spore discharge from the over-wintered cankers occurs 3 to 9 weeks after the petals have dropped from the blossoms.

The fruit becomes resistant by the middle of the summer and is seldom infected late in the season, but the Young leaves and shoots, especially water sprouts, remain susceptible throughout the growing season. The fungus remains alive in the twig cankers at least 3 years, and each spring produces a new crop of spores that infect the new growth.

Varieties vary in susceptibility. McIntosh, Maiden Blush, and Northwestern Greening are very susceptible. Rome Beauty, Wealthy, Yellow Newton, and Yellow Transparent are moderately susceptible. Delicious, Golden Delicious, Grimes Golden, Jonathan, Stayman Winesap, and Winesap are quite resistant, but their resistance is only relative and occasionally the fungus can seriously affect them. Ben Davis, once popular in the Midwest, and Oldenburg are extremely susceptible.

Apple seedlings grown for the production of nursery stock can be infected by the blotch fungus. The disease formerly was introduced into new areas in diseased nursery stock. That situation has been improved by spraying the seedlings with fungicides or by growing them in the Pacific Northwest.

Two types of blotch symptoms occur on apple leaves. Tiny white spots (one-sixteenth to one-eighth inch in diameter) form in the blade tissue between the veins; on veins and leaf stems petioles, the fungus produces elliptical, sunken, tan or buff lesions. Usually only one black dot the pycnidium or spore case forms in the center of each white spot. A variable number form in the buff-colored lesions on the veins and petioles.

Often the leaves remain uninfected even though the fruit and twigs show many infections. The white spot phase ordinarily does little damage, but the infection that develops near the base of the petioles frequently defoliates the trees by midsummer. The fungus may grow from the petiole infections into the adjacent twig tissue, causing a canker to form at the leaf scar.

Blotch infections develop on the twigs, water sprouts, and fruit spurs of the current year's growth. They first appear as dark, raised, roughly circular spots, studded with tiny projections caused by the formation of the pycnidia within the shoot tissues. As I mentioned, some of these infections are the result of the fungus passing from the leaf petioles into the twig tissues. Others, particularly those on the stem between the leaf scars, are the result of infections from spores.

The infections from spores are particularly numerous on the succulent, rapidly growing water sprouts. The center of the canker soon becomes slightly sunken and brown to tan. In the second year this central or older part of the canker is surrounded by a dark border of varying, width, indicating the extent of the advance of the fungus. Pycnidia are formed in this area. During the third season an additional boundary zone forms. Usually by then the canker becomes irregular and roughened. If the infections are numerous, several cankers may coalesce into roughened, cankered areas several inches long. They may eventually girdle the twigs.

As the fungus does not penetrate deeply into the twig tissues, the lesions are soon separated from the living tissues by a callus layer. The dead tissues finally crack into fragments and are sloughed off as the result of the tensions set up by the normal increase in diameter of the twigs. This process takes time and the roughened scar tissues can be seen encircling the larger twigs and branches for several years after spore production has ceased.

On the fruit the symptoms are quite dissimilar at different stages in their development. The common name of the disease refers to the mottled or blotched appearance of the fruit.

The earliest symptom (but one not frequently seen because it persists so short a time) is the isolated, usually dark-colored and semi-hemispherical, raised or blisterlike areas, one-eighth inch in diameter, on the young apples. The spots appear in late May or early June and slowly enlarge. Usually they develop fringed. margins. By midsummer the spot is slightly raised and dark and has irregular but distinct margins. Frequently the spot at this stage has a star-like appearance. Later several spots may merge, a depression of varying depth and diameter develops, and the fringed margins, so characteristic of the spots in midseason, disappear. If numerous infections are on the apple at the beginning of the season, their subsequent growth and fusion may cause unsightly dark areas, which cover one-half or more of the surface of the apple and completely destroy its market value.

The fungus does not produce a rot of the fruit tissues, because only the outer layers of cells are invaded. Badly infected fruit may crack, usually in three directions from a central point. Secondary fungi then can enter and rot the apple.

The control of apple blotch must have as its ultimate aim the elimination of the twig cankers, for they are the agency by which the fungus perpetuates itself from one season to the next. Spray materials used to inhibit the formation of the cankers also protect the leaves and fruit.

As the cankers can be present on young trees, all trees for new plantings should be thoroughly inspected as they are received from the nursery to be sure the disease is not being introduced at the time the orchard is established. The young trees should be thoroughly inspected again the year after they are planted. Any cankers that escaped the previous inspection can be removed by pruning, if they are in the smaller twigs, or shaved off with a sharp knife, if they are on the larger limbs or on the trunk of the tree.

Max W. Gardner demonstrated in Indiana that the development of the fungus could be retarded in young orchards by those measures. But the procedure is practical only during the first few years in the life of the orchard, for the trees soon get too large to permit the careful inspection needed.

On older trees many cankers can be eliminated by normal pruning operations. Trees that have been neglected for several years must be pruned thoroughly to remove the cankers and permit proper spraying.

Cutting out the cankers on the young trees and thorough pruning of older trees are helpful control measures, but at best they are only supplementary aids. The control of the disease on the leaves and fruit and the elimination of the cankers depend mainly on the use of fungicidal sprays.

During the period of spore discharge from the overwintered cankers, which starts approximately 3 weeks after the petals have dropped and continues for about 6 weeks, the young leaves, fruit, and twigs must be protected from infection. Many field experiments have shown that lime-sulfur solution and the wettable sulfur preparations used to control the apple scab fungus are only partly effective in controlling the blotch fungus. The standard procedure therefore, is to use bordeaux mixture (4-4-100) or ferbam (ferric dimethyl dithiocarbamate) at the rate of 2 pounds in 100 gallons of water. The first spray should be applied 2 weeks after the petals have fallen, followed by three applications at intervals of 2 weeks or only two applications at 3-week intervals.

If properly applied, those sprays will protect the leaves and fruit from blotch infections and prevent new cankers from forming on the current year's growth. Because the cankers formed the preceding season will ooze spores for at least two more seasons, however, the spray program must be carried on for at least three or four seasons. After that the fungus can be kept under control by one or two sprays with materials at half the strength I recommended each season.

An objection to the use of bordeaux mixture to control blotch is the injury that is apt to accompany the continued use of a copper-containing spray. In some seasons that injury to leaves might be worse than the injury caused by the blotch fungus. In other seasons the use of a copper spray 2 weeks after the petals have dropped may cause serious russeting of the fruit. Ferbam, therefore, is preferred. On light-colored apples it may leave a dark residue that must be removed, but that is a minor drawback, compared to the injury that may result from the use of bordeaux mixture.

JOHN C. DUNEGAN is a pathologist in the Bureau of Plant Industry, Soils, and Agricultural Engineering.