Diseases of Berries in the East

W. F. Jeffers.

Growers of raspberries, blackberries, and dewberries usually appreciate the severe losses that diseases may cause but often they are not aware of the ten percenters the less severe ailments that may make the difference between a profitable crop and an unprofitable crop.

With the present high cost of production and premium prices for high-quality berries, it is more essential than ever before that growers follow sound disease-control practices. Just as they make careful plans for fertilizing, pruning, and harvesting, they should take steps to prevent and stop diseases before it is too late. The alert grower of bramble crops learns to identify the diseases in order to insure profitable production.

MOSAIC is the most widespread and common virus disease of raspberries, yet so many symptoms and effects are associated with mosaic that it is hard to describe properly.

Apparently several different but related viruses can cause mosaic. Plants infected with a mixture of mosaic viruses usually show symptoms different from those infected with only one.

Two main types of mosaic diseases occur on raspberry. Green mosaic causes a mottled pattern of light and dark green areas in infected leaves. Yellow mosaic causes bright-yellow discoloration of part or entire leaves. Both attack black and red raspberries. Control measures for them are about the same.

Black raspberries generally respond to mosaic infection by a gradual reduction in growth and fruit production. Leaves are smaller in size, usually misshapen, and mottled. Severe infection may cause considerable puckering of leaf tissue between the veins so that blisterlike spots are formed. The entire plant may be severely stunted, and fruit will be small, dry, and nearly tasteless.

New canes often are somewhat stunted and have brittle tips. Leaves on infected canes often are bunched near the tip because of compacting of the new growth. Infected plants may produce fair crops of fruit for a few seasons after planting, but after that they are usually of little value.

The causal virus permeates roots, stems, and leaves. Therefore even though tip layers might appear healthy they will later develop symptoms of mosaic. In some districts symptoms of mosaic are masked by hot summer temperatures; leaves formed then may appear normal, but during cool weather later in the season mosaic is again easily seen. Sometimes the effect of mosaic is so mild that the plants are little damaged, but when the plants are grafted to healthy plants of another variety the new plant may become severely diseased.

Black raspberries may be infected with a type of mosaic that makes the leaves yellowish green and much smaller. Infected plants usually are stunted and produce poor fruit.

Mosaic infection in red raspberries usually results in yellowish-green discoloration of the leaves and general stunting of the plant. Often leaflets are malformed and blisterlike places are produced between the veins. Leaves formed late in the summer sometimes show small yellow spots on the upper surface. High summer temperatures may mask the symptoms. Infected plants gradually decline in vigor and ability to bear good fruit.

The use of the disease-free planting stock is of primary importance in control of mosaic. Growers who have disease-free plants of good commercial varieties should try to keep them so and use such stock or reliable certified stock for new plantings.

New plantings should be at least 500 feet away from other fields of brambles. Wild or escaped raspberries in the vicinity should be destroyed.

Because only an aphid that feeds primarily on raspberries can spread raspberry mosaic, new plantings should be far enough from other brambles to be protected from this weak-flying insect. A suitable insecticide can be used to kill aphids in the bramble patch.

New plantings should be examined several times during the first growing season and infected plants should be removed. Aphids that are present should be killed before the plants are carried out of the field. Otherwise they might spread mosaic to other plants.

Most black raspberries are very susceptible to mosaic. Some varieties of red raspberries are less severely affected than others and should be grown if mosaic is a serious problem. Indian Summer, Lloyd George, Antwerp, Herbert, Marcy, and Newburgh are among the varieties that are poor hosts for the aphid vector (Amphorophora rubi). Therefore they usually are not affected by mosaic. Latham is often infected with mosaic, but usually it is less severely injured than Cuthbert, Taylor, Chief, June, and others.

MILD STREAK is the most serious disease of black raspberries in some places. A virus trouble, it does not kill plants but causes them to produce poor fruit. The disease has become serious in Maryland. It is also bad in other parts of the East, but because of its apparent mild effect on the plant it often is not recognized as a disease. Red raspberries are not affected by the streak viruses.

Streak diseases get their name from the discolored stripes on canes of black-raspberries. The intensity of the streaking varies from faint, dull lines to bright, purplish ones. Severe streak causes a more drastic purple marking, which sometimes may cause much of the stem tissue to turn bluish. Mildly infected plants generally make good growth and live as long as healthy plants, but severe streak causes stunting and often kills plants within a year or two after infection.

THE FRUIT of infected plants looks dull, somewhat shrunken, and dry healthy fruit is bright and plump. Fruit from diseased plants is smaller than healthy fruit and shatters more readily a serious loss of quality and quantity that means lower returns to the grower.

Leaves of plants affected with mild streak often curl downward early in the season. Lower leaves of affected canes may die later in the summer and remain hanging on the stalk. Tips of new canes often are somewhat more curved than healthy tips. Clearing of the veins of lower leaves also may occur in streak-affected plants.

All or only a few canes in a hill may be streaked. In the Cumberland variety, cane streaks usually appear as dull, gray discolorations resembling areas where the normal bloom has been rubbed away. Later in the season the markings may become slightly reddened, but by fall they generally cannot be detected. Cane symptoms first become apparent soon after new growth begins in the spring. In the variety Naples the streaks are greenish brown. In Logan they are usually more of a violet or purple.

Fruiting laterals do not generally show symptoms of streak as well as new canes do, but some seasons they may clearly show the typical streaking.

One should remember that at times other than the fruiting season plants affected with mild streak may appear quite healthy to persons unfamiliar With the disease. Growers unknowingly may thus use infected tip layers in setting new plantings.

In striking contrast to mild streak, severe streak has a drastic effect on the plant. It causes new canes to be stunted and blue in color and to have curved, misshapen leaves. Such plants are usually noticeable and are removed by the grower or else they die within a year or two. Mild streak is by far the most serious of the two streak diseases, for infected plants live many years and are difficult to detect. Thus a planting can become badly diseased within a few years and produce nothing but poor fruit. Growers are reluctant to remove plants that appear healthy and continue to care for a badly diseased planting year after year in the hope that the quality of the fruit will improve.

THE USE of disease-free planting stock is of basic importance in controlling mild and severe streak. There is generally not much danger of severe streak in commercial planting stock as it is readily detected. Because mild streak is so hard to recognize, however, there is difficulty in certifying for freedom from the disease. Mild streak fortunately is not widespread and healthy plants can be had, Growers should learn to recognize the disease and obtain planting stock from areas where the disease does not occur. Careful inspection of fields at fruiting time will disclose the presence of disease, and healthy fields can be selected for planting stock. Recently infected plants cannot be recognized because symptoms usually take several weeks before they appear; therefore planting stock should not be taken from a field showing any amount of mild streak.

Selection of an isolated planting site is important in control of streak diseases. No vector has been proved to spread mild streak, but quite likely an insect is responsible. New plantings at least 500 yards from other raspberries generally can be kept free of the disease, while infection may build up rapidly in plantings next to raspberries. Mild streak occurs frequently when blackberries grow near raspberries. The two crops should therefore be separated. All wild or escaped brambles near raspberry plantings should be destroyed.

If a few streak-infected plants are present in a planting, they should be burned in place or covered with a burlap sack before being removed. That is necessary to prevent possible spread of a streak by insects from plants that would be carried through the patch.

VERTICILLIUM WILT, or blue stem, of raspberry is caused by a fungus (Verticillium albo-atrum) that lives in soil. The same fungus attacks tomato, potato, pepper, eggplant, blackberry, maple, barberry, common daisy, groundsel, pigweed, and other plants.

Because the fungus can attack part of a raspberry plant or the entire root system, the disease causes varying degrees of injury. Plants usually are not killed in a single season, but infection becomes increasingly severe for several years until the entire plant is killed. The main reason is that raspberries make many canes, which act almost as separate plants.

First symptoms of wilt appear in early summer, when the lower leaves of new canes, instead of being in prime condition, turn yellow and die. Soon other leaves begin to wilt and the cane turns blue, wilts, and dies. The blue color may involve the entire stem or may be in the form of one or more blue stripes extending up the stem. Several new canes may be affected or in more severe cases the fruiting wood may likewise wilt and die. Infected fruiting canes may fail to mature properly, and small, dry fruit results.

A characteristic symptom is evident when the lower stem or roots of a recently wilted plant are cut. Such parts will show a distinct brown discoloration of the woody tissues. Roots may be killed and rotted by the disease.