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| Figure 62. Mosaic virus symptoms on raspberry leaves. Note the mottled areas of dark green and light green or yellow. |
Red and black raspberries are susceptible to numerous viruses. Raspberries probably suffer greater infection and more serious damage from viruses than any other fruit crop in the United States. Virus infection in raspberries can reduce fruit yields 70 percent or more. There are four main virus-induced diseases of raspberries - mosaic, leaf curl, streak, and tomato ringspot.
Other disorders of raspberries can cause symptoms similar to viruses. Late-spring frosts, mineral deficiencies (such as iron and nitrogen), powdery mildew, pesticide injury, and feeding by leafhoppers, aphids, and mites can all cause symptoms similar to those caused by various viruses. Positive identification of a bramble virus or virus complex cannot be based on foliar symptoms alone. Greenhouse and laboratory tests using specific scientific techniques are required for positive identification of viruses.
Mosaic
This disease is caused by a virus complex (more than one virus involved). Viruses of the mosaic complex (Rubus yellow net, black raspberry necrosis, raspberry leaf mottle, and raspberry leaf spot virus) cause the greatest reduction in growth, vigor, fruit yield, and quality of any of the bramble viruses. No raspberry plants are immune, but black and purple varieties are damaged more severely than red varieties.
The symptoms of mosaic vary considerably, depending upon the variety grown, which virus or viruses of the complex are involved, and time of year. Symptoms are most evident on new canes during cooler weather of spring and fall. Symptoms may disappear in the summer when temperatures are high. This is an important point to remember when considering control of virus diseases. Even though symptoms may disappear temporarily, plants remain infected for life. Infected canes are usually short and less vigorous than healthy canes. Leaves are mottled with yellowish or light green spots on a darker green background (Figure 62). On more susceptible varieties, leaves become puckered with large, dark green blisters surrounded by yellowish or yellowish-green tissue. Leaves that develop in hot weather may be symptomless or show only faint mosaic pattern with yellow flecks in the normal green color. Leaves formed in late summer show a fine, yellowish, speckled mottling.
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| Figure 62. Mosaic virus symptoms on raspberry leaves. Note the mottled areas of dark green and light green or yellow. |
Mosaic-infected plants are often progressively more stunted each year. In addition
to leaf symptoms, the fruit yield is reduced and may be dry and seedy (often
crumbly) and lack flavor. On black and purple raspberries, the tops of newly
infected canes often curl downward, turn black, and die.
The raspberry mosaic virus complex is spread almost exclusively by one species of insect, the large raspberry aphid (Amophorophora agathonica). The aphid is widespread and feeds on the undersides of leaves near the tip of the canes. The aphids become contaminated with the viruses and can spread the viruses to healthy plants up to a quarter of a mile or more away. The mosaic virus can also spread by commercial propagation from infected plants and movement of the diseased nursery plants.
Leaf Curl
Leaf curl is less common than the mosaic complex, but it is considerably more destructive. Infected plants are worthless and should be destroyed immediately. The yield of infected raspberries can be reduced up to 70 percent. Infected black raspberries may degenerate and die after two or three years.
Leaf curl symptoms are easily recognized. Leaves on infected plants are uniformly small, dark green, crinkled, and tightly curled downward and inward. When diseased shoots first appear, they are pale yellowish-green, but they soon turn dark green, become stiff and brittle, and usually do not branch. Each year the plant loses more vigor and is progressively more dwarfed. Fruiting laterals are shorter and more upright than normal ones. Berries on infected plants may ripen prematurely and are small, dry, seedy, and crumbly.
The raspberry leaf curl virus, the causal agent of raspberry leaf curl disease, is spread exclusively by the small raspberry aphid (Aphis rubicola). Heavy populations of this aphid can cause severe inrolling of leaves even in the absence of the leaf curl virus. Winged forms of the aphid can transmit the virus to healthy raspberries from nearby infected brambles. Wind-borne aphids may spread the disease several miles.
Raspberry Streak
Raspberry streak, caused by tobacco streak virus, is generally a minor, but widespread disease. It is presently limited to northern Ohio, western Pennsylvania, and western New York. Streak affects only black raspberries.
The most obvious symptom of the disease is numerous purplish streaks that appear on the lower parts of infected canes. Usually, the streaks are less than an inch long. Terminal leaves on infected canes are often hooked or recurved, twisted or rolled, and darker green than normal. Leaves on the lower positions of the cane may show yellowing along veins and mottling. Fruits on infected canes are smaller than normal, dull, seedy, and crumbly, and lack flavor. The individual drupelets often ripen unevenly, giving the fruit a blotched appearance.
Tomato Ringspot Virus
This virus disease occurs only in red raspberries and is widespread in the major red raspberry-producing areas of the Pacific Coast and northeastern United States. Infected plants may appear normal, but they are usually somewhat less vigorous than healthy plants. The most obvious symptom of the disease is the production of small, crumbly berries that fall apart when touched. The crumbly berry is caused by the failure of some of the tiny fruitlets (drupelets), which make up the fruit, to develop.
The tomato ringspot virus can affect many other species of woody and herbaceous plants. This virus is transmitted through the soil by the dagger nematode (Xiphinema americanum).
Control of Virus Diseases
Always start new plantings with the highest-quality plants available. Use only certified, disease-free, virus-indexed stock. Avoid obtaining uninspected plants from friends or neighbors. Select a planting site that is sunny and fertile and has good air and water drainage. Destroy all wild and neglected raspberries and other brambles located within 600 to 1,000 feet of your planting site. Five hundred feet is the minimum.
Do not plant black or purple raspberries near old plantings of red raspberries, even though the red raspberries appear healthy. Red raspberries may have latent infections. This means that they can be infected, but do not show symptoms. Even though infected plants are symptomless, the virus can still be transmitted from them to healthy plants. If black and red raspberries are planted together, separate them as far as possible. If possible, plant black raspberries upwind from reds. The reason for this is the aphids that transmit viruses are generally blown or carried by wind rather than by active flight. Therefore, you do not want aphids to be blown from your red raspberries to your more susceptible black raspberries.
Go through the raspberry planting at least twice a year and remove all plants showing any virus symptoms. This should be done once about mid-June and again in late August or September. Before removing infected plants, kill all aphids on them by spraying infected plants with an insecticide a day or two before removal. Dig out the diseased plants, including roots, and dispose of them away from the planting site. In established plantings, where more than 5 to 10 percent of the plants show visible virus symptoms, removal of infected plants probably will not pay. In this case, maintain the planting until fruit yield becomes unprofitable, then destroy it. It is unwise to establish new plantings next to old, infected ones. Maintain strict aphid control at all times, especially in late spring and early summer when aphid populations are highest.
If the virus is transmitted by nematodes, the nematodes must be controlled to control the disease. Have soil tested for plant parasitic nematodes before planting. Samples should be taken in July of the year preceding planting. Spring samples taken when soils are cold are not accurate and do not give the grower sufficient time to apply a pre-plant nematicide. Information on collecting soil samples and submitting them for analysis is available from your county Extension office.