Red and black raspberries are vulnerable to a number of viruses and an infection can reduce yields by as much as 70 per cent. Mosaic, streak, leaf-curl and tomato ring-spot are the four main viruses of raspberries. There are other raspberry disorders that can cause symptoms similar to a virus such as powdery mildew, mineral deficiencies, feeding by aphids, mites and leafhoppers and genetic disorders (such as crumbly berries).
The mosaic disease is caused by a group of viruses. No raspberry plants are immune, but the black and purple cultivars are more susceptible to severe damage than the red cultivar. Depending on the cultivar grown, the symptoms of mosaic can vary considerably, which virus or viruses of the group are involved and the time of year. Leaves are mottled with light green or yellowish areas. On some cultivars, leaves can become puckered with large dark green blisters surrounded by yellowish green tissue. Symptoms may disappear with summers high temperatures and even though symptoms may disappear for a time, the plant will always be infected.
The raspberry mosaic virus complex is almost always spread by one species of insect, the large raspberry aphid (Amophorophora agathonica). The aphid feeds on the undersides of leaves near the tip of the cane. In addition to any leaf symptoms you may see, the amount of fruit is reduced and may be dry, seedy, crumbly and have no flavor. Newly infected tops of black and purple raspberries often curl downward, turn black and die. Mosaic infected plants are many times more stunted each year.
Raspberry streak affects only black raspberries and is caused by the tobacco streak virus. Numerous purplish streaks, usually less than an inch long, that appear on the lower portions of infected canes is the most obvious symptom. Tip leaflets are often hooked, twisted, curled and darker green than normal. The lower leaves may have some yellowing along the veins and mottling. Fruits are crumbly, seedy, lack flavor and size.
Raspberry plants infected with leaf-curl are worthless and should be destroyed. This virus is exclusively spread by the small raspberry aphid. Infected plants are easily spotted as the small, dark green leaves are crinkled and tightly curled downward and inward. Diseased shoots first appear a pale yellowish-green, then turn dark green, become stiff and brittle and usually do not branch. Berries on infected plants are dry, seedy, crumbly and lack size.
Tomato ring-spot only infects red raspberries. The most obvious symptom, of an otherwise healthy looking plant, is the production of a small, crumbly berry. This virus is transmitted through the soil by the dagger nematode (Xiphinema americanum).
To avoid these viruses, always purchase certified, disease free, virus-indexed stock. Avoid accepting plants from friends and neighbors. Choose a planting site that gets plenty of sun, is well drained, fertile, has good air circulation and is located 500 to 1000 feet from any other raspberry plantings. Plant black raspberries upwind from the reds because black raspberries are more susceptible to aphids than the reds. Those little aphids can be blown a fair distance.
Inspect your plantings 2 to 3 times a year for symptoms. Aphid populations are highest in late spring and early summer. Use an insecticide, following all label directions, spraying a day or two before removing the infected plant. This should kill any virus carrying aphids and prevent them from infecting any nearby plants.
As always, Happy Gardening!
The Purdue University Cooperative Extension Service can be reached at 499-6334 in LaGrange Co.