Members of the Fragaria genus, black and red raspberry species grow on canes in U.S. Department of Agriculture plant hardiness zones 2 through 7. Typical fungal diseases include anthracnose, blight, rusts, wilts and rots. Common viral diseases include raspberry leaf curl, raspberry mosaic, raspberry ring spot and bushy dwarf virus. These diseases almost all develop in cool, wet spring weather.
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Identifying Fungal Anthracnose and Blights
Anthracnose typically strikes in wet spring weather, infecting black or purple raspberries more than red raspberries. The fungus causes small purple spots up to about 1/8-inch wide on young canes. The spots sink in the center, turning gray with purple edges that often join together. The canes may crack and small, pitted raspberries ripen slowly.
Spur blight strikes strikes in wet, early-spring weather, affecting red raspberries more than black raspberries. However, the symptoms do not occur until mid- to late summer on overgrown, excessively vigorous canes. Chocolate-brown to purple blotches grow around buds, stunting them or causing them to produce weak canes the next year.
Cane blight also appears in early spring, causing purple to dark-brown cankers below wilted leaves on the main branches or canes. Spur blight strikes around buds, but cane blight infects whole stems, turning them purplish-black and causing them to crack. Cane blight typically strikes areas wounded by pruning and can cause failure of buds to develop, wilting of lateral shoots, and the death of canes.
Identifying Fungal Rusts, Wilts and Rots
Infected orange rust leaves turn yellow with rust-colored spores on the bottoms of leaves. The fungus moves to other parts of the plant, including the roots. The plants become spindly and lacking thorns. They will not recover and the plants will die the following spring.
The popular Heritage red raspberry cultivar ( Fragaria × ananassa "Heritage") is particularly hard hit by late leaf rust in which powdery masses of fine, light-yellow fungal spores appear on leaf stems and berry caps. There is no known way to control this disease.
Soil-borne vermicillium root moves up from the roots, causing leaves to turn yellow and drop beginning at the bottom of the canes. As the disease progresses upward, the canes turn blue and die.
Warm, wet weather also encourages fuzzy botrytis or penicilium mold to develop on ripening raspberries.
Identifying Viral Raspberry Leaf Curl
Raspberry leaf curl can cause a 20- to 70-percent drop in yield and black raspberries to die in two to three years. The tips of yellow and red raspberry leaves may curl downward the first year of infection or there may be no symptoms. The next spring, crinkled, stunted, rounded leaves near the tips curl tightly, turning downward. Pale, yellowish-green shoots of newly infected plants typically do not branch, becoming stiff and brittle and turning dark green. Infected plants grow small dry, seedy, crumbly berries. The plants grow more poorly each year. Black and purple raspberry plants show similar symptoms. Dwarf, stiff leaves arch upward, turning dark, greasy green. As the years pass, severely dwarfed, bushy young canes turn rigid, unable bend to the ground to grow roots at the tips.
Identifying Viral Common Raspberry Mosiac
Black raspberries typically suffer most by the common raspberry mosaic, a viral disease that can cause raspberry yield to drop by half. Infected red and yellow raspberry canes grow poorly in cool weather, developing mottled, puckered leaves with large, green blisters. The leaves droop as the blisters turn yellowish-green or yellow. Leaves that grow in hot weather show pale yellow spots or show no symptoms at all. Each year the plants get worse, growing dwarf, often-deformed leaves mottled with yellow, yielding fewer raspberries. Black raspberry leaves turn black and rotten and the plants yield fewer berries.
Identifying Ring Spot and Bushy Dwarf Viruses
Soil nematodes spread red raspberry ringspot, caused by the tomato ringspot virus. After one year of infection with no symptoms, diseased plants show yellowing veins and yellow rings on new canes. These symptoms disappear in the second year, but leaves grow slowly in spring, turning yellow. Diseased canes yield crumbly, deformed berries. The raspberry bushy dwarf virus infects both red and black raspberries, resulting in declining vigor and plant yield. The symptoms vary with the season and the cultivar. Some cultivars grow leaves with yellowing between the veins on the leaves, while others develop irregular patterns resembling oak leaves.
Controlling Raspberry Diseases
Plant raspberries certified by the U.S. Department of Agriculture as being virus- and nematode-free and resistant to fungal disease. Plant them in a fertile, sunny site with good drainage and destroy all neglected or wild raspberry or blackberry canes within 600 to 1,000 feet. Separate red and black raspberries by 150 feet to prevent them from infecting one another. Check for viral diseases in cool, cloudy weather and spray infected plants with 2 tablespoons of powdered Malathion per gallon of water and remove and destroy the plants one or two days later. To prevent infestations of disease-bearing nematodes, anchor a sheet of clear plastic on the ground in late spring and leave in place for at least two months to kill the nematodes by heat. Spray lime sulfur, useful for controlling fungal disease, when new leaves are 1/4- to 3/4-inch long in the spring or when the temperature is above 75 degrees Fahrenheit. To kill aphids, spray the foliage with a 1 to 2 percent of horticultural oil.