For many gardeners the raspberry crop is done for the year in August, while for others, there’s a new crop just setting fruit. Whichever kind of raspberries you have, the diseases are the same although fall-bearing or ever-bearing cultivars such as “Autumn Bliss” or “Caroline” escape many of the diseases.
Raspberries are infected by a variety of fungal and viral diseases that may affect fruit quality and yield. Some years the problems are more severe than others. The most common fungal diseases include anthracnose, cane blight, spur blight and gray mold. Viral diseases are less common and won’t be discussed in this article.
Anthracnose is seen on canes and leaves of some varieties of red raspberries and all varieties of black raspberries. In spring, small purple spots appear on young canes. They are about 1/8 inch in diameter, sunken in the center and turn gray with a purple border. Infected fruit remains small and slow to ripen. Affected leaves develop a shothole appearance. Anthracnose generally kills canes.
Powdery Mildew is a whitish powdery growth that appears on the bottom side of the leaves. It’s not considered a serious problem.
Spur Blight also affects leaves and canes. In mid-summer, purple to brown lesions appear on canes. They are centered around individual buds, which may fail to grow or produce weak shoots the following year. Wet conditions in spring favor the development of this disease, but the symptoms don’t become visible until mid to late summer.
Gray Mold, a very common and a serious disease of raspberries, causes fruit rot and blossom blight. The infection shows first on young flowers, but quickly spreads to fruit during cool, wet weather covering them with a gray or brown fungus. . It also infects leaves and canes.
Cane Blight usually appears on damaged canes that become infected by the spores. Infected canes show cankers that appear black, brown or gray and contain black, pimple-like spore cases. The infected canes tend to be brittle and often break near the infection. Ultimately the cane will die.
Raspberry Leaf Spot produces white, gray or tan spots on the tops of the leaves. Heavy infections may cause early leaf drop and increase the susceptibility to winter injury. Leaf spot symptoms are similar to anthracnose.
A limited number of chemical fungicides are registered to control fungal diseases that affect raspberries. If you choose to use a chemical, read the label carefully to make sure it is safe for use on raspberries.
The University of Minnesota recommends the following good cultural practices as the best way protect your raspberries from fungal diseases. Start with disease-free plants. Plant raspberries in well-drained soil or raised beds. Avoid overhead watering, over-fertilizing and wounding plants. Control weeds. Prune and thin canes annually during dry weather. Remove newly infected canes and destroy. Remove and destroy old fruiting canes after harvest and destroy them.
In addition, good air circulation promotes drying of foliage. Rows should be narrow with proper thinning and pruning of canes. Poor air circulation and the resulting wetness provide the right environment for fungal infections to occur. The old adage, “An ounce of prevention is worth a pound of cure,” is an excellent idea to follow when it comes to dealing with fungal diseases of raspberries.
Information is available from U of M Extension. Use this link: www1.extension.umn.edu/garden. For help with gardening questions, call 444-7916, leaving your name, number, and question.
A local master gardener will call to assist you.
