Blackberries Part 2-Diseases
In the April 2017 issue we talked about starting a blackberry patch: environmental requirements, selecting varieties, planting and harvesting. This month we will continue our discussion on potential diseases that may invade your blackberry patch. There are a number of diseases that can attack blackberries. Knowing what to look for is the first step in controlling problems that may arise in the blackberry patch.
Anthracnose is the result of a fungus that attacks the leaves and canes of the plant. You may first notice spots on leaves and/or canes. In late spring, the spots on the leaves are small with gray centers and purple margins. Leaf infection rarely causes defoliation, but the spots enlarge and become oval in shape and sunken. The infected tissue may eventually drop out and give the leaf a shot-hole appearance. On the canes, anthracnose symptoms first appear as small purplish spots on young canes (primocanes) and spurs. The spots enlarge and develop rather conspicuous borders (dark in color) with gray centers. In general, fruit on infected canes ripens abnormally. Infected canes may become girdled or cracked, causing either decline or death. If the planting is seriously infected, anthracnose is best controlled by removing and destroying all canes during the spring pruning.
Cane Blight – This disease is caused by the fungus, Leptosphaeria coniothyrium, which sporadically attacks canes of all Rubus species. Cane blight usually affects only canes that have been wounded in their vegetative year. All symptoms of cane blight occur in close association with wounds. Infection occurs in late spring or early summer through pruning — especially large pruning cuts — and insect wounds. In the spring, buds fail to break dormancy, lateral shoots wilt, or fruiting canes die when the fruit begins to ripen. Canes are usually brittle at the point of infection, and may break if bent. Symptoms appear late in the season on new shoots where plants have been pruned. Infected areas are brownish purple and develop from the cut ends. Branches originating in the infected areas wilt and die. Fruiting canes show a sudden wilting of branches when the fruit begins to ripen. Weakened canes are more susceptible to winter injury.
Symptoms of cane blight. Following infection, dead and dying floricanes are observed in the spring and summer. Dead canes may have a silvery to gray appearance.
Septoria Leafspot is caused by a fungus – Mycosphaerella rubi. The symptoms are similar to anthracnose leafspots. Spots tend to remain small with light brown or tan centers. Tiny black specks visible with a hand lens develop in the centers of leafspots. Chemical control is not usually necessary.
Rosette (fungus – Cercosporella rubi): This disease is also called double blossom or “witches’ broom.” Symptoms appear in the spring as bunches or clusters of foliage at terminals or along fruiting canes. Flower buds are larger and redder than normal. Petals may be purplish, and sepals are much elongated. Infected flowers do not set fruit. Control rosette by removing infected canes as soon as they become noticeable. Destroy all wild berry plants in the vicinity. Remove and burn all fruiting canes soon after harvest and keep plants adequately spaced for good air circulation. Where heavy infection has occurred, mowing all canes to the ground may be necessary.
Verticillium wilt causes the leaves to turn yellow, starting at the bottom of the canes and progressing upward. Infected canes are stunted and eventually wither and die.
Verticillium wilt. Photo Credits: Bernadine C Strike, University of California & Natural Resources.
Phytophthora Root Rot The fungi that cause Phytophthora root rot live in the soil and occur under wet and poorly-drained soil conditions. Blackberry plants that have wet feet are often predisposed to Phytophthora root rot. Excess water not only promotes susceptibility of roots to this disease, but also aids the fungus in moving to new infection sites.
Disease symptoms may first become noticeable in the spring, initially occurring in areas of the planting that are low or poorly drained. Foliar symptoms can include marginal browning, red or purple coloration, and/or chlorosis. Off-color leaves may also be smaller than normal. Infected plants show low vigor, developing fewer canes than usual; the canes that are produced may be weak and stunted. Stressed plants become more susceptible to other diseases, as well as to winter injury. Severely infected plants collapse and die. None of these symptoms alone are definitive for Phytophthora root rot since other factors, such as prolonged flooding or canker diseases, can result in similar symptoms. Diagnosis requires a careful examination of the main roots and crown of dying (not yet dead) plants. The tissue beneath the root epidermis or bark is white on healthy roots, while a typical reddish-brown discoloration is evident with Phytophthora-infected roots. Often a clear line of demarcation can be observed between diseased and healthy portions of the root. A laboratory test is often required to confirm the presence of Phytophthora.
Did you know that the Extension Office can send a sample from your infected plant to the plant pathologists at Virginia Tech, who will then send you a report with their diagnosis? If you’re interested in this, contact the Help Desk first to find out how to prepare your sample. You can call (434) 872-4580 or email: firstname.lastname@example.org. The Extension Office is located at the Albemarle County Office Building on 5th Street Extended, 2nd floor. Find out more at piedmontmastergardeners.org/contact.
Orange rust is also a concern on blackberries. In the spring, the undersides of the leaves are covered with bright orange fungal growth. Unlike all other fungi that infect blackberries, the orange rust fungus grows “systemically” throughout the roots, crown and shoots of an infected plant, and is perennial inside the below-ground plant parts. Once a plant is infected by orange rust, it is infected for life. Orange rust does not normally kill plants, but causes them to be so stunted and weakened that they produce little or no fruit. Key control methods are cultural practices such as removing infected plants early in the spring and eradication of any wild blackberry plants growing near the planting.
Orange rust-infected plants can be easily identified shortly after new growth appears in the spring. Newly formed shoots are weak and spindly. The new leaves on such canes are stunted or misshapen and are pale green to yellowish. This is important to remember when one considers control, because infected plants can be easily identified and removed at this stage. Within a few weeks, the lower surface of infected leaves are covered with blister-like pustules that are waxy at first but soon turn powdery and bright orange. This bright orange, rusty appearance is what gives the disease its name. Rusted leaves wither and drop in late spring or early summer. Later in the season, the tips of infected young canes appear to have outgrown the fungus and may appear normal. But appearances can be deceptive! At this point, infected plants are often difficult to identify. In reality, since the plants are systemically infected, the infected canes will be bushy and spindly in future years, and will bear little or no fruit.
Crown Gall (Agrobacterium tumefaciens): Wartlike growths (galls) appear on the roots or crowns of infected plants. Galls may range in size from that of a pinhead to several inches in diameter. Plants are weakened and yield dry, poorly developed berries. Galls are caused by bacteria present either in the soil or on planting stock. The bacteria enter the plant only through wounds or growth cracks.
An on-line blackberry diagnostic tool developed by N.C. State University can be found at the following link: https://projects.ncsu.edu/project/berries/diagnostic_tool/roots_and_crown/crown_and_roots_have_galls.php.
The key to controlling diseases in your blackberry patch is sanitation. The following suggestions will improve your chances of a healthy blackberry patch and improve your chances of producing a more bountiful harvest.
- Avoid “wet feet” by selecting a well-drained location. Consider a raised bed to reduce the likelihood of root diseases.
- Select disease-resistant varieties.
- Only disease-free plants should be planted. Before planting, inspect all plants and cut off and burn any old or diseased stems.
- Remove as many wild blackberries growing nearby as possible. The folks at the Tennessee Agricultural Extension Service recommend a minimum distance of 300 feet.
- If cane diseases become a problem, cut all plants at the soil line and destroy. For varieties with a double blossom, cut canes back to 12 inches above the ground immediately after harvest.
- Remove all old canes soon after harvest.
- Keep the blackberry patch free of weeds. Weed removal allows good air circulation, which helps reduce conditions favorable for disease development.
Blackberries are a wonderful garden addition. With a little care, a properly established blackberry patch will provide you with years of fresh gourmet and healthy treats.
Thanks for joining us in The Garden Shed. We look forward to your visit next month.
“Small Fruit in the Home Garden,” https://www.pubs.ext.vt.edu/426/426-840/426-840.html
“Field Identification and Management Strategies of Common Diseases in Blackberry Production,” https://extension.missouri.edu/greene/documents/Horticulture/Blackberry/BlackBerryDisease16.pdf
“Cane Blight of Blackberry,” http://extension.uga.edu/publications/detail.cfm?number=C894
“Management of Important Blackberry Diseases in Arkansas,” https://www.uaex.edu/publications/PDF/FSA-7563.pdf
“Rosette (Double Blossom) of Blackberry,” https://www.ars.usda.gov/ARSUserFiles/5276/BlackberryRosetteInformation.pdf
“Cane Diseases of Brambles,” http://plantclinic.cornell.edu/factsheets/canediseasesbrambles.pdf
“Diseases of Small Fruits,” https://ag.tennessee.edu/EPP/Redbook/Small fruit diseases.pdf
“Blackberries and Raspberries in Home Gardens,” https://extension.tennessee.edu/publications/Documents/SP284-C.pdf