Cankers can occur on many plant species, as explained in issue number 16 of this newsletter. Roses in Illinois are severely injured by cankers following winter stress. We'll have to watch woolly-worm coats to determine the severity of weather to expect this upcoming winter, but it is unlikely that we will be blessed with two mild winters in a row.
Three canker diseases (brown canker, stem or common canker, and brand canker) are common in Illinois and are generally confused with winter injury or other problems. The first symptoms of canker are small, roundish lesions on the canes. The spots are pale yellow, reddish, or bluish purple. They gradually enlarge, turn brown or grayish white (often with a darker margin), and may partially or completely girdle the cane. Complete girdling results in dieback or poor growth of the plant parts above the affected areas. Cankered areas are sprinkled with black, speck-sized, fungal fruiting bodies. When left unchecked, infections may spread downward into the crown, causing entire rose plants to wilt, wither, and die. Infection occurs chiefly through variety of wounds, including thorn abrasions. Infections may also occur on the leaves and flowers.
Some simple steps now can help minimize problems with rose cankers. Good sanitation is critical to controlling these cane diseases. Prune the canes in fall and in early spring, according to the type and cultivar grown. Remove and burn or haul away with the trash all infected, dead, and weak parts of canes, as well as infected leaves, flowers, buds, and hips. When pruning cankerous stems, cut back to a strongly growing shoot or branch at least two to three inches below any sign of infection. Before each cut, dip the shears in a disinfectant such as 10% chlorine bleach or 70% rubbing alcohol. Use sharp tools to make clean, slanting pruning cuts no more than 1/4 inch above a node.
When planting new rose bushes, use only top-quality, disease-free plants from a reputable nursery. The plants should be free of cane bruises or colored spots. Bargain roses are often infected. Maintain plants in high vigor by proper planting, spacing, fertilizing, watering, winter protection, and thorough spraying with fungicides. Start treatment as the buds break open in the spring and continue at seven- to ten-day intervals into September or early October. The fungicides that control black spot usually control cankers as well, so no additional spraying is required. Adding a spreader-sticker material to the spray, however, helps wet the canes for better protection.
Consult Report on Plant Diseases No. 626 for more information on rose cane cankers. Chemical options are listed on page 38 of the Illinois Homeowners' Guide to Pest Management and page 104 of the Illinois Commercial Landscape and Turfgrass Pest Management Handbook, 1998-1999.