Rose cankers appear any time of year but especially when the plants are under stress. Three canker diseases (brown canker, stem or common canker, and brand canker) are common in Illinois and are generally confused with weather injury or other problems. Cane infections may approach 100 percent (all canes infected) where control measures are not practiced. Identifying particular canker species is not important, but it is important to identify a problem as a canker. The first symptoms are small, roundish lesions in 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, fungus-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 a wide variety of wounds, including thorn abrasions. Infections may also occur on the leaves and flowers.
Management of rose cankers starts with good sanitation. Prune the canes now and in early spring according to the type and cultivar grown. For example, it is important to leave shrub rose stems intact as long as possible, deferring pruning until spring. 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 2 to 3 inches below any sign of infection. Some suggest that pruners be disinfected between cuts, but that can be quite time consuming and possibly unnecessary. Prune in dry weather and disinfect pruners at least between plants. Use sharp tools to make clean, slanting pruning cuts no more than 1/4 inch above a node.
New plants should be 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 when purchased. Maintain plants in high vigor by proper planting, spacing, fertilizing, watering, winter protection, and thorough spraying with fungicides. Start as the buds break open in the spring and continue at 7- to 10-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 helps wet the canes for better protection. Consult Report on Plant Disease, no. 626, “Rose Cane Cankers,” for details. This report is available in Extension offices or on the Vista Web site, accessible through the Plant Clinic site, http://plantclinic.cropsci.uiuc.edu/.