Even in the drought-stressed northern parts of the state, it is difficult to find new diseases to write about. Rose cankers appear at any time of year but especially when 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. Where control measures are not practiced, cane infections may approach 100 percent with all canes infected. Identifying the particular canker species is not important, but identifying a problem as a canker is important. 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 in fall and in early spring, according to the type and cultivar grown. Remove and burn or haul away 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. Before each cut, dip the shears in a disinfectant such as 10 percent Clorox or 70 percent rubbing alcohol. Use sharp tools to make clean, slanting pruning cuts no more than 1/4 inch above a node.
Plant 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 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. Don’t stop now though. Adding a spreader-sticker material to the spray helps wet the canes for better protection. Consult Report on Plant Disease No. 626 for details concerning rose cankers.