Blight & Cankers on Rose Cane

Bacterial Blight

• Roses grown in the wild or under cultivation in greenhouses and gardens are susceptible to bacterial blight caused by Pseudomonas syringae. The disease is more prevalent during prolonged spring showers. Initial symptoms include brown spots on foliage, and blooms that enlarge in size to cover more plant surface. Left untreated, the disease kills infected flowers and smaller canes and causes cankers and streaks on larger canes.

Cankers

• Three types of cankers affect rose canes -- brand canker caused by Coniothyrium wernsdorffiae, stem blight and canker caused by Coniothyrium fuckelii and brown canker caused by Cryptosporella umbrina. Symptoms of brand canker include tiny red spots on canes that enlarge and turn brown to contrast with the green cane. Small fruiting bodies appear on the spots before they turn brown. Left untreated, tiny slits appear on the epidermis of the cane above the lesions. Stem canker manifests itself as tiny yellow or red spots on canes that gradually increase in size, causing the canker to appear light brown and feature a deeper brown margin. The tissue within the canker cracks and emits black sooty mold. Brown canker causes tiny raised spots on the canes of the current year's growth. As the lesions enlarge, their centers turn whitish-gray and are surrounded by a purple margin. Groups of tiny spots grow so close together they coalesce into large patches.

Damage

• Growers often mistake bacterial blight for frost damage or winter kill because the area above and below the infection on the cane appears black. Upon peeling the blackened bark, the tissue appears moist and brown. Brand cankers appear on outdoor growing roses. Left untreated, the cankers enlarge and girdle the cane, causing death. Stem cankers appear on greenhouse and landscape roses. It girdles the stem, causing he portion of the cane between the cane tip and cankered area to die. The whitish-gray patches of brown canker are surrounded by tan cankers during the winter or spring that grow several inches in length and girdle the cane.

Management

• Prune rose canes with symptoms of blight and dieback. Collect and destroy clippings and plant debris. Use a copper-based fungicide during fall or spring to prevent the disease and control spread. Spraying a registered fungicide from spring through fall will control the spread of cankers. Apply the spray every week and ensure all parts of the rose plant are covered.

Control

• To control the chances of blight and cankers on roses, space plants sufficiently apart to provide good air circulation. Avoid over-fertilizing roses and irrigating from above, which causes the foliage and canes to remain wet for prolonged periods. Use sterilized pruning shears and form sharp, straight cuts to reduce chances of wounds. Avoid working on canes when they are wet.