Mark Windham was a distinguished professor of ornamental pathology at the University of Tennessee, Knoxville. After retirement, Mark and his wife Karen love to walk beaches, visit with old rose friends, and enjoy grandchildren. These are a series of columns he is writing for various ARS rose publications. We will be adding about one a month for the next two years.
Mark’s Mayhems and Maladies
Jan 2025 – Stem Canker Wants to Steal Your Roses!
There is nothing more discouraging than to sharpened your pruners and dressed for spring pruning only to find that brown/gray/black stem cankers have killed many of your roses’ canes. This lethal disease starts after roses have gone dormant. While you are snug under a quilt with hot chocolate or tea and assuming your roses have “settled down for a long winter’s nap”, your roses can be “stolen” by this disease. The suspects for causing this malady include the fungi Alternaria spp, Botryodiplodia theobromae, Botrytis cinerea, Coniothyrium fuckelii, Cryptosporella umbrina, and Trichothecium roseum.
Symptoms of stem canker usually start as small yellow areas on the canes. Cankers may have zonate (target like) patterns of gray and brown. Some cankers remain all year at less than an inch in length and are limited by nodes. Other cankers may grow across nodes and may be longer than 4 inches. Spores are produced on the surface of cankers and can be spread to other canes by wind or splashing rain. Canker fungi usually invade canes through freeze cracks, damage from canes rubbing together during wind events, pruning cuts, and other injuries such as humans pushing canes together while moving between roses.
Application of nitrogen fertilizers late in the growing season, leads to more and larger cankers. One or two weeks before your last Fall rose show or if not showing roses, then 4-6 weeks before the first frost day for your area, stop adding nitrogen fertilizer. Ensure you’re your pruners are sharp bypass pruners. Minimize late fall pruning. Anvil pruners may crush stem tissues at the cutting site and lead to a greater likelihood of canker formation. Fungicides have not been effective in preventing stem canker and are not recommended for canker management. During winter months, inspect your roses for stem canker at least twice and again at spring pruning. If cankers are observed, remove them immediately. When making the cut to remove a canker, cut at least one inch below the visible edge of the canker discoloration of pith tissue. Failure to detect and reduce canker incidence during winter months can lead to high lost of roses to this disease.
A. Stem cankers ravaging different dormant rose cultivars in early March in Knoxville, TN. Note the blacken stems with intermittent gray and brown cankers. B. A close-up of a terminal stem canker at an old pruning cut. The small black flecks on the brown tissue (look like black pepper flakes) are fungal fruiting bodies that release spores.
