by Nanette Londeree, Master Rosarian
This time of year can be a glorious one in the rose garden – plants are generally big and healthy and producing flowers with deep colors that last longer on the plant. Even with this general vigor, you’re likely to see some dieback – that seemingly sudden demise of a small stem, cane or even a very large cane. Healthy green canes develop a dull brown discoloration in the pith that can extend all the way to the surface on one or more sides of the cane. So, what is going on?
Dieback is defined as “the dying of the outer portions of a plant due to disease or weather damage; death of part or all of the woody portion of a plant.” Dieback causes death of the terminal areas of the cane usually extending down from tips of stems or canes. It can be isolated to a small section of a cane, like a portion of cane left above a pruning cut, or an entire cane, down to the bud union. Dieback can occur any time of the year and is found wherever roses are grown. It may be the result of some sort of injury to the cane; a mechanical cut or break followed by a fungal infection or a penetration from a boring insect.
It may also be a result of the age of the cane, weather induced (in colder climates) and in some cases, related the variety of rose. It is more likely to occur on plants that are stressed or less vigorous. After our cool, wet spring this year when blackspot was rampant in many gardens, you may have seen an increase in the amount of dieback.
Flatheaded borers, Chrysobothris spp., may kill canes or an entire plant. Larvae are white and up to an inch long with enlarged heads. Adult beetles do not significantly damage roses. Eggs tend to be laid on stressed rose plants, especially in bark wounds caused by sunburn or disease. The Raspberry horntail, Hartigia cressoni, larvae are white, segmented caterpillars up to one inch long that can cause tips of canes to wilt and die in spring, reducing second cycle blooms. Adults are wasplike, black or black and yellow, and about 1/2 inch long.
Some dieback is simply a matter of age. As a cane ages, it will develop a grayish brown bark, similar to a tree. The canes productivity will continue to diminish, and you may actually see a progression from greenish-gray to yellow to brown.
To prevent, or at least minimize the occurrence of dieback, buy only the best quality disease-free plants from a reputable nursery, keep your plants in good health with adequate sun, water, air circulation, rich, well-draining soil and maintain good garden sanitation. Prune your roses carefully using sharp pruners to leave a clean, sloping cut just above nodes. Don’t leave long stubs of internodal tissue to become suitable sites for fungal development. If you see a rose that exhibits dieback, prune out all the apparent infected parts, down to a point that you see healthy green pith. Make your cuts with sharp shears well below diseased areas. After cutting out the cane, before proceeding to another disinfect your pruners by dipping the blades into a 3% bleach solution (right out of the bottle) between each cut. Make sure to dispose of all material; don’t add it to your compost pile.