Stumped by Cane Canker?

by Rita Perwich, Consulting Rosarian, San Diego Rose Society

Problems in the rose garden can be disheartening: damaged rose blooms are disappointing but blooms are easily deadheaded and roses rebloom; leaf damage is troubling, but leaves can be cut and pulled off and the plant readily grows new leaves. Damaged canes pose a much more serious and critical issue. On every occasion the cane must be cut and sometimes it must be removed right down at the bud union. Like surgery, pruning out canes or portions of canes is unnerving. Unlike the regrowth of blooms and leaves, the growth of new canes is not a given. Unfortunately, even the experts are stumped and puzzled about the causes of cane canker. The unknown always raises fears and anxieties, and without a firm diagnosis of the pathogen involved, there is confusion on how to treat the problem.

Symptoms to Watch For

Diseased canes can range in color from black to yellow to brown to purplish-red. The disease often shows up in spots or splotches or zonate patterns. Stem cankers and dieback can be just a few inches long or they can spread down the entire cane.

Which Pathogens Cause Rose Canker?

There are some names that we hope we will not encounter in our rose garden: Alternaria spp, Botryodiplodia theobromae, Botrytis cinerea, Coniothyrium sp. Cryptosporella sp. and Trichothecium roseum are some of the unwelcome and unrelated pathogens sometimes associated with rose canker. But Dr. Mark Windham, a distinguished professor in the Department of Entomology and Pathology at the University of Tennessee, explains that identifying and pinpointing the exact causal pathogen of a canker is complicated because of the following:

1. The fungal pathogen responsible for the dieback may not be easy to pinpoint even when examined under a microscope. This is because the pathogen found in the wounded cane is sometimes not the pathogen that caused the canker but a secondary invader that has outgrown the initial causal pathogen;

2. The symptoms of rose canker are not always the same even on the same rose;

3. The same canker can be different in appearance in different regions of the country;

4. Some cankers can remain at less than an inch, others can extend beyond a node into the internodal region and be four or more inches in length;

5. Signs of the fungi (the physical, visible presence of the pathogen) are apparent on some cankers but not on others;

6. The variations in symptoms and sign formation suggest multiple causes of rose canker.
   

Can Fungicides Help?

The inability to point to a specific pathogen as the cause of rose canker is a big problem when rosarians are trying to find a control strategy such as an appropriate fungicide. Rosarians have discovered to their frustration that application of fungicides are not always effective as a preventive or remedial strategy. Dr. Windham explains:

1. For a fungicide to be effective, you need to know the causal pathogen’s identity;

2. It is possible (but less likely) that a bacteria is the initial causal agent of rose canker. This possibility is one explanation why fungicides may not be effective;

3. Some pathogens become resistant to fungicides in as little as three generations of sporulation which could be less than three days in the case of Botrytis cinerea;

4. Timing and application procedures used for other fungal rose issues cannot be assumed to apply to rose canker;

5. Most canker issues are associated with pruning wounds. Systemic fungicides may not be sufficient or may be impeded in translocating to the canker due to the wound site on the plant.

A Challenged Plant is More Susceptible to Cane Disease

Cankers are more likely to occur on plants that have been weakened by pest, disease, sunburn or poor nutrition. Dr. Windham suggests that most cankers are associated with wounds, often during pruning, that open the inner cambium to disease.

Good Rose Culture Helps Prevent a Challenged Plant

In order for disease to take hold, there must be a “disease triangle” that consists of a susceptible host, a virulent pathogen and a conducive environment. When we manipulate one or more legs of this triangle, we may be able to change the outcome of the disease. Since roses are a susceptible host, and fungal pathogens that cause cane disease are present in the rose garden, especially in rainy years, we must take actions to prevent a favorable environment for rose disease. We do this by the following:

1. Planting our roses in the sun and spacing and pruning them to allow adequate exposure to air circulation and light;

2. Applying amendments and organic mulch to the soil;

3. Irrigating in the morning since many fungal pathogens proliferate and thrive on humidity. Drip irrigation is best;

4. Deadheading any fungal-infected flowers and leaves immediately and disposing of fallen leaves and petals to remove disease from the garden. The spores from botrytis and black spot and downy mildew can start on the blooms and leaves and invade the canes;

5. Using sharp and clean bypass pruners and loppers and avoid wounding the epidermis with improper cuts which allows penetration and infection by disease. Our cuts should be 1/4-inch above a node to speed callus formation;

6. Disinfecting the blades of our pruners between each cut when cane canker is present. Use disinfectant wipes or spray the blades with a solution of 1 ¼ teaspoons of Lysol disinfectant concentrate mixed with 8 ounces of water;

7. Pruning out damaged canes immediately and at least 2 to 3 inches below the infected portion.

Spotting cane canker in the garden is jarring. We can be heartened by the fact that the experts are working on determining the causes of rose canker and how to minimize its impact in our gardens. In the meantime, we must be pro-active, wide-eyed and alert in our rose gardens.

Cane Canker is varied in appearance, photos by Rita Perwich.