Keeping These Beauties Healthy
Hariette Henry – Halton Master Gardener
There is no question that Canna Lilies are stunning plants, grown either as the focal point within a container or massed at the centre of an island in a city boulevard. These plants are relatively problem free owing to their shorter growing period but there are some common diseases that can afflict them so it would wise to be able to identify and treat these diseases to avoid losing these lovely plants in the future. The most common diseases you may encounter are:
1. Canna Rust
This is a common fungal disease caused by the fungus Puccinia thaliae. Symptoms include spore-like orange spots visible on the plants leaves. Sometimes the spots may spread onto the stems. As the infection progresses, the upper parts of the leaves develop a blackish-brown appearance. Such leaves are prone to fall prematurely. Precautionary measures such as making sure your Canna is getting six hours of sunlight a day; providing good air circulation; watering the plants at the soil level as opposed to spraying the leaves; ensuring the soil bed drains quickly; and not over-watering, are all measures that you can take particularly in the hot, humid weather we are experiencing to maintain a healthy plant. If the rust appears, the affected foliage should be pruned-off immediately and disposed of (not in the compost pile). To stop the spread of the Canna rust, you can use commercial anti-fungal sprays. Spray both the upper and underside of the leaves. Using copper or sulfur-based fungicidal preparations is recommended. Make sure that the spray lists Canna rust on the label and follow the application directions.
Cannas can also be susceptible to viruses, particularly if they’ve been subject to Canna rust. Following are the most common Canna viruses:
- Yellow mottle virus
- Bean yellow mosaic virus
- Yellow streak virus
- Aspermy virus
In the case of these viruses, the leaves develop a flecked or puckered appearance and may display green or yellow irregular splotches. This is usually accompanied by streaks appearing along the crown of the leaves. These viruses do affect the blooms which become discoloured. These viruses are fairly rare and unfortunately there is no known remedy. Elimination of the diseased Cannas is the only solution. The diseased plant along with any others within touching distance should be dug-out and disposed of in waste collection.
3. Canna Botrytis
Blight is another problem that can affect Canna Lilies. It usually impacts older stems and leaves.It is caused by the Botrytis cinerea fungus. This fungus appears as a white growth on the canna leaves. The leaves develop dark spots. The flower buds are covered in a thin, gray sheet of fungal spores. As with rust, the best way to avoid or at least control blight is through good cultural practices such as watering in the early morning, maintaining good air circulation around the plant and pruning away spent decaying foliage. Retail fungicides can be used to control symptoms but cannot eradicate the fungal spores. Copper-based fungicides are recommended for use in this situation.
Other Canna Problems
These diseases are, of course, not the only problem to potentially affect the health of Canna Lilies. They can be attacked by a host of insects such as Japanese Beetles once they have been weakened by pathogens. The best course of action is buying from a reputable grower and of course maintaining good cultural practices.
Over-wintering Cannas in Canada
Though Cannas are not native to Canada, (their native range extends from South Carolina to Argentina and includes the Caribbean islands), there’s no reason we can’t preserve these tender rhizomes for future seasons. The process is relatively simple:
- Once their summer beauty has faded and leaves are yellow, died back or killed by frost and before the ground freezes, cut back the dead foliage to approximately 2 inches.
- Carefully dig your Cannas with a shovel or spade.
- Continue digging and cut all around the clump.
- Lift the rhizome clumps out of the ground and shake off any excess soil.
- Rinse the rhizomes to remove dirt.
- Air dry the rhizomes in a well ventilated area at 20-26 C/70 -80 F for at least a week.
- Store in a cool, dark and humid place (fruit cellar or basement) with good ventilation in a cardboard box covered in vermiculite or peat moss.
- Check throughout the winter and discard shriveled, diseased or insect-infested rhizomes.
- In spring, replant outdoors after all danger of frost is past. Alternatively, you can get a jump on the growing season by potting them up early indoors.