Popular for its bright, seasonal foliage, the canna lily is essentially a disease-free plant. Some damaging canna diseases have been identified, however the occurrence is limited. Their immunity is mainly due to their shorter growth period. Still, you should know how to identify the common canna diseases and treat them.
TIP: Our expert gardening advisor, Rachel Klein adds, "An important part of owning healthy cannas is buying strong plants from the nursery. Inspect your cannas before buying, checking for discoloration within the leaves, yellow leaves, puckering, and stunted growth."
Canna lilies' seasonal flowers exhibit a wide range of colors, ranging from reds and yellows to dull shades of white. Canna lily grows up to 6 feet and is easy-to-prune.
TIP: Rachel notes, "Dwarf canna lilies are becoming very popular due to their height of three feet or less. They can be afflicted by all of the same conditions as their larger relatives. Disease resistant varieties of canna include Fire Dancer and Tropicana."
Disease 1: Canna Rust
This is a common fungal disease. It is caused by the Puccinia fungus. Long periods of humid weather, low air circulation, and waterlogged soil propagate canna rust.
Easy-to-identify symptoms include spore-like, orange spots visible on the plant's 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.
Treating Canna Rust
This fungal infection cannot be comprehensively eradicated. It has a seasonal, developmental pattern and it re-surfaces among most cannas. The best approach is taking precautionary measures. Make sure your canna is getting at least 6 hours of direct sunlight a day. When planting, space your cannas at least 1 foot apart to promote good air circulation. During the wet season, don’t spray water on the leaves. Ensure that the soil bed drains quickly and do not over-water. If the rust appears, the affected foliage should be pruned-off and burnt immediately. The leftover ashes should not be mixed with garden soil, as they may contain fungal spores.
TIP: Rachel advises, "When pruning off infected foliage use sharp clean gardening shears. After pruning, wipe down the shears with a cloth or rag moist with bleach. This will keep your garden tools sterilized so you do not spread your garden ailments."
To arrest the spread of canna rust, 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.
Fungicides should be used minimally since they are known to affect the nutrition uptake ability of the canna lily. [If the canna is heavily infected and does not respond to fungicides after 10 days, you should consider digging up the plant and rhizome completely and destroying it before it infects others.]
Disease 2: Canna Viruses
Canna lilies suffering from canna rust often become susceptible to the destructive canna viruses. Common canna viruses include:
- Yellow mottle virus
- Bean yellow mosaic virus
- Yellow streak virus
- Aspermy virus
The leaves develop a characteristic, flecked or puckered appearance and may display green or yellow irregular splotches. This is usually accompanied by streaks appearing along the crown of leaves. The bloom becomes discolored, distorted, and small. Foliage growth is stunted and the plant eventually dies. When more than one symptom presents itself the condition could be more severe.
TIP: Rachel cautions you, "The yellow mottle virus is of special concern because it can be passed from parent to seed."
Treating Canna Virus Infection
An effective remedy is not known for these rare viruses. Elimination of diseased cannas is the only solution. You should dig-out the entire plant from its soil bed and burn it. Plants within touching distance of virus-infected cannas should also be dug-out and burned.
Disease 3: Canna Botrytis Blight
Botrytis Blight of cannas usually affects the older stems and leaves. It is caused by the botrytis cinerea fungus. Initially, the mold attacks the stems but first symptoms appear on the leaves. The disease spreads quickly in humid, rainy conditions.
Botrytis blight first appears as a white growth on the canna leves. During the growing season, the leaves develop dark spots. The flower buds are covered in a thin, gray sheet of fungal spores. Spores are spread by wind and water. Young stems might show discolored streaks along the base.
To prevent blight, water your cannas in the early morning to allow them time to dry before evening. Water the soil closely and do not allow the water to splash up onto the foliage. Retail fungicides can only cure the symptoms but cannot eradicate the fungal spores. The infection spreads through older, weathered foliage. Hence, pruning-away spent, decaying foliage is a recommended form of disease control. Deadheading your cannas as soon as their blooms begin to fade is a very important practice. Blighted sections of the plant should be pruned-off. It is important to keep up with pruning and deadheading measures because as infected flowers and foliage fall, they infect other parts of the plant, and other plants. Ensure that the leaves surrounding the infected/pruned sites are sprayed with an anti-fungal solution. Again, copper-based fungicides are ideal.
TIP: Rachel emphasizes, "In all of these cases the most important measure is identifying and destroying infected leaves and plants before they can act as a source of infection for others."