Cannas or canna lilies are tropical and subtropical herbaceous perennials. When grown in their native habitat, they flower the entire year. The plant grows from a rhizome, and when grown in cold climates (frost and freezing conditions) the rhizome is dug up and stored during the winter season and planted again in the spring. Cannas are susceptible to canna rust, botrytis blight and canna viruses.
Leaves of the canna are flat and broad, and their color can be green (also variegated green with white, cream, pink or red), brown, maroon or bronze. The iris-like flower grows through the base of the leaves or false stem. They come in red, orange and yellow. Some plants have a combination of those colors. Cultivars range in height from 2½ feet to 10 feet, but they rarely grow above 4 feet. Cannas can be grown in full sun or partial shade. They prefer well-drained, rich organic soil, but they will tolerate most soils as long as they are well-drained and fertilized.
Canna rust is a fungal disease caused by the Puccinia fungus. Symptoms of the disease are orange spots that appear on the canna leaves. These spots can also spread to the stem of the plant. Progression of the disease results in blackening/browning of the upper part of the infected leaves. These infected leaves may fall off prematurely. This disease generally occurs during humid weather and when the soil is extremely wet.
Controlling Canna Rust
The best defense against canna rust is to make sure that the plants are grown in well-drained soil and spaced far enough from each other to allow air flow between the plants. Once the fungus is present, avoid sprinkling and overhead watering the plants, because this may spread the fungal spores, thus spreading the disease. Infected leaves should be removed as soon as you notice the presence of the disease. These leaves should be destroyed--do not put them in your compost bin. A fungicide can be applied to protect the plants that have not yet been infected.
Symptoms and Control of Botrytis Blight
Botrytis blight or gray mold is another fungal disease that can attack the canna plant as well as a large variety of annuals and perennials. Just like canna rust, this fungal disease occurs during humid/wet conditions. Symptoms of the disease are a gray/fuzzy mold that appears on the older blossoms and leaves of the plant. The infected flowers and leaves should be removed to avoid spreading the infection. In some instances you may have to remove the entire canna plant. It is important to avoid overhead watering and to provide adequate air circulation between plants. Botrytis blight overwinters, so clean up the dead plant material in the fall and discard it. If weather conditions seem conducive to botrytis blight in the spring, you can apply a preventative fungicide.
Symptoms of canna viruses are spotted and streaked leaf crowns, stunted growth and discolored blossoms. There is no known cure for canna viruses. Infected plants should be dug up and destroyed.