Canna lilies can grow to 6 feet tall and produces blooms of many colors---reds, yellows, oranges and whites---throughout the summer. Cannas grow in temperate-tropical climates and will die back in the cooler months, but should begin to poke above the ground in mid-spring. Canna lilies are remarkably free of disease, and can actually be quite hardy, as they propagate quickly and will spread within a single season. But there are some diseases that haunt these flower-garden staples.
Like rust diseases on any other plant, canna rust may be caused by standing water or soil that does not drain well. The rust is a fungus that shows up as orange spots on the leaves of the lilies. This is most often caused by over watering the plant or planting it in a spot that holds water. Cannas should be planted in well-draining soil and full sun. If canna rust appears, spray with a mild fungicide.
If a gray, fuzzy substance appears on your canna blooms, it is likely a mold called Botrytis. This mold does not spread and may be controlled by removing the affected blooms and disposing of them. Botyris usually appears in humid conditions and affects older blooms.
Canna lillies may be infected by canna viruses, many of which may affect similar plants such as gladiolas, freesia or other lillies. Flecking, streaking or puckering on the leaves may be indications of a viral infection. The viruses cannot be controlled and will cause plants to become weaker as they age. Plants with viruses may be removed or burned to avoid transmission to other plants.
Called leaf-roller caterpillars, these insects will sew themselves to the leaves of a canna lily and eat the leaf from the inside out. Leaves may look as if they are torn or as if they have been through a bad wind storm. Treat the affected plants either by removing the leaves or spraying with insecticide.
Attracted by the scent of the canna's flowers, Japanese beetles may cluster on canna leaves and eat them. The leaves will have holes or, if untreated, may be eaten down to the stem. Japanese beetles also attract other beetles, so they should be eliminated as quickly as possible. Remove infected leaves and dispose of them far away from healthy plants. You may also treat leaves with contact insecticide. An effective way to control Japanese beetles is to flick them into a cup of water yourself and let them drown before discarding.