Cannas are relatively free of disease, however virus problems have been exacerbated in recent years by growers ignoring disease and shipping virus-infected rhizomes. Symptoms of virus infection range from mild to severe; resulting in streaks or spots in the former case and stunted growth as well as twisted and distorted foliage and blooms in the latter.
Cannas are susceptible to Canna Yellow Mottle Virus, Bean Yellow Mosaic Virus, and Tomato Aspermy Virus. Over years, affected cannas will lose vigor, become unsightly, and flower before plants are mature. These viruses are spread by aphids and other sap-sucking insects. There is no treatment for viruses. Destroy (burn) affected plants and start again with healthy rhizomes. Keep any new introductions separate until their health can be established.