University of Vermont Extension 
Department of Plant and Soil Science

Fall News Article
line
CANNAS AND VIRUSES
 
Dr. Leonard Perry, Extension Professor
University of Vermont
 
           
Cannas are a tender perennial, popular and grown by gardeners for their large attractive leaves and large flower spikes.  Being tender, their roots need to be overwintered indoors in cold climates.  Within the last ten years, virus diseases have become widespread in cannas throughout the world, so if you have these perennials you may not want to overwinter them but rather discard them this fall after tops are killed by frost. 
          
If you're not familiar with the canna, it is the only genus in the Canna family.  Although tropical, most of the 100 or more cultivars (cultivated  varieties) have been developed as hybrids  in temperate climates. There, they are grown outdoors in summer for their bold foliage, coarse visual texture, tropical effect, and flower spikes in colors usually from red to orange to yellow.  Leaves are green, reddish bronze, or striped.  Depending on cultivar and conditions they may range from 2 to 6 feet or more tall.  They prefer full sun, or at least 6 to 8 hours a day, and warm temperatures.  They may tolerate some drought but many also can be grown as aquatic plants.  Although tops will die back if frozen, the roots (rhizomes) may survive in the ground if mulched where air temperatures stay above zero degrees (F). 
           
Where temperatures freeze outdoors, plants are cut back after frosted, and then dug, or allowed to dry in their pots, and held overwinter in a non-freezing location.  Then water in spring, increasing the amount as growth resumes.  This is the process that has been used until recently when at least three viruses have been found overwintering in such plants, and spreading through plants worldwide.  This has resulted in many growers no longer selling cannas, and many collections having been destroyed.  Larger and reputable specialty growers, and some botanical gardens such as Longwood in Pennsylvania (longwoodgardens.wordpress.com) have obtained or selected virus-free plants and are propagating from these.
           
Two of the viruses, the bean yellow mosaic and canna yellow streak, are in the potyvirus group which is not passed along in seeds.  So plants with these could be grown from seeds and be clean of virus, until reinfected.  The canna yellow mottle virus is in the badnavirus group which can be passed through seeds-- unusual for viruses-- so continues the disease and its spread.
           
What you'll see with virus-infected cannas first are short, light green streaks in leaves parallel with the veins.  Close inspection shows lots of speckling.  These streaks grow larger, then turn to dead (necrotic) streaks, with lighter colored streaks in the flower petals.  As more of these dead veins appear, leaves turn a more general yellow (chlorotic) and the plants will be less vigorous.  These symptoms are more easily seen in green than bronze cultivars.  It is easy to see in some such as 'Phaison' with light green streaks against the red instead of the usual pink striping. In other cultivars, such as 'Pretoria', it is difficult to see with the usual yellow streaks appearing more sandy in color. If you're not sure, watch the leaves as they unfold in spring.  If they have such streaks or blemishes they are likely infected.
           
The only effective solution so far for infected plants is to destroy (not compost) them in trash bags, or to bury or burn them. Destroying infected plants will slow the spread of this disease, and help prevent future "clean" plants from becoming infected.   Although you may not see aphids on cannas, these can spread viruses.  If a low level of virus, you may not see symptoms or they may be mild and plants may continue to grow normally.
            
If you suspect a virus infection, and have more than a few plants severely infected, you may want to have a laboratory test done as several other problems such as nutrition may mimic virus symptoms.  Check with your local state university diagnostic lab to see if they can do such virus testing (www.npdn.org), or tell you where to send samples.
           
Underwatering or root stress may result in brown leaves.  Spider mites may cause speckling and eventual browning.  Blackened leaves, beginning with rust-colored spots on undersides, is likely canna rust disease-- most prevalent under hot and humid conditions.  
           
Then, when buying future cannas, make sure the sources indicate if the plants are virus free and discuss how they have ensured this.  Even if you buy virus-free plants, they may get infected from sick plants nearby.  If you like the appearance of cannas, but avoid or discard them due to virus disease, consider some of the other large-leaved tropical plants as alternatives such as ornamental bananas, elephant ear's, and ornamental gingers.

Return to Perry's Perennial Pages, Articles uvmext logo