Cannas in Malaysia are susceptible to what looks like a viral disease. The symptoms in Malaysia are stunting of plants, patches of dead tissue on young leaves, reduction of flower-size to half the normal size, and progressive stunting and weakening of the clump or bed until no more shoots appear. Separating out the rhizomes and cleaning them before transplanting does not help unless the disease is detected before it has spread to all the rhizomes.
At the early stage of infection, the bed should be dug out and the pieces of rhizome bearing swollen buds separated out for replanting. When new shoots emerge, the plants that are infected can be recognized by dead patches of tissue on their leaves. Such plants should be eliminated immediately, leaving healthy plants for starting new colonies. The virus takes several weeks to spread through a bed of cannas so there is time for such corrective action if one is alert to the symptoms of infection.
The virus does not travel in the air or soil. Plants in the next bed a short distance away may remain free of infection. I think the best way to prevent infection is to remove all canna shoots as soon as they have finished flowering. Such spent shoots should be cut off at their base. A regular once-a-week pruning programme will improve ventilation of the canna bed and deter white flies from settling on the leaves. White flies suck sap and are probably the agents spreading the virus in Malaysia.
In temperate countries, where cannas are sold as rhizomes, the canna virus has been disastrous for the canna trade because it is impossible to tell whether a rhizome is infected until it is grown out. In Malaysia and elsewhere in SE Asia, cannas are sold as full-grown plants, usually flowering, so it is immediately apparent from the condition of the plant whether it is infected.