Tuesday, 10 July 2007

Canna virus

Like all animals and plants, the Canna genus is susceptible to certain viruses, which may result in spotted or streaked leaves, in a mild form, but can finally result in stunted growth and twisted and distorted blooms and foliage. Known species of virus, confirmed by Dr Rick Mumford, senior virologist at the Central Science Laboratories in England, are:
  1. Canna yellow mottle badnavirus (CYMV) infecting canna species.
  2. Bean yellow mosaic virus (BYMV) infecting cannas, gladiolus, freesia and many legumes.
  3. Tomato aspermy virus (TAV), causes mosaic in cannas, but it has not been reported affecting cannas in the UK.
  4. Cucumber mosaic virus (CMV), cannas are susceptible to this virus, but none found yet in England.
  5. Canna yellow streak virus (CaYSV), this was recently discovered by scientists at the Central Science Laboratory in England. Dr Rick Mumford is quoted as stating “Typical virus symptoms include flecking, mosaic, leaf streaking and necrosis, which in severe cases render plants unsaleable.”
To put this in context, humans have dozens of virus to contend with, and the Dahlia enthusiasts have to live with over twenty different virus's, but they have learnt to live with them. Whenever an affected specimen is spotted, then it is burnt. In addition, the commercial growers do not attempt to sell affected stock. The latter is still not happening in the Canna world, and we are still seeing many commercial growers selling their badly affected stock. Once the commercial growers stop spreading this viral pollution, the genus will stabilise and the virus will stop being a major topic of Canna conversation.

Overall, very little is known about the Canna viruses, but the following points are generally accepted:
  • It manifests itself in rust coloured streaks or mottled markings on the leaves and in colour breaks on the flowers.
  • Sometimes leaves are slightly distorted and puckered.
  • Like many plants under stress, affected cannas will flower very early in the season and before the plant is full height.
  • Over the years the canna will loose vigour and become increasingly unsightly.
  • Some are spread by aphids and other sap sucking insects.
  • At one time it was thought that Cannas may have the ability to outgrow the virus, this has not been proven. It may be that stressed plants simply recover, and as the stressed plant exhibits similar leaf damage to virus it was assumed wrongly that the plant had virus.
  • Most authorities advise to burn all affected cannas and start again.
  • Keep any new introductions well away from potentially infected stock.
The point about plant stress being mistaken for virus is important, and I would recommend that you do not destroy plants until you are sure that they have virus. I quarantine any suspects, so that they cannot pass on virus, then they are spoilt rotten. Best soil, best sun aspect, best protection against hard weather, best food, best watering possible.

After a month, or so, of this life of luxury, if they are not looking healthy, then they are presumed to have virus and are destroyed.

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