Thoughts on virus from Hart Canna

CANNA VIRUS DISEASE
A guest article authored by Keith Hayward, Hart Canna

Most gardeners with a particular interest in cannas will by now be well aware of the advancement of canna virus which was unknown not many years ago, and which is now a worldwide epidemic. It has infected virtually all, if not all canna growers in most countries of the world. Most private collections in many countries are thoroughly infected, and some have been totally destroyed by it. Many good heritage varieties have been lost, maybe for ever.

At Hart Canna, the low point of our own involvement with canna virus disease was 2005, when we became aware that most of the cannas we were selling were infected, and most of the plants in our National Collection were infected. In case this reads like a shameful admission, it should be realised that, at that time, the world's major suppliers of cannas in the Netherlands, France, the USA, Israel, Australia, were thoroughly infected with virus disease. It was difficult to buy a healthy canna anywhere. Any canna rhizome or plant purchased in any garden centre, hardware store or nursery was virtually certain to be diseased (and we think, still are). All cannas grown in municipal planting schemes were clearly diseased (and still are). Many canna enthusiasts had collections of cannas that were totally diseased, and some were quite unaware of it (and some still are).

At that time we faced a difficult commercial decision. Should we continue in business selling diseased stock, or should we decide not to sell diseased cannas, in which case we would have nothing to sell. We decided that we would not knowingly sell any plants with disease, and if this was to be the end of our business then so be it. Eight years of work would come to nothing. Then providence took a hand. We managed to find from various sources a number of varieties that appeared to be reasonably healthy. Also, a customer who had become a friend gave us a van load of healthy rhizomes. So we were able to put together a catalogue of some 40 varieties which we were reasonably confident were healthy. This was a reduction in the over 100 varieties that we had been selling previously, but even so it meant that we were able to continue in business in 2006.

We built on that new beginning for our 2007 catalogue. The cannas that we sold in 2007 were grown from stock that was reasonably healthy the previous year. The same policy has been adopted for our 2008 sales. We hope that the worst of this virus crisis is behind us.

But not all growers and suppliers have decided to throw away their diseased plants and to start again with healthy plants. Most of the "big boys" and many of the "small boys" in the business are continuing to grow and sell diseased cannas. They will say that nobody notices that the plants/rhizomes that they buy are diseased; that they still get good sales, that the diseased cannas still produce a good crop of flowers, and that not many people complain anyway. The result is that most of the commercial supplies of canna to our retail outlets still continue to be diseased.

So what is this disease that has caused all this trouble? Firstly is should be said that most plants are susceptible to virus diseases. Apples, bananas, strawberries, potatoes, orchids, dahlias, lilies, daffodils, and most other plants, even mushrooms, they can all get virus disease. But there are strict rules about food crops, because food is deemed to be important. Ornamental plants like cannas are deemed to be unimportant, and there are few restrictions on the importing and exporting of diseased plants and rhizomes. So, diseased cannas find their way to our retail outlets. If it was just one or two, it wouldn't matter very much, because you could rogue out individual diseased plants, as gardeners do with e.g. dahlias. With cannas, virus hit hard, and it hit everywhere. You couldn't rogue out the odd one that showed the disease, because they all had it.


HERE ARE SOME FREQUENTLY ASKED QUESTIONS (FAQ):

• What do diseased cannas look like? Some pictures are shown below. The initial symptoms are light green speckles on the leaves, and short light green streaks that are parallel with the leaf veins. As the year progresses, so it gets worse and worse, and the pale green streaks in the leaf become dead streaks. The plant looks very diseased. By the end of the year, all the leaves, even the new leaves, are distorted, twisted, and streaked with dead areas. You also get white streaks in the flowers.

• Do all diseased cannas look the same? No. It is usually easy to see in green varieties, more difficult in bronze. It is very easy to see in C. ‘Phasion’, as short bright green streaks against a red background. It is very difficult to see in C. ‘Pretoria’ - the normal yellow stripes just look a little sandy. Sometimes it seems to produce corrugations rather than speckles and streaks.


• Can it be confused with any other condition? The first leaf that sprouts from a rhizome can be naff anyway - watch for the second leaf. Red spider mite causes leaf browning which superficially can look like virus. Root stress, due to transplanting plants that are growing, or using poor compost can cause leaves to have brown edges.


• Can it kill cannas? It seems to weaken them so that many don't survive the winter.


• Can all cannas get it? Yes. No varieties are immune in spite of what some folk say. Bronze varieties get it just as much as green.


• Can it be cured? No, except by the scientific procedure of meristem propagation (we have built a laboratory at our nursery with this intention) but it is not easy.


• How is it spread? Aphids are believed to be the main vector, even though you don't usually see aphids on cannas. I know from my own experience that Red Spider Mite doesn't spread it.


• Can it be spread by e.g. knives used for pruning? Maybe. Don't risk it.


• How do you sterilise knives? We have a pan of continuously boiling water. Whether it works or not we don't know.


• Can it be spread by touching a healthy canna after touching a diseased canna? Maybe.


• Can it be maintained in the soil from one year to the next? My personal view is no, it can't.


• Can cannas grow out of it? A plant pathologist would say no. My own view is that occasionally they can.


• Can it spread from other plants to cannas? I haven't noticed this happening. A healthy population of cannas usually remains healthy.


• Can it be spread in seeds? Maybe not, or only a small percentage of seeds carry it.


• Can the symptoms be masked by good growing conditions? Some people say so. Personally I don't think so - you just need to look closely.


• What is the virus called? There are a number of viruses that have been identified in diseased cannas: Canna yellow mottle badnavirus (CYMV) infecting canna species. Then there is Bean yellow mosaic virus (BYMV) infecting cannas, gladiolus, freesia and many legumes. Tomato aspermy virus (TAV), causes mosaic in cannas, but it has not been reported affecting cannas in the UK. Cucumber mosaic virus (CMV), cannas are susceptible to this virus, but none found yet in England. Canna yellow streak virus (CaYSV), recently discovered by scientists at the Central Science Laboratory in England. Dr Rick Mumford, senior virologist at CSL is quoted as stating "Typical virus symptoms include flecking, mosaic, leaf streaking and necrosis, which in severe cases render plants unsaleable."

• How quickly and how far can it spread? My own view is that it often spreads to cannas alongside diseased cannas. It doesn't seem too good at jumping distances of even a few yards. Commercial growers grow fields of cannas all crowded together which is the ideal conditions for it to spread.

• Are the varigations in variegated varieties caused by virus? I think not. I had samples of C. ‘Phasion’ and C. ‘Pretoria’ tested, and they were determined to not have virus disease.
  

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  How can virus disease be identified? Virus particles can be seen under an electron microscope. The particular type of virus is identified by immunological tests. Not many laboratories are able to do this.

PHOTOS

The beauty of cannas is the perfect foliage. Healthy specimens a beautiful shape and should have no blemishes at all. If a canna grows new leaves that are already blemished as they unroll, then suspect virus. The main symptoms, as these pictures show, are pale green speckles and streaks. Later in the season, these pale areas die and turn brown, and then the plant looks very sick. The flowers also show pale speckles and streaks.

Notice the pale streaks parallel to the leaf veins. A closer view would show lots of speckles.

See the speckles and streaks.

This leaf of Phasion (Durban, Tropicanna) should be burgundy coloured, with pink stripes. Virus replaces the pink stripes with light green stripes.

The streaks are now beginning to turn brown.

Diseased C. 'Centenaire'. The leaves now have dead areas.

Diseased C. 'Wyoming', late in the season. It is now looking awful.

A field of diseased cannas in the Netherlands.

References:

Wikipedia article on Canna Virus

Hart Canna on the Web