Welcome to the Garden Web and thanks for posting. Your English is fine.

Your concern for virus is well taken but we might better understand your plants conditions with a photo attached. First let me say that in general canna virus infections typically do not affect only a portion of the plant. In other words, if the affected plant is "growing from the same planted rhizome" the entire plant should show the same symptoms.

To be on the safe side I would suggest you remove the affected plant. You can transplant it to a distant location away from your other cannas and observe it for this season or simply destroy it and remove the risk.

The cannas you are growing from seed have very low risk of having virus unless it is transmitted by other virused canna plants.

Regards,

Kent

Hello, and thanks for your reply.
You're right, I should have posted photos. Here they are :

Some leaves show weird yellow spots (photos 1-3):
http://imgur.com/lY0R1sL
http://imgur.com/bIpZg7F
http://imgur.com/yDNRphV

However in the middle of this leaves, some of them look quit normal to me (they were eaten by slugs and snails, but the colour seems right) (photos 4-5)
http://imgur.com/1EYxrpg
http://imgur.com/DRMLxry

On the other side of the bunch the leaves look ok (photo 6)
http://imgur.com/yBLdhGI

Do you think the first photos show the symptoms of a virus ?

Best regards,
Tanguy

Here is a link that might be useful: photos

Thank you for posting your canna photos. Unfortunately, in my opinion the first three photos definitely show typical potyvirus infection symptoms. Assuming your other cannas appear healthy I would remove these plants. We can only hope none of your other cannas have been infected.

One of the real dilemmas for canna growers is the fact that at times and under prime growing conditions the virus symptoms seems to disappear. The plants rapid growth seems to outgrow the rate of the virus infection and in effect mask the infection. The sad truth is the plant is still infected and capable of passing the virus on via sucking insects to healthy cannas. There is no cure for the infected plant or the rhizome it grows from.

If there is silver lining to this story it may be that I have not found in recent years virus transmission from seed collected from virused cannas. That is not a scientific confirmations but only my personal experience. Unfortunately seed harvested from our hybrid cultivars due not breed true.

Good luck,

Kent

Thanks for these informations.
I guess I will remove this plant as soon as possible. I just hope it didn't infect the nearby plants. I will see how they evolve through the summer.
Two more questions :
- Is it safe to plant new cannas on the same spot next year ?
- What should I do with the tools afterwards ? Is it safe to just wash them after I used them to dig out the rhizomes ?

Best regards,
Tanguy

The most important issue is to be sure to remove each piece of the rhizome. If you leave this area vacant for the balance of the season there should be no issue with replanting cannas there next year.
Your tools should not be a problem if you simply wash off the soil and let them thoroughly dry.
I hope this all works out for you.

Kent