Are you referring to nematodes? Nematodes are very difficult to control.

Chrysanthemum eelworm (not nematodes) will attack Penstemon. No cure, remove and burn the affected plants.
But to be sure it IS eelworm, you can take a few leaves and cut them up and drop them into a tube of water. Leave over night and the worms will come out and be seen wriggling around in the water.
Rather unusual thing to be afflicted with though, except where people grow lots of Chrysanthemums unhygenically.

Erm, Owdboggy's right, but I have to point out that eelworms are, in fact, nematodes of some variety. We tend to use 'eelworm' when one is being a pest and attacking something, and 'nematode' when we use an organic treatment for something like vine weevil, but they're all nematodes.

I was trying to avoid being pedantic! :)

Pedantry or clarity, hmm, sometimes its difficult to tell...

Still does not get anywhere with whether this is an eelworm/nematode infection though.

It's all very educational though.Thanks folks, I didn't know they were all nematodes either..

That's why I mentioned it - I only found that out myself about a month ago, Steragram, when I was researching something else.

Ah, good old serendipity strikes again!