I'd remove the plant - the yellowing between veins could mean magnesium deficiency, but as the plant next to it is fine, I'd assume this one has a virus, though usually you'd see some distortion of the leaves as well as the mosaic yellow patterning.

Can I interject here Bamboo? If there is just yellowing of the leaves, is it just magnesium deficiency and can that right itself with some application or other...cannot remember WHAT! High potash? If there is deep veining to the leaves, is that more serious?

Thanks!

You can right magnesium deficiency, but why would two dahlias growing together not both be suffering magnesium deficiency - doesn't make sense, unless they're in pots. Epsom salts is good for magnesium shortage.

I was thinking the same Bamboo. They are not in pots. I have 4 other varieties of dahlia in the same area and the pale pink one is the only one suffering This is why I think it might be a virus; but how do I finf out ?

You don't, I'm afraid, you just have to assume - plants with viral infections usually look distorted, with yellow streaking, veining or mosaicing, but given that this one has performed in this way for 3 years, I think you can safely assume its a fault with this plant, and dispense with it.

I have the same problem...that's why I wanted to know! I have marbling on both plants though so should I get rid of both?

Thanks for input. Guess what my job in the garden is tomorrow !

If you've just got yellowing between the veins, Izzy, you could try epsom salts, but its a bit late to worry about it this year. Although if the leaves on yours are distorted as well as mosaiced, its more likely to be a virus. Is it the same variety as Roseberry's problem one?

why dont you keep it and plant it in a pot of compost next yr and see how it grows in it

No, Bamboo...not really distorted but mosaiced definitely. Perhaps I should lift them and put them in a pot, Speedy 10.
Should I lift them anyway?...have not grown them before!

I f it's a virus where does it come from ? Is it the earth or a grub or an insect or something!

It's inside the plant, and it is transmitted from one plant to another by sucking insects, such as leafhoppers or aphids, also by dirty pruning shears! Once it's there, there is no cure, and the only way to propagate the plant without transmitting it is by cell culture--terribly technical method difficult for home gardeners.