What are the strange growths on my dahlia?
Lynda C Smith, by email
Stefan says: You have the lovely pink, peony-flowered variety ‘Fascination’, but when you lifted the tubers, you found strange growths and wonder if you should destroy the tubers. Sadly, I fear the answer is yes, and as soon as possible, because what you have found are the classic symptoms of a disease problem called leafy gall.
Although you might not have seen it before, it’s common and widespread on many ornamental plants and, apart from dahlias, carnations, pelargoniums, gladioli, nasturtiums and sweet peas are probably those most often affected, although it does occur on many others. It’s caused by bacteria that live in the soil but although the disease has been known for many years, there’s still some mystery about how the infection comes about. There is some evidence that it can be transmitted on the seeds of nasturtiums and sweet peas. Once inside the plant however, the bacteria cause normally dormant basal buds to start into growth.
You then see the symptoms you’ve photographed: masses of short, thickened or otherwise distorted shoots, usually at or near ground level, on plants that may otherwise appear normal, although there may be some general stunting. It’s similar in some respects, although much more contagious, to the problem called crown gall, where bacteria causes hard, knobbly swellings at soil level on woody plants.
It’s important to destroy the affected plants – not in the compost bin – and try to avoid planting dahlias, or any of the other highly susceptible species, on the contaminated plot for as long as possible. You should also disinfect your hands in the way we have become used to with
Covid-19 (although, of course, the bacteria won’t infect people), and also any tools that have been used. This is to prevent spreading the bacteria to other plants. Never take cuttings from infected plants but buy fresh new stock and raise them in clean, new compost.