The disease was confirmed on samples from a private garden in Yorkshire and is thought to be the first case of the year. The disease, which causes damage on Impatiens wallerina leading to wilting and severe defloration of certain colours, has not been seen in nurseries this season.
STC research scientist Dr Martin McPherson said: "There had been no sign anywhere this season but suddenly this week we have had an outbreak. I think we can expect to be hearing of a few problems over the next few weeks.
"If the wet weather continues it is going to be an increasing problem. It is just to let people know that it is out there."
McPherson called for growers to send in samples if they witness any symptoms.
STC has alerted the British Protected Ornamentals Association (BPOA), which has issued a report to members. BPOA technical committee acting chairman Wayne Brough said: "We don't want to ring alarm bells because it hasn't been found on any nurseries this year. Impatiens have gone the whole way through the production chain without any problems and most Impatiens left the nursery weeks ago."
It is not clear how the disease arose, though STC speculates it could be from plants bedded into bad soil or from an airborne source.
It said bedded plants are at greater risk due to the likelihood that amateur gardeners and local authorities do not use fungicides.
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