Outbreaks have been widespread in UK parks and gardens throughout August, leading to the defoliation of busy Lizzies.
The British Protected Ornamentals Association (BPOA) and Stockbridge Technology Centre (STC) are investigating the extent of the problem in commercial nurseries and are urging bedding plant producers to send back surveys that were issued last month.
ADAS consultant Wayne Brough said: "The disease has been found more on nursery sites this year, where before it emerged at garden centres. It has been worse than usual and started a lot earlier. Plants were on the nursery for longer and became less spaced out as stock was not sold (because of poor weather)."
BPOA technical committee chair Michael Smith agreed: "(Downy mildew) got out early because of ideal climatic conditions for it in April. Before we knew it, there were cases all over the place."
Growers controlled their outbreaks with professional fungicides, unavailable to the amateur market, but symptoms emerged in garden centres and gardens later in the season.
BPOA marketing committee chairman James Alcaraz said all the growers he spoke to at the Four Oaks Trade Show told him their product had "gone out clean".
Reports have said it is a new strain of Impatiens downy mildew, but Alcaraz argued: "It is not a new strain. We know what it is and how to treat it. Idle talk shouldn't be spread."
But according to STC director of plant pathology Martin McPherson: "There does appear to be a difference. This year, resting spores are being seen for the first time. They appear as blackened streaking in the stem tissue.
"It means this disease is fundamentally different. Whether it's a new strain is up for debate.
"There is the potential it could establish in wild Impatiens. It would be a huge concern if it did."
The pathogen responsible for the infection, Plasmopara obducens, first appeared in the UK in 2003 and was believed to have come in on seed supplied from the US.
McPherson said it is not known whether the disease is now being spread via seed, soil, air or vegetatively, or if it overwinters.