The ash dieback infection was identified on three ornamental shrubs – which are in the same family as ash – by staff at Westonbirt Arboretum.
However, the Forestry Commission does not believe these findings will have a “significant impact on the environment”.
It continues to urge people to report suspected sightings of the disease.
Prof Nicola Spence, the UK’s chief plant health officer, said: “Since 2012, the Government has invested more than £6m into ash dieback research.
“These findings highlight the importance of the Forestry Commission’s reporting system, Tree Alert, and of arboreta and other plant collections, which play crucial roles in supporting the UK’s world-leading plant health sector.
“Landscapers, gardeners and tree practitioners should be vigilant for signs of ash dieback on these new host species, and report suspicious findings through Tree Alert.”
In 2016, research suggested the ash tree was likely to be wiped out in Europedue to ash dieback and an invasive beetle called the emerald ash borer.
The government is working to better understand the biology of the infection and has invested in the “world’s largest screening trial for tolerant trees”, according to the commission.