Defra confirmed an outbreak of Koi Herpesvirus (KHV) disease in the top pond at RHS Garden Hyde Hall near Chelmsford, Essex. The site has been issued with a Designated Area Order prohibiting movement of fish to and from the garden's ponds.
Hyde Hall curator Ian Le Gros said: "KHV spreads rapidly through water during warmer temperatures and there is no treatment or licensed vaccine to prevent or cure possible infections. As a precaution, all RHS gardens have ceased accepting donations of fish from visitors, as the introduction of fish from other courses is a potential source of infection."
The RHS followed advice from the Environment Agency, Defra and the Centre for Environment, Fisheries & Aquaculture Science. Fish carcasses have been incinerated and a four-year monitoring and testing regime will swing into action.
Hyde Hall representative Sue Carter said a value could not be placed on the fish that had been killed: "We still have about 12 large koi left but this has been devastating for us - it's so sad."