For nearly 80 years, one of the world's finest collections of rare plants was "lost", hidden behind the walls of an immense garden whose existence had been forgotten.

Then, a mere decade and a half after they were rediscovered, it appeared that the 70 camellias and 350 rhododendrons growing in the Lost Gardens of Heligan, in Cornwall, were to be lost again – this time, irretrievably.

Four years ago, the plants, all more than 90 years old, and probably dating back to the middle of the 19th century when they were gathered by Victorian plant hunters, were given a death sentence by the Department for the Environment, Food and Rural Affairs (Defra) after contracting a lethal fungal disease called sudden oak death. Staff at Heligan watched in despair as the mature rhododendrons were left dead or dying.

Defra ordered every rhododendron in the lost gardens to be destroyed, but there are specimens unique to Heligan which, if lost, could never be replaced.

Three of the plants attacked by the fungus died, but in a landmark project at Duchy College, North Cornwall, researchers cloned four others, creating disease-free plants which are being replanted in Heligan.

Specimens are going on sale in the Heligan plant centre, so that the rhododendrons can be grown outside Cornwall for the first time. This is a precaution, because the Heligan site is still infected and the new plants may one day be destroyed by the fungus.

The disease was first observed in the 1990s when it took a grip on the wild oak population of California, where the climate is similar to Cornwall's.

"It's very exciting to be able to use science and horticulture in such a pioneering way to save these remarkable and beautiful plants that have such rich heritage," said Ros Smith, Duchy College's micro-propagation project manager.

"In micro-propagation we take a clean part of the infected plant. The material is then taken into the lab and placed in pots of jelly containing chemicals and nutrients, which produce new shoots. We are in fact cloning the plant."

Peter Stafford, Heligan's managing director, described the breakthrough as "a fine example of the kind of pioneering work with nature we excel at in the Lost Gardens of Heligan".

Heligan's 80 acres of walled pleasure gardens date back to when the Tremayne family were the local squires. The shape of the garden was designed at the end of the 18th century, and the collection of plants was built over several generations.

One of the biggest contributors of rhododendron seeds was a collector named Joseph Dalton Hooker who spent the years from 1848 to 1850 travelling around Darjeeling, and making three excursions into the remote area of Sikkim in search of rhododendrons and other new plants.

In 1914, all 22 of the male staff signed up with the Duke of Cornwall's Light Infantry and went off to a war from which only six returned. The house was taken over by the War Office. It was returned to the family after the war, but they rented it out to tenants who could not afford the upkeep of the gardens, which were left to grow wild behind their high walls.

In 1990, John Willis, a member of the Tremayne family, who had inherited the gardens, had a chance meeting with Tim Smit, a Dutch-born businessman and record producer, and his colleague, John Nelson. They hacked their way into the gardens with machetes and uncovered a botanical wonderland, which they then developed as a tourist attraction.