Field Air is working with the Corangamite Catchment Management Authority to spray Phospite - a salt-like chemical - over steep terrain in the Otways where a deadly mould is threatening grass trees, banksias and Victoria's floral emblem: the native heath.

Phytophthora cinnamomi (once known as cinnamon fungus) has microscopic spores that can destroy plants that have taken decades to grow - and is described as a 'biological bulldozer'.

It is transported on tyres and shoes - and is already a huge problem in Woowookarung State Park (Canadian Forest), the Brisbane Ranges National Park and the Otways.

"Threatened species such as Southern Brown Bandicoot, Long-nosed Potaroo and Swamp Antechinus all rely on grass trees for shelter and protection," small animals consultant Dr Barbara Wilson said

"By helping the grass trees to survive, we are helping to protect these mammals."

But the authority admits the phospite trial is no silver bullet.

"Once dieback is discovered in a tree, it can't be eradicated, but it can be controlled," CCMA Environmental Programs Manager Jessica Miller said.

"Applying phospite to the plants doesn't kill the disease - but it does boost the plants' immunity.

"It's a little like an immunization."

"This trial will give us a real insight into the effectiveness of aerial spraying phospite across a large area."

The Federally-funded program involves using the agricultural chemical in its liquid form - and at a stronger rate.

This is poured into a 1500 litre tank over the wings of the 'Air Tractor', which is then fed into a series of tubes for spraying.

"We're targeting the leaves, not the ground," Chief Pilot Stephen Rossington said.

"And it'll have to be reapplied every three to five years."

The CCMA said earlier trials showed phospite was effective in the battle against the mould during a Deakin University trial in the Otways 20 years ago - and again in the Western Australia's Stirling Ranges.

Ms Miller said State Government departments were mapping Phytophthora infestations across Victoria