Vineyard owners have been warned that producers with particularly pinched pockets may fail to spray and that disease could spread to nearby vines.

Ken Bray, who heads the Hunter Valley Wine Association's viticulture sub-committee, points to a Pokolbin vineyard untended since the March harvest where marshmallow weed and wheaten grasses now grow mad because the owners have not spent the money to slash.

"It [disease] has been on people's minds since there's been an oversupply of grapes," said Mr Bray. "It's a bit like swine flu. It could spread."

Since wetter conditions replaced drought, the diseases most likely to hit the Hunter are airborne. Powdery mildew attacks the leaves, while downy mildew erodes the bunches, often leaving an unwanted taint in the fruit, which could also be susceptible to nibbling pests, Mr Bray said.

About 20 per cent of vines across the nation may need to be pulled out to make the industry viable. Wine Grapegrowers of Australia has commissioned a report to see if research backs the alarming figure, its chief executive, Mark McKenzie, said.

"It is an issue everywhere … Some people are doing the sums and saying if they don't get a contract for their fruit, they are not growing it," he said.

"Unless they can find a buyer [for a vineyard], they put it to sleep. They mothball it and revive it later, or choose to do nothing and walk away," Mr McKenzie said.

There have been suggestions that state or local governments should require vineyard owners to remove their vines if they plan to cease commercial operations.

But others opt for quicker fixes. Mr McKenzie knows of an owner in McLaren Vale in South Australia who offered to spray his neighbour's vines for free.

Legal complexities, including possible trespassing offences, face grape-growers who might be tempted to spray neighbours' vines without permission. It is not great for relationships, either, Mr Bray said.

"There are insurance issues in taking equipment onto someone else's property, and the responsibility for spraying with [a chemical]."