Conservationists warn today that mistletoe, favourite plant both of pagans and stealers of Christmas kisses, could vanish from the nation's halls and doorways within 20 years.

The National Trust fears that the decline of traditional apple orchards, where mistletoe thrives, may lead to the parasitic plant disappearing – or becoming much harder to obtain.

It is leading a campaign to prompt orchard owners and gardeners to think about nurturing the plant and wants householders to make sure they buy sustainably sourced mistletoe.

The trust says that in the cider heartland – Somerset, Gloucestershire, Herefordshire and Worcestershire – traditional orchards have declined dramatically in the past 60 years. Many that survive are not tended, hastening the death of the trees and then the plant.

Jonathan Briggs, an environmental consultant and a leading mistletoe expert, said: "Mistletoe is doing well right now. Those older orchards are probably yielding more mistletoe than they used to because it's not being controlled.

"But because the mistletoe is not being controlled, fast forward 10 or 20 years and the orchards won't be there. The mistletoe will accelerate the trees' deaths and it seems inevitable that we will have a shortage of mistletoe in 10 or 20 years."

Briggs - who keeps mistletoe in his house in Gloucestershire all year to ward off evil spirits - said mistletoe would migrate to bigger, non-orchard trees and so become harder to reach. This could turn it into a much more expensive product or prompt people to rely on cheaper foreign imports. "A Christmas kiss could become more expensive," he said.

Peter Brash, an ecologist at the National Trust, said it would be a "sad loss" if mistletoe declined or became harder to buy. "Mistletoe is part of our Christmas heritage and has a special place in a wonderful winter landscape," he said.

Brash said people should check where their mistletoe came from. "Ensuring your mistletoe comes from a sustainably managed, British source is good news all round. You will be supporting a small, homegrown industry, while helping to ensure a future for mistletoe and the creatures that are dependent upon it."

As well as brightening up the house at Christmas time, mistletoe provides winter food for birds such as the blackcap and mistle thrush. It also supports six insects, including the rare mistletoe marble moth, some sap-sucking bugs and the affectionately named "kiss-me-slow weevil" (Ixapion variegatum).