I doubt it - the organism which causes this overwinters in the ground - it won't mind having to move up a bit to get at your onions, membrane or no membrane.

Agree with Bamboo not a good idea unless you can grow in a friend or neighbour's garden

Thanks Bamboo and Moon growe for your answers, disappointed i wont be able to grow onions again. Would it be possible to grow them in grow bags in a cold greenhouse?
Thanks in advance,
Roger Hales.

I've no idea about a cold greenhouse, but it should be possible to grow them in a different area of soil in your garden, if you have it. I don't know enough about onion growing to know if they will cope in gro bags, but I wouldn't have thought the depth of compost in the bag would be sufficient if the bag were laid sideways.

I wouldn't recommend trying to grow onions in gro bag.