Vegetable gardeners whose crops have in the past been decimated by diseases are always on the look-out for varieties labelled disease-resistant'.

But do they actually work? Have seed companies really made progress in stopping blight, mildew, clubroot and other diseases?

The latest research by Gardening Which?, the Consumers' Association magazine, which trialled 14 varieties - five flowers and nine vegetables - and enlisted the help of nearly 300 amateur gardeners, proved hugely disappointing.

The verdict, in the June issue of the magazine, concludes: "We were hoping that at least some of the so-called resistant varieties would be sufficiently less affected so that we could recommend them. Alas this is not the case, at least with most of the varieties currently available."

While breeders are constantly working hard to identify the genes which make plants resistant, disease can mutate almost as quickly as the production of new varieties. By the time the seed actually goes on the market, the disease may have changed.

Researchers tested the most common diseases including potato blight (which also affects tomatoes), clubroot, rust, powdery and downy mildew. Most plants were raised from seed, and each was grown alongside a similar but non-resistant variety. The disease was introduced either on plants from a garden or as fungal spores and monitored throughout the summer.

The newest blight-resistant tomato, Fantasio', was grown alongside the non-resistant Ailsa Craig' and while the latter was worst affected, with up to half the developing fruit damaged, between 20-30 per cent of Fantasio' fruits were also hit.

Cabbages and swedes were tested for clubroot, which causes the roots to swell and burst, greatly reducing crop yield.

Tests found that the resistant swede Marian' showed severe symptoms, while the resistant cabbage Kilaxy' (Suttons) was less affected than its non-resistant counterpart Marner Large White'.

Two flower varieties were tested for rust - which weakens plants as they show symptoms of powdery orange, yellow, brown or black spots on the leaves - the antirrhinum Rust Resistant Mixed' and non-resistant Liberty', and the hollyhocks Antwerp', which claims resistance, and Pastorale', which doesn't.

Tests found that there was no difference in the rate of infection for the hollyhocks, while the resistant antirrhinum actually suffered more.

Tests on phlox Blue Paradise', which is said to be resistant to mildew, and the non-resistant phlox Fujiyama', showed that both were badly affected by mildew, while there was also no difference in the result of tests for downy mildew on the so-called resistant butterhead lettuce Milan' and the non-resistant All the Year Round'.

The best bet is to stick to your favourite varieties and try alternative ways to prevent disease.

Keep tomatoes away from potato crops and ensure foliage stays dry by watering the soil only.

If you have clubroot-infested soil, lime it regularly to keep the soil above pH7, as clubroot prefers acid soil, improve drainage and start brassicas off in pots of compost.

If your flowers are suffering from rust, remove badly affected leaves, don't use a lot of nitrogen fertiliser, which produces lush soft growth, and consider crop rotation for your veg.

The full report is in the June issue of Gardening Which? For details on how to receive three issues of the subscription-only magazine for £3, telephone 01992 822800 or visit www.which.co.uk/gwtrial