From David Shaw in blighty North Wales.
There are two reasons why the official scores indicate Sarpo varieties are no better/worse in their official scores. than other varieties.
1) The scores are out of date. Testing in government laboratories is done before the variety is commercialised - the the case of Sarpo Mira, 2002 and Axona, 2004. The blight population in UK has changed since then. New strains have become common and one strain, Blue 13, is particularly virulent and can overcome most of the foliage resistance of high scoring varieties like Lady Balfour, Orla and Setanta. Blue 13 is not able to overcome the resistance of most of the Sarpo varieties. Retesting by government labs and and field trials by the likes of Sarvari Research Trust will update resistance scores. This takes time and money and many of the less popular varieties will not be tested in the near future.
2) Official scores for foliage blight and for tuber blight are based on a few experiments in the laboratory and are not always a good indication of what happens in the field. A good example is Axona which is said to be only moderately foliage resistant (0 - 9 score of 6). In field trials, Axona is still one of the most resistant varieties. Blight infections spread more quickly on Axona than on Sarpo Mira but only in severe blight conditions does this have an effect on yield. Incidentally, the scores given sometimes depend on the publication you look at. This may be a copy error. Sarpo Mira (score of 7 for foliage blight on the Briish Potato Variety Database) is still used as a standard in blight trials throughout Europe and is found to give the highest scores consistantly.
The newer Sarpo varieties are an attempt by the breeders to select potatoes with a range of maturities, skin colour and cooking/taste characteristics, different from the first Sarpos to be introduced (middle maincrop and high dry matter). The newer ones do have lower scores for foliage blight than Sarpo Mira but in most seasons, with a moderate to low blight pressure, the resistance of these new varieties is adequate and still better than most other so-called blight resistant varieties.
David