Today's New Scientist has a letter from Andrew Sanderson, Co Durham who grew Sarpo Una and Sarpo Mira. He says that the Sarpo varieties suffered from blight whereas the variety Kestrel was unaffected. He makes no mention of the crop of tubers he harvested from each variety.
The blight scores recorded in the 'official' database for Kestrel is 5 for foliage blight and 3 for tuber blight (scale 1-9 where 1 is susceptible and 9 is highest resistance). On the same scale, Sarpo Mira scores 7 (foliage) and 9 (tubers); Sarpo Una foliage is 6 and tubers 5. In our own trials we find similar results except that Sarpo Mira usually scores 8 on foliage.
Now, what could be going on here? Have the Sarpo varieties changed in their resistance due to evolution of the blight pathogen? Before we jump to that conclusion there are other possible explanations. The short note does not say if plants were all grown close together in a garden plot or separated spatially. Microclimate varies across even a small plot so that after rain, foliage may dry more quickly in one microsite and slower in another. This would affect the severity of blight infection.
It is assumed that the blight was the late-blight disease caused by Phytophthora infestans. But dark spots on the leaves can be due to other diseases and pests or even to nutrient deficiency in the soil. Sarpo Mira often shows deficiency symptoms on lower leaves that are not disease related and do not seem to affect yield.
If Andrew has an experimental bent, he could try growing the same varieties from fresh certified seed and if similar results are obtained, samples could be sent to SRT for positive identification. Even better, he could give half the seed of each variety to a friend and have them grown on a different site and compare results.