I t has become increas ingly evident tha t the to ta l percentage of degenera t ive diseases in a s train of po ta toes is not an accurate measure of the value of the strain for seed. Th i s is especially evident when the amoun t of diseases is quite large, tha t is, more than is usual ly found in certified stock. Of ten one strain will ca r ry only leafroll and ano ther only mosaic, yet their total percentages of diseased plants are the same. An in teres t ing quest ion to table stock growers is ,--~Vhich has the grea ter effect in decreasing yields ? in Bulletilr 126 of the Storrs Agr icul tura l E x p e r i m e n t Station, the average yields of hills of the same strain of potatoeS, effected with mosaic or leafroll, are given as 0.58 and 0.33 pounds respectively. This indicates tha t leafroll is much more impor tan t than mosaic in reducing the yields. Some recent data corroborate this opinion. In g rowing over one hundred strains of po ta toes each year in the source oi seed tests at Storrs, condit ions are propi t ious for the spread of degenera t ive diseases, for there are a lways at least a few strains car ry ing large percentages of those troubles. Near ly all the plants usually become infected if g rown in the test plots for four or five years. New seed of a certain strain of Green Mounta ins has been obtained f rom Maine each year since 1922. Seed has been saved at d igg ing t ime each season, so that the 1925 series of plots contained new, one year old, two year old, and three year old substrains. S t range ly enough the s tock g rown here for three years outyielded by twen ty -e igh t bushels per acre that g rown here for two years. Th is was so unusual tha t a reason was sought to explain it. T h e total percentages of diseases were pract ical ly the same,--40.5 and 39.5, but the diseases cons t i tu t ing that total were ent irely different. These are shown in the fol lowing table and indicate that the twen ty four per cent difference in leafroll was much more impor tan t than the 22.5 per cent difference in mosaic.