Experiments on the Spread of Rugose Mosaic and Leaf Roll in Potato Crops in 1946

A series of experiments on the spread of potato rugose mosaic (virus Y ), and leaf roll, which has been in progress on a uniform plan since 1943, was ended in 1946. Mean values for thirteen centres in England and Wales showed that in 1946 69% of the infections with virus Y and 48 % of those with leaf‐roll virus reached the tubers of Majestic potatoes by the beginning of August. There was usually little subsequent increase of rugose mosaic, but a late increase of leaf roll was associated with a relatively high initial spread. Three‐quarters of the virus Y and over half the leaf‐roll infections occurred within five plants distance of the source. There was no close correlation between the spread of either virus and the maximum number of Myzus persicae , either apterous forms on the plants or alate forms caught on adhesive traps, but centres with high trap catches in July and August showed pronounced late season spread of leaf roll. There were marked differences at different centres in the relative spread of the two viruses. The amount of spread and the gradients from source of infection of the two viruses are compared over the period 1943–6.