152 Some Economic Aspects of the Virus Diseases in Potatoes. 
potato growing. Mr. J. France, of Linthwaite, Huddersfield, 
kindly placed his garden at the writer’s disposal for this test. 
TABLE I. 
(Average weight of produce per plant at Weetwood, Leeds) 
Interveinal 
Variety. 
Healthy. 
mosaic . 
Crinkle. 
Leaf -roll. 
lb. oz. 
lb. oz. 
lb. oz. 
lb. oz. 
President 
.. 1 13 
1 8J 
1 8J 
— 
Majestic 
.. 1 14 
1 9 
10J 
— 
Arran Victory . 
.. 1 14 
1 3i 
15 
TABLE II. 
(Average weight of produce per plant at Linthwaite) 
Variety. 
King Edward 
Kerr’s Pink 
Majestic ... 
Healthy. Mild mosaic. Leaf -roll. 
lb. oz. lb. oz. lb. oz. 
38 — 13 
3 4 — 
43 2 15 
Seeing that virus diseases cause such drastic reduction 
in the yield of individual plants, it follows that the more 
diseased plants there are in a crop, the lower will be the 
yield. Insects spread the diseases from plant to plant in the 
field. The potato-growing districts of England have large 
numbers of insects, particularly aphids, and, therefore, virus 
diseases are there spread quickly. The stocks rapidly become 
totally infected with the diseases, the yield falls, and the 
growers must obtain fresh stocks from areas where insects are 
scarce, viz. Eastern Scotland or Northern Ireland. It is no 
uncommon experience to find that one diseased plant will 
infect sixteen surrounding plants in a lowland area, whilst 
in Scotland and northern Ireland the disease rarely spreads to 
more than one surrounding plant. Nevertheless, ordinary 
Scotch seed frequently contains up to about 30 per cent, 
of virus-disease-bearing tubers, and rarely has less than 
about 12 per cent. English growers, therefore, start with a 
considerable amount of disease in their stocks, and it is 
greatly to the credit of the Scotch and Irish seed raisers that 
they have put on the market virus-free potato seed before 
most English growers have realised the need for it. Such 
virus-free seed has been given the rather inappropriate name 
of ‘ Stock Seed,’ and is certified by the respective Ministries 
of Agriculture of Scotland and Ireland as having less than 
one per cent, of virus disease. It is the purpose of this paper 
The Naturalist 
