SOME ECONOMIC ASPECTS OF THE VIRUS 
DISEASES OF POTATOES. 
I5i 
JOHN GRAINGER, PH.D. 
Field naturalists are probably much more familiar with 
virus diseases than may at first be imagined. There is a 
very common leaf -mottling ‘ Mosaic * disease of the dock, 
and a similar malady on dog’s mercury. If the naturalist 
be also a gardener, his raspberries are almost certainly 
attacked by a mosaic disease, his black currants are susceptible 
to ‘ reversion,’ whilst his potatoes are attacked by several 
distinct virus maladies, all of which reduce the yield severely. 
A virus is an extremely small particle, probably about a 
millionth of an inch across, and forty times smaller than the 
average bacterium. It is infectious and multiplies within 
the host -plant (or animal) which it attacks. Its effects 
are not, however, commensurate with its size, as will be seen 
later. Many philosophical studies have been made upon 
viruses, but their practical significance, though not neglected, 
is not fully realised. The potato crop is a very important 
one in Yorkshire, about 60,000 acres being grown in the 
county. Infection with virus disease makes it necessary for 
growers to spend large sums of money on the purchase of 
fresh stocks of seed from a relatively virus-free source ( i.e . 
Scotland and Northern Ireland). 
The chief diseases are Crinkle, Interveinal Mosaic, Mild 
Mosaic and Leaf -roll. Crinkle causes a considerable lobing 
of the margins of infected leaflets, with waving and crinkling 
of the lamina. A mottling of well-marked light -green areas 
also appears. The whole plant is stunted. Interveinal 
Mosaic also produces a mottle upon the leaves, but the light - 
coloured parts are more diffuse than in crinkle, and are found 
between the main lateral veins. There is no malformation, 
and the plant is not greatly reduced in size. Mild Mosaic 
gives only a very faint mottling and no malformation. Leaf- 
roll causes the plant to become very dwarfed, and to appear 
generally chlorotic. The leaves stand rather stiffly in a 
vertical position, and the leaflets are rolled in a funnel- 
shaped manner. In the early stages of infection the lowest 
leaves have their margins only incurved. 
Some idea of the reduction in yield caused by particular 
viruses can be obtained from Table I, which shows the average 
yields of about 40 plants of each kind grown at Weetwood, 
Leeds, on heavy land not particularly suited for potato 
growing. Table II shows the relative yields of healthy 
and diseased plants grown on a light soil more suitable for 
1933 July 1 
