79 
the mixture, and full directions for its preparation and 
use are given by the Board of Agriculture in Leaflet 23. 
Once the disease is evident in the field it is too late to 
attempt to control it by spraying, but the first application 
should be made some time before the first outbreak of 
disease is expected, z.¢., about the end of June or beginning 
of July in most districts. The mixture should be well 
stirred before use and both surfaces of the leaves of the 
plants should be thoroughly sprayed. Three or four 
sprayings at intervals of a fortnight or ten days should 
generally be sufficient to prevent any serious epidemic. 
The cost of such sprayings is relatively small, working 
out in normal times to about 25/- an acre for three applica- 
tions (ze, 2d. per square rod). Apart even from the 
prevention of Late-Blight spraying has been proved bene- 
cial to the plant; for sprayed plants-usually retain their 
leaves at least a fortnight longer than unsprayed. In 
certain districts of America where spraying is regularly 
adopted the increased profit is usually from £3 to £5 per 
acre. 
The subject of the resistance of different varieties of 
potatoes to this disease has received the attention of ex- 
perimental growers for many years. Darwin himself was 
for some years interested in the matter, and as a result, 
various species of wild potatoes growing in South America 
were experimented with and used for crossing. At the 
present time, therefore, something can be done to avoid 
the disastrous effects of the Late-Blight disease by grow- 
ing varieties known to be resistant to this disease. It must 
be said, however, that though a variety is highly resistant 
in one locality it may be equally susceptible in another 
district where the conditions of environment differ. It is 
also notorious that disease-proof varieties lose their resist- 
ance after a few years, either because the plant, in time, 
deviates from the original type, or because the fungus 
becomes slightly modified so that it is able to break down 
the resistance. Needless to say, a variety which is resistant 
to Late-Blight disease will not necessarily be resistant 
to the Wart disease and other diseases of the potato. 
Since the Late-Blight disease usually affects the crop most 
seriously towards the end of the season, it follows that 
early varieties do not suffer from the disease to the same 
extent as the main-crop and later varieties. By, there- 
fote, selecting varieties proved to be resistant in recent 
