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, i.e., 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 o>f 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 (i.e., 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- 

 fore, selecting varieties proved to be resistant in recent 



