SPRAYING PROBLEMS 143 



zoospores liberated therefrom germinate on the leaf; 

 the germ tube enters a stoma and a fine weft of 

 mycelium adpressed to the cells grows into the air 

 spaces and absorbs nourishment from them. With- 

 in forty-eight hours aerial spore-bearing threads grow 

 out from this internal spawn-weft, branch sparingly, 

 and bear conidia at the tips of the branches. The 

 conidia soon become detached, and are capable of in- 

 fecting neighbouring leaves or plants. Since myriads 

 of these conidia are formed on each diseased spot, the 

 chance of infection is great, and when conditions are 

 favourable it is easy to understand how rapidly the 

 epidemic spreads. As if touched by some magician's 

 wand, the aspect of the whole country-side changes 

 from green to brown and black, and within ten days, 

 in severe cases, there is nothing left but a waste of 

 rotting vegetation. If the disease run its course the 

 haulm perishes, and from that time forth tuber de- 

 velopment is arrested. In the Kingsbridge and Barn- 

 staple districts of Devonshire the haulm in early, 

 mid-season, and late varieties was dead before the 

 end of July, 1917, rendering a serious loss in the yield 

 inevitable. If we can prevent this loss, which exceeds, 

 on an average, 2 tons per acre, in disease-prevalent 

 areas we have increased food production, or, if a 

 certain tonnage of potatoes were required, there would 

 be a commensurate economy in land and labour which 

 could be utilized for other crops. 



Potato blight can be prevented by spraying with 

 Burgundy or Bordeaux mixture. These remedies 

 were first used in France. It was the practice to 

 dust the vines along the tracks through the vine- 

 yards in France with a mixture of copper sulphate 

 and lime to prevent theft. It was observed that the 

 powdered vines retained their leaves, whereas the 

 leaves of vines not so coated were destroyed. Mil- 



