APPENDIX 459 



by the bacteria present in the soil. Dr. Smith considers 

 that the rapid spread of the disease is caused by insects 

 of various kinds feeding alternately on diseased and healthy 

 plants. To prevent such an epidemic, when the disease 

 is known to be present, the crop should be thoroughly 

 sprayed with an insecticide, or better, with Bordeaux 

 mixture containing an insecticide, such a wash being pro- 

 tective against both insects and Phytophthora infestans. 



Diseased tubers should be gathered and burned, not 

 buried, nor thrown on the manure heap. 



When the disease is present and shows a tendency to 

 spread, the crop should be lifted at the earliest oppor- 

 tunity, as by so doing many of the tubers may be saved 

 if allowed to dry thoroughly on the land, and afterwards 

 used as soon as convenient. 



Potatoes showing indications of an internal brown- 

 coloured ring should not be used for ' sets.' 



Smith, Erwin F., U. S. Dept. Agric. Bull., No. 12 

 (1896). 



Massee, G., Journ. Board of Agric., ix. p. 308, pi. xi. 

 (1902). 



