514 DISEASES OF CULTIVATED PLANTS 



matter in the tissues infested, hence the course of the disease 

 can be clearly followed in the haulm and tubers. 



The presence of the parasite is first indicated by the 

 wilting and shrivelling of the leaves. Soon afterwards dark 

 brown streaks appear on the haulm ; these streaks gradually 

 extend downwards and pass along the underground branches 

 into the tubers. Once in the tuber, the disease shows as an 

 imperfectly formed pale brown ring, situated at some little 

 distance from the outside of the tuber, and corresponding to 

 the vascular bundle zone of the tuber. As the disease 

 progresses the brown ring becomes broader and darker in 

 colour until finally only the skin remains, the entire starchy 

 portion of the tuber having crumbled to powder and the 

 myriads of bacteria it contained are liberated in the soil, 

 where they live until an opportunity is offered for infecting 

 another crop. 



The leaves and stem are as a rule first infected, the bacteria 

 gradually following the vascular bundles of the stem down 

 into the tuber. In some instances infection of the tubers 

 may take place directly by 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 protective 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 opportunity, 

 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 ring should not be used for 'sets.' 



Smith, Erwin F., U.S. Dept. Agric., Bull. No. 12 (1896). 



Blackleg of potatoes. This disease is due to a bacterium 

 called Bacillus phytophthorus. It has been known for some 

 time on the Continent, but fortunately it is as yet rare in 

 Britain; there 'is reason, however, to fear that it is spreading 

 with us. It is very destructive in its effects, and in Germany 



