BLIGHTS 



501 



I One-year-old plants may exhibit blackened areas in the crown, and 

 black streaks which run down into the tap root. As the plant grows 

 older, this blackening increases until the whole crown becomes in- 

 volved, and either the crown buds are destroyed or the root is no 

 longer able to perform its functions, and the plant dies. 



So far as our present observations go, the disease appears to run its 

 course with the first cutting, and those plants which have suflicient 

 vitality throw out a good growth for the second and third cuttings. 



Cause of the disease. — If a small piece of the yellowish green, 

 watery tissue from a diseased plant, or a fragment of the dried exudate 

 is placed in a drop of clean water on a glass sUde, there will appear on all 





Fig. 144. — Pseudomonas medicaginis. Twenty-four hour culture on nutrient agar; 

 stained with aqueous fuchsin; X 1000. {Original.) 



sides of it, after half a minute, a dense, milky cloud, which can be seen 

 readily with the naked eye, and which slowly diffuses out into the drop. 

 When this preparation is examined under the low power of the micro- 

 scope, this milky zone easily resolves itseK into swarms of motile 

 bacteria. 



The organism grows readily upon the ordinary culture media and 

 pure cultures of the germ, inoculated into scarified stems of healthy al- 

 falfa plants, produce the disease in seven to nine days with typical 

 symptoms. 



