252 DISEASES OF ECONOMIC PLANTS 



To prevent the spread of the disease, and to lessen the 

 danger from subsequent attacks, a spray of Bordeaux 

 mixture, 5-5-50, appHed as soon as the disease is noticed, 

 is needed. Careful supervision and spraying to check 

 renewed attacks of the fungus, particularly if the weather 

 continues to favor the fungus, should not be neglected. 

 New growth should also be protected by the use of Bor- 

 deaux mixture. 



If subject to this disease in previous years, it is well to 

 spray before the disease appears; also throughout the 

 season, reaching all sides of the leaf with the fine spray 

 and using high pressure to make the mixture stick to the 

 smooth onion leaves. 



Wet lands are more subject to this disease and should be 

 avoided if possible. It is recommended to rake and burn 

 the dead tops, to destroy the infective spores within them, 

 and to practice crop rotation. 



Vermiculariose (Vermicularia circinans Berk.). — The 

 outer scales of the bulb after harvest often show black 

 growths of pecuUar and striking character. They consist 

 of central black dots or very small rings, around which 

 from one to several larger rings are arranged concentric- 

 ally. Under moist conditions these markings enlarge 

 rapidly by the production of more rings or wavy hnes, 

 or, under some circumstances, the whole area may be 

 overgrown with black. Later the disease may extend 

 through the outer bulb scale and into the next and suc- 

 ceeding layers to considerable depth, causing a dry, black 

 rot. 



The black spots and rings are due to the pycnidia of a 

 fungus, each pycnidium closely studded with numerous 



