Diseases of Plants 293 



metal vessels, an earthenware dish should be used. 

 Mercuric bichlorid is a deadly poison and should not be 

 used by children. 



A " black rot " of cabbage (also called " brown rot," 

 " stem rot," and " dry rot ") is caused by a bacterium. 

 Young plants are killed ; older ones lose their leaves and 

 become rotted. As young plants may become infected 

 in flats or in seed beds, these should be watched. Any 

 plants showing signs of the black rot should be de- 

 stroyed. The disease is seed-borne ; hence it is advis- 

 able always to disinfect seeds of cabbage with mercuric 

 bichlorid as directed above for destroying the germs of 

 bean blight. 



All the diseases mentioned above are combated by 

 destroying diseased plants, by keeping flats and frames 

 clean, and by crop rotation. 



THE DOWNY MILDEW OF THE CUCUMBER 



Through the eastern and the southern states the 

 downy mildew is a very destructive disease of the cu- 

 cumber and of all the vine crops related to it. The first 

 sign of attack is the appearance of yellowish spots on the 

 leaves. These rapidly enlarge until the leaves are almost 

 or entirely yellow or brown, when they soon die. The 

 older leaves are attacked first, and the disease progresses 

 toward the tips of the vines until the plants are either 

 killed or very much stunted. 



The disease is due to a fungus, which cannot itself be 

 detected with the naked eye. But an examination of the 

 discolored areas under a microscope will reveal colorless, 

 branched, and thread-like stalks of the fungus pro- 



