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- 
