254 



DISEASES OF ECONOMIC PLANTS 



wise sound: or again several scales may be rotted. If the 

 outer scale is rotted, the bulbs are designated as " slippery 

 onions " by the growers. The rot does not spread from 

 scale to scale within the bulb. In storage it does not 

 progress rapidly in dry onions, but wet onions do rot 



-Onion affected with soft rot; longitudinal and cross section. After 

 Stewart. 



rapidly. The only suggestion toward control is to keep 

 the onions dry in storage. 



Black mold (Macrosporium parasiticum Thuem.). — Fre- 

 quently onion leaves which have been injured by some 

 cause, also old stems or seed stalks, assume a velvety, black 

 coating which often covers them entirely. This condition 

 follows only upon previous injury, weakening, or death, 

 from some other primary cause, chiefly as a sequel of the 

 downy mildew. Though not a primary disease its pres- 

 ence extends the injury started by other causes, breaks 



