288 Diseases of Truck Crops 



one application will suffice; two to four applica- 

 tions may sometimes be necessary, especially in low, 

 wet, and poorly drained lands where the disease 

 is severest. 



Smut 



Caused by Urocystis cepuloi Frost. 



Next to blight, smut is the most important disease 

 of the onion. The trouble is found wherever onions 

 are grown, and it does not seem to be limited by 

 climatic or soil conditions. 



Symptoms. Smut is at first characterized by dark 

 spots on the seedling leaves (fig. 54 e). When held 

 up to the Ught these spots are opaque. Later longi- 

 tudinal cracks appear on one side of the spot, which 

 widen, exposing within a fibrous mass covered with a 

 black powder made up of the ripe spores of the fungus. 

 Yotmg infected seedlings usually die early. Those 

 which survive later show smut pustules on the leaves 

 and the outer scales of the bulbs. The disease may 

 be carried from infected fields with the seed, with 

 infected manure, and by man himself on infected soil 

 particles adhering to his shoes or implements. 



The Organism. The spores of the fungus are able 

 to retain their vitality for a long time, possibly 

 twelve years. The spore baU (fig. 54 f) is made up of 

 sterile cells and spores. The latter (fig. 54 g) germ- 

 inate in the same way as spores of other smuts, see 

 com smut, p. 252. 



