FUNGI AND FUNGOUS DISEASES OF PLANTS 253 



before the smut appears on the surface, its mycelium grows 

 more or less throughout the corn plant. Every part of the 

 host may be infested by the mycelium, which grows wholly 

 by means of food material derived from the host. In addition 

 to this food tax levied upon the host by the parasite, the corn 

 grains and tassels are often occupied and eventually destroyed 

 by the smut. Finally the spores may fall to the ground and, 

 after a period of dormancy, germinate and produce a short 

 hypha, which bears spores that may serve to infect the next 

 crop of corn with the parasite. 1 



241. Oat smut. The behavior of this parasitic plant (Usti- 

 lago avenae) (fig. 197) is similar to that of corn smut. The 

 ripened spores may lie upon the ground, adhere to the grains, 

 or remain upon the straw until there are favorable conditions 

 for growth. Probably the grain used for seed is one of the 

 chief means of spore distribution. It has been found that by 

 treating seed oats for a brief period with hot water (132 to 

 133 F.) or with water containing -^ per cent formalin the 

 smut may be killed. 



It is possible for both corn and oats to mature grain even 

 while infected with smut, but usually partial or total destruc- 

 tion of the grains results. Some kinds of corn and oats seem 

 to be more resistant than others to attacks from the disease. 

 The importance of preventing the growth of smut by treatment 

 of seed oats and corn and by a search for resistant varieties 

 of oats and corn is evident. 



242. The rust fungi. The rusts are among the most widely 

 distributed and most destructive plant parasites. Many kinds 

 of plants are susceptible to attacks from rusts. Indeed, a given 

 kind of rust may live for a time upon one kind of host plant 

 and later upon another kind. In each stage the rust presents 

 a different appearance; because of this, in earlier studies 



1 "Corn Smut," Ind. Agr. Exp. Sta., 1900. 



"The Smut of Grain Crops," Bulletin 122, Minn. Agr. Exp. Sta., 1911. 

 "The Smuts of Wheat, Oats, Barley, and Corn," Farmers' Bulletin 507, 

 U.S. Dept. Agr., 1912. 



