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.! 
241. Oat smut. The behavior of this parasitic plant ( Usti- 
lago avenue) (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 ;‘5 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. 
