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The Bulletin. 



on the aerial roots, the leaves, the stalk, the ears and the tassel. They 

 may reach large size, but sometimes are so small as to be readily over- 

 looked. 



Cause. — Oat smut is caused by the fungus Ustilago avenae; stinking 

 smut of wheat by Ustilago foetans; loose smut of wheat by Ustilago 

 tritici; and corn smut by Ustilago zeae. The fungi grow in the tissues 

 of the plants, drawing nourishment from them, but not giving external 

 evidence of their presence until conditions are suitable for their fruit- 

 ing, when they form the striking masses of dust-like spores. 



Fig. 9. Smut of Corn. Ear infection, with the grains enlarged and distorted. 



Dissemination and Infection. — The dark dust-like spores disseminate 

 all of these, but in rather different ways. Oat smut and wheat stink- 

 ing smut spores lodge on the grain in the field or become mixed with it 

 in threshing, remain alive on the surface of the seed during storage, 

 germinate with the seed on planting, and form infection spores that 

 enter the very young oat or wheat sprouts just after they emerge from 

 the seed, and before they have appeared above ground. This very early 

 period is the only one when infection by these two smuts can occur. 

 Once inside the young sprout these fungi grow upward in the growing 

 points and become established in many or all of the buds that produce 

 shoots in the stool. No effect is seen until the smutted heads appear, 

 months after the infection of the plants. 



Spores of loose smut of wheat, on being scattered, may sift into the 

 open flowers of nearby heads. The spores germinate immediately in 

 the moisture there, and the fungus grows directly into the very young 

 seed, and establishes itself in the tissues as the grain passes through 

 the milk and dough stages, becoming dormant as it reaches maturity. 

 The fungus remains alive inside the seed during storage, and resumes 

 growth in the seedling plants. It, too, grows upward in the shoots, and 

 is first apparent in the smutted heads. 



