302 FIELD CROPS FOR THE COTTON- BELT 



the cotton-belt. The " crown " rust is not a serious disease. 

 It usually appears a little earlier than the stem-rust and 

 little injury is noticed until the latter appears. 



There is no known treatment for rust. Some varieties 

 are more resistant to attacks of rust than others and the 

 only relief lies in the growing of these resistant sorts, chief 

 of which is the Red Rust-proof type. These so-called 

 rust-proof oats are not entirely rust-proof as there is nearly 

 always a considerable amount of rust on the plants. In 

 almost any variety there are some plants more resistant 

 to rust than others. As this rust resistance is heritable 

 to a greater or less degree, the possibility of breeding up 

 rust-resistant strains is great. To what character of the 

 plant rust-resistance is due has not been definitely estab- 

 lished, but most authorities agree that the cause is physi- 

 ological rather than morphological. 



369. Oat smut. This disease occurs in two closely 

 related forms both of which are noticeable exclusively 

 in connection with the flowering or seed-producing parts 

 of the plant. The most common and destructive form 

 (Ustilago avence) is known as loose smut in that it reduces 

 the entire flower-cluster or inflorescence into a black, dusty 

 mass of spores (Fig. 47). In the other form (Ustilago 

 Icevis) known as closed smut, the disease destroys only 

 the kernels, changing them into black masses of spores, the 

 glumes not being attacked. This form, therefore, remains 

 inclosed or hidden. In each of these forms infection occurs 

 only during the young seedling stage. The mycelia subse- 

 quently grow throughout the entire tissues of the develop- 

 ing plant, finally maturing the spores (or seed) in the 

 flowering portion. As these diseases are propagated from 

 year to year by the spores that are carried over on the 

 seed, they are easily controlled by various seed treatments, 



