PLANT DISEASES 165 



with formaldehyde and wheat with hot water to kill the smut 

 spores. A different mode of application is illustrated in the 

 control of onion smut. In this case a formaldehyde drip 

 attached to the seed drill kills the smut spores in the soil im- 

 mediately surrounding the seed and thus protects the young 

 onion plant from infection. 



A second method is that of spraying the surface of the 

 leaves, fruits and branches of the host plants with a fungicidal 

 compound such as Bordeaux Mixture or lime sulfur. By this 

 means the spray compound forms a protective layer and 

 poisons any spores which may alight on the host as soon as 

 these spores begin to germinate. For example, apples are 

 sprayed to control scab and other diseases, and potatoes 

 are sprayed in the Northeastern States to control late 

 blight. 



A third method is that of selecting disease-free seed. This 

 is practiced to control corn root-rot, potato diseases, and 

 sweet potato diseases. In some cases, as in that of corn 

 root-rot, seed testing is an important item, while in other 

 cases it is necessary to secure seed from fields or from regions 

 where certain diseases are not present. 



A fourth method is that of varying planting dates so that 

 crops will escape certain diseases. For example, the earlier 

 spring wheat is planted the less will be the loss from wheat 

 rust. Another example is the growing of seed beans during 

 the hot weather of the fall in Louisiana when the temperature 

 is so high that it controls the anthracnose disease. 



A fifth method is that of eradicating other plants, usually 

 worthless, which may harbor plant diseases destructive to 

 crops. For example, the common barberry serves as host of 

 wheat rust and is being generally destroyed for that reason; 

 likewise the red cedar is the host of apple rust. Certain 



