CHAPTER XI 

 PLANT DISEASES AND INSECT PESTS 



PLANT DISEASES 



Prevalence. — Plants are affected by diseases just as 

 animals are, and one of the very serious problems of the 

 farmer is to learn to know the diseases of the crops he grows, 

 and the most practical means of combating them. Many 

 volumes have been written about plant diseases, and there 

 is much that is still unknown about them. Plant diseases 

 are caused by parasitic plants that grow in or on the useful 

 farm crops and steal their living from them. Only a few 

 of the more common diseases of the principal farm crops 

 can be mentioned in a book of this kind. 



Loss. — An immense loss is sustained every year from 

 plant diseases. The loss on every farm from this cause is 

 very great, much greater, we believe, than most farmers 

 reaHze» Every year there is some loss from disease in 

 nearly every crop grown, but as a rule the disease is not 

 noticed unless it is quite bad. For example, one can very 

 seldom, if ever, find a field of wheat entirely free from rust; 

 yet the rust is usually unnoticed until a season occurs which 

 is very favorable to it, and a great loss is sustained. Such 

 a season occurred in Minnesota in 1914, and wheat rust 

 caused a loss of at least 30 per cent in the wheat crop, or a 

 money loss of over $15,000,000 in one state in one year. 



Rust affects a great many of the common farm crops, 

 such as wheat, oats, barley, alfalfa, and rye. As indicated 

 above, the loss is often enormous. At present there is no 

 known remedy except to select varieties of crops that are 

 capable of resisting the disease, and selecting early-maturing 

 varieties that are likely to ripen before the rust becomes 

 prevalent enough to seriously injure the crop. Winter grain 

 crops, because they ripen earUer, are less subject to rust 

 than spring-sown crops. 



Grain smuts are diseases that affect barley, oats and 

 wheat. The black or naked heads of grain, commonly seen 



