PLANT DISEASES 291 



to infection, may escape. Varieties of wheat resistant 

 to rust are known. The group of wheats known as 

 durum, sometimes called macaroni wheats, contains a 

 great many varieties which are actually resistant to 

 rust. Some of the durum wheats are not at all resist- 

 ant, but a large number of the varieties have demon- 

 strated their resistant powers in experimental fields 

 as well as in actual epidemics. Unfortunately these 

 durum wheats are not adapted to all the localities 

 where other wheats can be grown so that they cannot 

 be recommended in place of the spring and winter 

 wheats in every case. In the raising of any crop all 

 the factors tending to the production of the most 

 profitable yield must be considered, and rust is only 

 one of the factors. Durum wheats do not command 

 so high a price as spring and winter wheats and the 

 farmer rightly takes this into consideration. The 

 fact remains, however, that many durum wheats can 

 be grown which will resist severe epidemics of rust. 

 It remains for the farmer to decide whether the rust 

 is a sufficiently important factor to warrant the growing 

 of this sort of wheat. Many experiment stations, as 

 well as the United States Department of Agriculture, 

 are now attempting by plant-breeding methods to de- 

 velop varieties of spring and winter wheats which have 

 resistant powers against rust and which still retain 

 the other desirable characteristics of these wheats. 

 The history of plant breeding has already shown that 

 it is possible in some cases even in a very short time to 

 develop by selection or crossing varieties which are 

 specifically resistant to certain diseases, and this is 

 hoped for in the case of rust of cereals. 



