ENEMIES AND DISEASES 159 



Enemies and Diseases. Like most plants alfalfa has 

 its enemies and diseases which are troublesome under 

 certain conditions in some sections. Alfalfa dodder, a 

 parasitic plant which twines itself around the alfalfa plant 

 and lives upon its juices, often ruins fields of alfalfa. The 

 dodder seed is nearly the same size and color as the alfalfa 

 seed and is not readily noticed. When alfalfa seed comes 

 from infested fields it is contaminated with this pest. If 

 sown with the alfalfa seed, the dodder will not be noticed 

 until discovered in the growing field. When considerable 

 areas of the field are found to be infested it is best to plow 

 the field and run to corn or grain crops for a series of two or 

 three years before reseeding to alfalfa. Many of the states 

 now have seed inspection laws and a penalty is attached 

 to selling seed contaminated with mustard, dodder, buck- 

 horn, and other obnoxious weed seeds. 



In seed-producing sections leaf spot is quite common, 

 and often the crop is materially reduced or entirely ruined. 

 When leaf spot, mildew, or rust makes its appearance it is 

 best to cut the crop at once for hay. The succeeding crop 

 may be entirely free from these diseases. 



Gophers and prairie dogs often injure fields of alfalfa 

 by burrowing in the ground and eating the alfalfa roots. 

 Trapping and poisoning the animals seem to be most ef- 

 fective. Blister beetles, army worms, and grasshoppers are 

 destructive, at times. 



EXERCISES 



1. If a 2o-acre field is seeded at the rate of 16 pounds of 

 alfalfa seed per acre and the seed costs 20 cents per pound, what 

 is the cost of the necessary seed ? 



2. Bring to school for class use specimens of nodule-bearing 

 roots of sweet clover, red clover, and alfalfa. 



