322 Diseases of Economic Plants 



usually turn yellow and fall off. Spots upon the stems show 

 the same general character as those upon the leaves. 



The lower leaves, and the leaves of older plants, are most 

 subject to the leaf-spot. Plants late in the season, as of 

 the second and third cuttings, suffer much more than earlier 

 in the season. These crops, especially in a dry season, if 

 allowed to stand a little too long before cutting may be badly 

 spotted, and the loss in hay, through fallen leaves, may be 



4^ 



1 L' 3 4 



Fig. 168. — ■ Four alfalfa leaf-spots due to: 1, Ascochyta; 2, Pseudope- 

 ziza; 3, Cercospora; 4, Stagonospora. After Stewart and others. 



considerable. The plants are never killed by this disease, 

 though young fields which have not yet become well estab- 

 lished may be ruined. 



Its mode of dissemination is unknown, and seed treatment 

 is useless as a preventive. 



The only practicable treatment is to mow down badly 

 diseased plants with the hope that the new shoots which 

 spring forth may overcome the disease. If the disease 

 appears just before cutting time, the mowing should be 

 hastened a few days in order to avoid loss from leaf shedding. 



Yellow leaf-blotch ^'"^ (Pyrenopeziza medicaginis Fcl., 

 Sporonema). —This disease is known from Vermont to the 

 Pacific, in Europe and South America, and under certain 



