358 FORAGE PLANTS AND THEIR CULTURE 



Rust ( Uromyces striatus) is another common leaf 

 disease recognizable by forming small spots of reddish 

 spores. Macrosporium sarcinceforme occurs frequently 

 in the East, forming pale circular spots bearing scattered 

 black spores. 



Two mildews also occur occasionally, especially in the 

 shade ; namely, powdery mildew (Erysiphe trifolii) and 

 downy mildew (Peronospora trifolii). 



None of these leaf diseases has as yet proven to be 

 of serious consequence. 



Anthracnose (Colletotrichum trifolii) occurs on alfalfa 

 in Virginia and Maryland, causing spots on both the stems 

 and leaves. These are at first purplish, then brown. The 

 stems are frequently girdled by the spot and then die, 

 and the whole plant may succumb. 



Alfalfa " yellows " is a disease of unknown cause. It 

 occurs quite commonly in Virginia and other Eastern 

 States. The leaves turn gradually to an orange-yellow 

 color, and the plant then ceases growth. When this 

 happens, it is best to cut the crop at once, even if but a 

 few inches high. It has been suggested that the disease 

 is probably related to the mosaic disease of tobacco, 

 known to be transmitted by a species of aphis. A species 

 of leaf-hopper seems to be constantly associated with 

 alfalfa yellows. 



421. Insects. Insects have thus far not proved a 

 serious menace to alfalfa culture in America, but locally 

 and in occasional seasons a large amount of damage may 

 be caused by grasshoppers or by caterpillars. The 

 recently introduced alfalfa leaf weevil may, however, 

 prove to be a serious factor to contend with. 



Grasshoppers are the most injurious insects to alfalfa 

 in the West at the present time, but the area of destruction 



