242 



Diseases of Economic Plants 



i:)acteria. The slimy, offensive decay usually begins at the 

 lower end of the main root and progresses upward. See 

 carrot soft-rot. 



Rust (Puccinia tragopogonis (Pers.) Cda.). — This true 

 rust is of somewhat general distribution, but 

 is not harmful. 



Powdery-mildew (Erysiphe sps.) is com- 

 mon, but not serious. Southern-blight (Scle- 

 rolium) is injurious in the South. 



SPINACH "■•** 



Blight.-'^'^ — This is the most destructive 

 spinach disease, causing a loss in eastern 

 Virginia estimated between S200,000 and 

 $400,000 annually. The diseased leaves are 

 mottled and deformed, and the plants 

 stunted. In general, the symptoms are much 

 as in the mosaics with the exception that 

 the plants are eventually killed. The disease 

 is due to a virus, and can be artificially 

 transmitted from plant to plant. In nature, 

 infection seems to be carried by plant lice 

 (aphids), and not to reside in the soil or to 

 be carried by the seed. Spraying to kill the 

 plant lice may be the means of conquering 

 this trouble. 



Anthracnose {Colletotrichum spinadoe E. & 

 H.). — Blotches appear upon the leaves, first 



Fig. 132.- White- as small, moist, watery areas, associated with 



rust upon sal- slight local wilting, followed soon by the ap- 



Aft h'^ 1] ^ pearance upon either side of the leaf of small, 



brown acervuli. The spots then change to 



gray and dry up. The disease spreads rapidly from plant to 



plant and is very destructive both under glass and in the 



open, rendering the plant unfit for use. 



White-smut (Entijloma ellisii Hal.). — The attacked 



plants are unmarketable, due to lack of uniform green color 



