662 THE FUNGI WHICH CAUSE PLANT DISEASE 



O. omnivorum Sh.'"^' '-^-*'' 



Mycelium dirty yellow; sometimes whitish when young, grow- 

 ing in the vascular bundles of the host; hyphse forming strands 

 and spreading from them, producing a rather dense arachnoid 

 layer on the surface of the host and bearing 1 to 4 branches arising 



Via. 448.— S. rolfsii, sterile mycelium growing on carrot. After Stevens and Hall. 



and growing at right angles from the same point near the ends, 

 3 to 5 ;u in diameter, tapering toward the ends.. 



It causes root rot on almost any kind of plant including among 

 its hosts a large variety of trees. The first description was by 

 Pammel in a Texas Bulletin; a later one was by Shear. The fungus 

 destroys the smaller rootlets, cortex of older roots and invades the 

 vascular system and medullary rays, resulting in wilt and death. 

 It may be seen as dirty yellowish strands or as a thin weft 

 superficially. Sclerotia-like Ijodies appear on the roots often at 



