402 DISEASES OF ECONOMIC PLANTS 



rapidity on hot days following rain, not so rapidly in con- 

 . tinuous dry weather. Especially after a rain, living plants 

 surrounded by dead ones may show symptoms of the 

 disease in the form of dense sterile mycelium upon the tap 

 root. All diseased plants have diseased roots marked by 

 injured rootlets and shrunken tap roots, accompanied by 

 depressed spots which are at first bordered by red dis- 

 coloration. The causal fungus is also apparent here, at 

 first as a white mold which later turns brown or yellow 

 and is finally accompanied by wart-like sclerotia. En- 

 largements, from which new roots are put forth, often 

 occur near the soil surface. Even the Unt of the diseased 

 plants is affected, the fibers being wider and larger, and 

 the spirals fewer and more uneven, than upon healthy 

 plants. The disease is truly of the soil, and the diseased 

 soil centers enlarge yearly as in other soil diseases. It 

 is subject to the modes of dissemination suggested on 

 page 65. 



Especial precautions should be exercised against the use, 

 for the purpose of legume inoculation, of soil which may 

 possibly be infested with the disease. This warning is 

 particularly necessary in view of the fact that alfalfa is 

 affected by the same disease, and the causal fungus is now 

 present in many alfalfa fields. 



As to treatment, rotation with immune crops in con- 

 junction with deep fall plowing is recommended. Shear 

 and Miles' say: " Rotation with immune crops at Terrell, 

 Tex., resulted as follows : 1904, planted to cotton, about 95 

 per cent of which was killed by root rot ; 1905, planted to 



> Shear, C. L., and Miles, Geo. F., U.S. Dept. Agr. Bur. Plant Indus. 

 Circ. 9, June 19, 1908. 



