298 DISEASES OF ECONOMIC PLANTS 



stunted. The tips of the rootlets are brown or black, and 

 the entire root system is involved later. The decayed 

 roots become so brittle that they are severed and remain 

 in the ground when the plant is pulled. Numerous 

 lateral rootlets are put out to replace these lost, but these 

 also succumb sooner or later. The leaves of diseased seed- 

 lings on rich soil are usually abnormally dark green. They 

 may, however, in poor soil be of a sickly yellow color. 



In the field the diseased seedlings may remain for weeks 

 without growth; some yellowing, wilting, and dying. 

 Some in light soil may survive and yield a fair, but late, 

 crop ; in heavy soil few survive to be of value. When 

 large roots are attacked, the effect is generally limited to 

 the outer surface, where a brownish, scurvy appearance is 

 produced. 



Sick plants in healthy soil or healthy plants set in sick 

 soil result in diseased crops, and the development of the 

 disease when the causal fungus is present is favored by 

 alkaline fertilizers.' 



To avoid loss from this disease only uninfected plants 

 should be used; to prociu-e these an uninfected seed bed 

 must be secured and maintained. 



An infected seed bed can be rendered safe by disinfec- 

 tion by the means suggested under soil disinfection, select- 

 ing the method best applicable to the conditions. Surface 

 firing is the method most widely used in connection with 

 the tobacco crop. Formalin has also proved successful 

 in many instances. 



The following measures conduce to reduction though 



• Gilbert, W. W., U. S. Dept. Agr. Bur. Plant Indus. Bui. 158, 

 1909. 



