4 BULLETIN 765, U. S. DEPARTMENT OF AGRICULTURE. 



The symptom of this disease is, then, a decay of the root system, 

 resulting in a stunting of the plants, roughly proportional to the ex- 

 tent of the decay in the roots. The effect of the disease on the root 

 system and the growth of the plant is shown in figure 1. Very fre- 

 quently the plants make no growth during the season, on account of 

 this disease. Curiously enough; root-rot rarely kills the plants in the 

 field. Aside from becoming stunted and yellow, they may show wilt- 



Fig. 1. — Plants of Burley tobacco grown on "tobacco-sick " soil (A) and in soil free from 

 disease (5i, showing the relative number of roots. These plants were grown under 

 identical conditions, except that the soil in which plant B was grown was sterilized, so 

 as to kill the parasites causing the disease. 



ing. even in very moist soils, on days when tobacco in healthy soil 

 and other crops show no wilting. The depleted root system is not 

 able to take up water as fast as the leaves lose it, causing temporary 

 wilting. It is not. however, a typical wilt disease, since the leaves 

 do not remain wilted. Crops with root-rot suffer from ;% drought,'* 

 therefore, very much more quickly than healthy crops. 



The importance of this disease is shown by the fact that careful 

 observations over a considerable part of the tobacco-growing sections 



