THE BROWN ROOT ROT OF TOBACCO AND OTHER PLANTS 



11 



Taking all these results into consideration, the writers have not 

 been able to compare satisfactorily brown root rot with any previ- 

 ously described phenomenon in plants. Whatever the fundamental 

 explanation may be, in a general sense the malady seems to result 

 mainly from the influence of one crop upon another. In this re- 

 spect it shows considerable resemblance to the injurious action of one 

 plant species upon another when grown in association or when 

 one precedes the other, as described by various workers already 

 referred to who have been concerned with the problem from an 

 agronomic standpoint. 



Unfortunately, sufficient details are not given by most of these 

 workers as to the symptoms, if any, exhibited by the roots of the 

 plants which are injuriously affected, to make possible a satisfac- 

 tory comparison with brown root rot, as described in this bulletin. 



Fig. 9. — Influence of soil temperature on the severity of brown root rot on tobacco 

 plants. Soil affected with brown root rot was placed in the jars of series A 

 (above) and the same soil sterilized in series B (below). Tbe plants were grown 

 at approximately 18°, 22°, 26°, 30°. 34 c , and 36° C, respectively, by immersing 

 the jars in water kept at the stated temperatures 



PLANTS AFFECTED 



In the absence of a known causal agency the determination of the 

 host range of brown root rot is obviously uncertain. It was deemed 

 important, however, to make some observations along this line, even 

 though conclusions could only be based on gross root symptoms. 



Such determinations are much more readily made with some plants 

 than with others. It was found especially difficult to draw satis- 

 factory conclusions on the grasses and grains, since roots on these 

 plants apparently die off readily from natural causes. 



Most attention has been given to crop plants which might profit- 

 ably be grown in rotation with tobacco. In most cases these plants 

 have been grown in brown root rot soil, both in the field and in the 

 greenhouse, with tobacco plants as controls. (Fig. 11.) After 

 growing in the soil for a sufficient length of time to give abundant 

 signs of the disease on tobacco, the plants were removed with the 

 roots, which were washed free from soil and examined for lesions 

 by floating out in water against a white background. 



