SOIL REACTION AND 

 BLACK ROOT-ROT OF TOBACCO 



Bv P. J. ANDERSON, A. VINCENT OSMUN and W. L. DORANi 



Black root-rot probably causes greater loss to the tobacco growers of the 

 United States than does any other disease. In the Connecticut Valley, accord- 

 ing to published records, it has caused varying amounts of damage since 1906. 

 It was probably prevalent long before that time, Init had escaped attention. 

 Within the last decade its ravages have been reduced considerably through 

 improved cultural practices, based on a better knowledge of the effect of cer- 

 tain environmental factors on the virulence of the disease. Nevertheless it 

 still takes its annual toll from the crop and, as one of the causes of "tobacco 

 sick" soils, is an important factor in tobacco production in Massachusetts. 



SyMPTOivrs OF Black Root-Rot 



The first symptom of the disease which the grower notices in tlie field is 

 that the tobacco doesn't grow. The plants remain stunted, with narrow, tliick, 

 tough leaves and either a starved yellow color or, where the nitrogen sujjply 

 is high, a \ery dark green color commonly called "black" by the tobacco man. 

 On hot days the leaves wilt and "flag" more quickly than healthy tobacco 

 plants. The dwarfed plants "top out" prematurely. Only rarely is a field 

 equally affected in all parts. Usually there are "patches" from a square rod 

 to several acres in extent, where the tobacco is short while in other parts of 

 the field ihe growth is normal. In the diseased "patches" the plants are fre- 

 quently very uneven in growth. 



From the above ground symptoms, however, it is not possible for even an 

 expert to be sure that this is black root-rot. Other troubles such as brown 

 root-rot, lack of fertilizer, or water-logged soil may produce the same appear- 

 ance. One must dig the plants and wash the soil from the roots to see the 

 lesions which are the unmistakable signs of black root-rot. Normal young 

 roots are white, but on a diseased plant many are black (brown at first), 

 either throughout their length or frequently only in segments, with other 

 segments appearing normal. Most of the ends of the small roots are black, 

 indicating that in digging the plant, the root broke at this point and the de- 

 cayed end was left in the soil. The tissue of the smaller roots is rotted 

 through, but on the large roots tliere occur enlarged, rough, scurfy lesions 

 which may or may not kill the interior tissues. Frequently the tap root is 

 entirely rotted off at the bottom and there is an increased number of laterals. 

 This results in a brush-work of intermingled brown, black and white small 

 roots just above the blackened end of the main root. The brown lesions are 

 caused by the same organism {Thielavia basicola Zopf)2 which later produces 

 the black ones, the color depending on the age of the lesion. The character 

 which distinguishes this disease from all other tobacco diseases is the coal 

 black color of parts of the roots. I'lie reduced root system is unable to se- 



1 During the first year of this investigation the work was conducted bj- G. H. Chap- 

 man, formerly research professor of Botany jit this Station. 



2 McCormick (Conn. Agr. Exp. Sta. Bui. 269. 1925) considers this Th,cl,iiiv;>sh haskola 

 (Berk.) Ferraris. 



