18 



ROOT DISEASES. 



BLACK ROOT-ROT. 



Description. — Black root-rot is one of the most widespread dis- 

 eases of tobacco and undoubtedly has caused more losses to tobacco 

 growers as a whole than any other disease with which they have to 

 contend. This disease frequently appears in the seed bed to" a serious 

 extent (PL IV) , but the most important damage occurs in the fields 

 through reduced yield and poor quality of tobacco. 



The disease proper is limited to the root system and the base of 

 the stem below the surface of the soil. The roots are distinctly 

 rotted and usually distinctly black in parts, but in severe cases the 

 marked absence of roots as a result of the disease is especially 

 noticeable (fig. 11). This depletion of the root system, together 

 with the diseased condition of the remaining roots, naturally results 

 in the starvation of the plant from lack of food and water. The 

 symptoms on the aboveground parts of the plant are, therefore, only 

 marked stunting of growth, usually with a yellowish or chlorotic 

 appearance, commonly with premature budding out. Such symptoms 



Fig. 12. — A field of tobacco varieties, showing- differences in susceptibility to black 

 root rot. Many varieties of tobacco are very susceptible to the disease, as illus- 

 trated by these alternate rows of ordinary White Burley, which have made little 

 growth in comparison with the resistant types in neighboring rows. 



are not ordinarily distinguishable from those produced by low soil 

 fertility, lack of soil moisture, and certain other unfavorable con- 

 ditions for plant growth, or in some instances symptoms produced 

 by other parasites. Examination of the root system must therefore 

 be made to determine with certainty the presence of the disease. The 

 importance of this similarity of symptoms lies also in the fact that, 

 more often than not, low soil fertility or dry weather has been held 

 responsible for stunted crops where black root-rot or other plant 

 parasites have been the actual cause of the conditions existing. ' 



It frequently happens, however, in the case of crops infested with 

 black root-rot that a very uneven growth develops. This condition 

 may result either from different degrees of infection on the seedlings 

 when transplanted or inherent differences in the resistance of individ- 

 ual plants to the disease where impure seed is used. Less striking 

 unevenness may result also as a consequence of the uneven distribu- 

 tion of the parasite concerned or from irregular environmental con- 

 ditions favorable for infection existing in the soil. Very uneven 



