TOBACCO RESISTANT TO BLACK ROOT ROT 



Kinds of Havana Seed Tobacco and Their Susceptibility 

 to Black Root Rot 



Havana Seed as a type of tobacco consists of man\- strains sufficiently alilce to 

 be distinguisliable collectively from other t>pes, but differing sufficiently to 

 make it possible to distinguish readily between many if not all of them. 



One important difference among the strains is their degree of susceptibility to 

 black root rot, ranging from moderate to severe susceptibility on the part of 

 much the greater number of strains to high resistance on the part of a relativelv 

 small number. The former are known generally as common or regular Havana 

 Seed; the latter, as black root rot resistant Havana Seed, or sometimes by the 

 shorter name resistant Havana Seed. The strains of regular Havana Seed 

 probably resulted from farmers first selecting from existing strains seed plants 

 that seemed to meet their needs under the conditions prevailing on their farms, 

 and continuing the process year after year according to the same standards. 

 Havana Seed tobacco manifests differences in many of its minor characteristics 

 readily under different environmental conditions. Continued selection of seed 

 plants year after year according to the ideals of different farmers, therefore, could 

 produce strains of tobacco which differ genetically in the minor characteristics 

 such as differentiate the strains of regular Havana Seed, but would not be likely 

 to produce changes in major properties of the strains. Since there is good reason 

 to believe that the original stock from which the present strains have developed 

 was inherently susceptible to black root rot in moderate to severe degree, it is 

 improbable that high resistance could have accumulated in any of these strains 

 by the method by which they probably were produced. 



The strains of black root rot resistant Havana Seed have been produced by 

 artificial methods designed to create strains possessing high resistance to the 

 disease. For the greater part they have been bred from strains well suited to 

 provide not only high resistance to black root rot but other desirable properties 

 as well. As a result of thorough testing to determine the degree of their resistance 

 to black root rot and their type, quality, and yielding capacity, and of selecting 

 and propagating onh- those strains that seemed to fulfill these objectives, the 

 number of strains of Havana Seed that are highly resistant to black root rot and 

 otherwise acceptable under Connecticut Valley conditions is few. 



Economic Importance of Black Root Rot in the Connecticut Valley 



The economic importance of black root rot is determined largely by the extent 

 to which it reduces yields and quality, and consequently the cash value, of to- 

 bacco crops. 



The literature contains numerous statements to the effect that black root rot 

 has been and still is one of the most highly damaging diseases of tobacco in the 

 Connecticut Valley. Anderson (1,2) states that black root rot was probably the 

 most serious of all diseases of tobacco in the Connecticut Valley during the first 

 twenty-five to thirty years of this century. Johnson (6), writing some years 

 back about the importance of black root rot in the country as a whole, states 

 that it undoubtedh- has caused more loss to tobacco growers than any other 

 disease with which the\- have had to contend. 



