TOBACCO RESISTANT TO BLACK ROOT ROT 



acidity, which is unfavorable to the de^'elopment of the disease. On large acre- 

 ages, the soil reaction has been increased by the use of alkaline fertilizers and 

 lime until it is favorable for the development of black root rot; and after soil has 

 been made alkaline, there is no practical way known to reduce its alkalinity 

 readily. As a result, the protective principle of disease control is not fully appli- 

 cable in the control of black root rot of tobacco. 



As has been pointed out, the causa! organism of black root rot is already present 

 in practically all tobacco land in the Connecticut Valley. Since it can live in- 

 detiniteh' in decaying vegetable matter in soils in the absence of tobacco, a pro- 

 longed period without tobacco would be required to eliminate the organism from 

 the soil by starvation. Rotation of tobacco with other farm crops, to aid in the 

 starvation of the organism, is ordinarily not advisable because the residual effects 

 of many farm crops promote the development of brown root rot which might 

 be as harmful to tobacco as black root rot. Neither is it practical for tobacco 

 farmers to fallow their land for long periods of time. The regular practice in 

 the Connecticut Valle^• is to grow tobacco continuously on the same land year 

 after year, and this not only maintains but often increases the infestation of the 

 causal organism of black root rot in tobacco soils. As a result, the eradicative 

 principle is not applicable in the control of black root rot. 



Fortunately the circumstances that prevent the use of exclusion, protection, 

 and eradication in the control of black root rot do not affect the use of immuniza- 

 tion for the purpose. And since immunization was found to be the only principle 

 on which fully effective control of the disease could be based, it was undertaken 

 to disco\'er resistant specimens in acceptable strains of regular Havana Seed, if 

 possible, and from them to develop new strains. If suitable specimens could 

 not be found, it was proposed to produce, b\' artificial breeding procedures, new 

 strains of Havana Seed, highly resistant to black root rot and capable of producing 

 good yields of tobacco of good t\pe and qualit\ in both infested and uninfested 

 soils. 



EXPERIMENTAL PROCEDURE AND RESULTS 

 Materials and Methodsi 



Previous knowledge of the nature of regular Havana Seed tobacco made it 

 seem unlikely that specimens possessing high resistance to black root rot could 

 be found. However, in order that nothing of value might be overlooked, numer- 

 ous strains were tested under severe black root rot promoting conditions on the 

 Massachusetts Agricultural Experinient Station farm. Also, in cooperation with 

 several leading cigar and tobacco manufacturing companies who sponsored the 

 work, many strains were tested on an outlying farm. In addition, the progeny 

 of unusually vigorous Havana Seed tobacco plants found in various localities 

 and suspected of possessing more than usual resistance to black root rot were 



■ The investit;ationaI work reported in this bulletin was done in part while the writer was employed 

 by the Bureau of Plant Industry. United States Department ol Agriculture, workins; in coopera- 

 tion with Massachusetts and Connecticut Agricultural Experiment Stations, and in part while 

 he was employed by Massachusetts Agricultural E;<periment Station. 1 he writer is indebted to 

 Dr. W. W. Garner, formerly of the Bureau of Plant Industry, Dr. James Johnson of the Bureau of 

 Plant Industry, and Dr. P. J. Anderson of the Connecticut .Agricultural Experiment Station for 

 advice and materials used in the work. 



