TOBACCO RESISTANT TO BLACK ROOT ROT 



thick, tough leave?. Badly diseased plants "top-out" or produce their inflores- 

 cense prematurely. 



Black root rot seldom attacks a whole field of tobacco uniformly. Patches of 

 diseased plants may be several acres in size, but are usually smaller. Different 

 patches in the same field, or even different parts of the same patch, may be dis- 

 eased in different degrees of intensity and accordingly manifest the distinguishing 

 symptoms in various degrees of development. 



Certain other diseases of tobacco have more or less similar above-ground symp- 

 toms, and it is not always possible, even for a person who is thoroughly familiar 

 with black root rot, to distinguish the disease with certainty by the above-ground 

 symptoms alone. It is by the presence of the black spore masses on the roots 

 that the identity of the disease can be most easily determined with certainty. 



Factors in the Development of Black Root Rot of Tobacco 



There are four major factors that largeh' determine the occurrence and de- 

 velopment of black root rot: the degree of infestation of the soil by the causal 

 organism, soil reaction, soil moisture and temperature, and the degree of suscep- 

 tibility of tobacco to black root rot. 



The causal organism of the disease occurs more or less prevalent!}' in most of 

 the land on which tobacco is or has been grown in the Connecticut Valley. The 

 organism has a rather wide range of suscepts, consisting of a few economic 

 and numerous wild plants, and occurs in other than tobacco land. Because 

 tobacco has been grown continuously for long periods on much of the tobacco 

 land, there is usually sufficient inoculum in the soil to cause moderate to severe 

 attacks of black root rot whenever other conditions are favorable to its develop- 

 ment. 



Relatively low soil acidity (high pH value) is favorable, and relatively high 

 acidity (low pH value) is unfavorable to the development of black root rot. 

 Under most weather conditions in the Connecticut Valley, the disease ordinarily 

 causes moderate to heav>' damage to tobacco at pH 6.0 and higher, but is seldom 

 serious at soil reactions below pH 5.6, and is not likely to develop at soil reactions 

 of pH 5.2 or lower. Because of the nature of the parent material from which 

 they were formed, most soil t>pes used to grow tobacco in the Connecticut Valley 

 were originally sufficiently acid to be unfavorable for the development of black 

 root rot. With reasonable precautions in the use of fertilizers and soil treatments, 

 their acidity can be maintained at levels unfavorable to the disease. However, 

 as a result of liberal treatments with alkaline fertilizers and lime, the reaction of 

 large acreages has been increased until it is now within the range of acidit\- that 

 is favorable for the development of black root rot. 



The causal organism of black root rot is able to grow at soil temperatures from 

 10° to 40^^ C. (50" to 104° F.) (10), but is apparently able to infect tobacco only 

 within the lesser range of 15"^ to 32'^ C. (59" to 89.6" F.), and severe infection is 

 limited to the still smaller range of 18" to 23" C. (64.4" to 73.4°F.) (8). Any ability 

 of the causal organism to infect tobacco at soil temperatures below 18" C. (64.4°F.) 

 is of little practical importance because soil temperatures as low as this ordinarily 

 do not occur in the Connecticut Valley after tobacco is transplanted into the field. 

 The ability of the organism to infect tobacco lessens rapidly at soil temperatures 



