MASS. EXPERIMENT STATION BULLETIN 432 



higher than 26° C. (78.8° F.) and ceases almost entirely at 32° C. (89.6° F.) (8). 

 This is of some practical importance because soil temperatures higher than opti- 

 mum for the development of the disease occur during portions of some tobacco 

 growing seasons, especially during the latter part. For this reason, tobacco that 

 has become infected by black root rot earh' in the growing season sometimes re- 

 covers sufficiently to \ield fairh" good crops. Frequently, however, when soil 

 temperatures are low, especially in heavy soils which warm up slowly, tobacco 

 may become too severeh" diseased to recover much. 



Soil moisture may have a dual effect on the development of black root rot. 

 High soil moisture near the saturation point is intrinsically favorable to the de- 

 velopment of the disease (8). A moisture content of less than saturation has 

 little or no direct effect on the development of the disease (8); but evaporation of 

 soil moisture lowers soil temperature; and, other things such as soil texture, 

 structure, etc., being equal,, the greater the amount of soil moisture evaporated 

 the lower the soil temperature becomes. And, as has been explained, the lower 

 soil temperatures are favorable to the development of black root rot. 



Immunity of tobacco to black root rot would, of course, eliminate it as an 

 important disease of the crop. However, none of the many strains of Havana 

 Seed are immune and only a very few are highly resistant to black root rot. 

 Strains susceptible to the disease make up the major portion of the acreage of 

 Havana Seed grown in the Connecticut Valle>' each year. 



Factors That Determine the Nature of an Effective Control Measure 

 for Black Root Rot of Havana Seed Tobacco 



The principal purpose of the foregoing accounts of Havana Seed tobacco and 

 of black root rot has been to show two things. First, Havana Seed comprises so 

 large a part of the total tobacco crop in the Connecticut Valley and especially in 

 Massachusetts that large fluctuations in its production would cause large fluctua- 

 tions in the total tobacco yield and consequenth' in the cash income obtained 

 from tobacco each year. Second, black root rot causes sufficient damage to 

 Havana Seed to reduce the production of this type of tobacco materially below 

 its potential capacity for production. If there were an eft"ective, easy-to-use 

 method of controlling the disease, this loss could be prevented. Such a control 

 measure would provide one of the greatest possible improvements in the pro- 

 duction of Havana Seed tobacco in the Connecticut Valle\'. 



The nature of black root rot and the adaptability of the causal organism to 

 prevailing environmental conditions, determine the principles of disease control 

 that may be used in devising and developing an effective control measure. Of the 

 four fundamental principles of disease control — exclusion, protection, eradication, 

 and immunization — all except the last are either wholly or largely inapplicable in 

 the control of black root rot of tobacco. 



The general distribution of the causal organisni in Connecticut \'alley tobacco 

 soils renders the exclusionary principle of disease control inapplicable. 



Black root rot, as already pointed out, is a disease of the roots of tobacco 

 plants. Since the roots are shielded by soil, protective fungicides cannot be used 

 directly to control the disease. However, farmers who still have tobacco land 

 of pH 5.3 or lower can protect their crop indirectly by maintaining this soil 



