TOBACCO INVESTIGATIONS. 27 



bed will go far towards controlling the trouble, but it is improbable, 

 owing to its nature, that it will ever be entirely eradicated. A good indi- 

 cation as to whether an infection is from the seed bed may be obtained 

 by making a note of the time, after planting, of the first appearance of the 

 disease in the field. If the disease is noticed at any time within a period 

 of two weeks after setting, one may be sure that the infection came either 

 from the seed bed or during the transplanting. If it appears after this 

 length of time it is usually the resultant of a field infection. If all the 

 leaves show the trouble it may also be stated that the infection came from 

 the seed bed or from the conditions under which the plants were set. If it 

 appears on some of the upper leaves at a later period the infection occurred 

 in the field, and soil conditions should be looked into. A full discussion 

 of the mosaic disease will be found in Bulletin No. 175 ^ of this station, 

 a copy of which will be mailed on request to the director's office. 



Thielavia root-rot, or the ordinary root-rot of tobacco, is probably the 

 most widespread trouble of fungous origin we have in our tobacco fields, 

 and as a primary and secondary cause of many of our "sick" fields it is 

 of great importance. 



This disease appeared to be more destructive in 1916 than usual, although 

 the amount could not be compared with that of 1915, as no extended 

 examinations were made during that season. Many fields presenting 

 an unthrifty appearance were studied with the idea of obtaining data 

 as to the presence of root-rot, but in some cases only slight infections were , 

 found, the root systems of the plants not being parasitized sufficiently to 

 account, in our opinion, for the general unthrifty appearance of the field. 

 However, some cases were observed where the plants on entire fields were, 

 to a large extent, badly infested with the root-rot fungus, and unquestion- 

 ably these fields are in need of immediate treatment looking toward the 

 eradication of this trouble. In nearly every case these heavy infections 

 were on fields long used for growing tobacco, and the soil reaction and other 

 factors were rather abnormal. 'It would appear from the observations 

 made during the season of 1916 that in our fields the causal organism of 

 Thielavia root-rot is widely distributed, but produces noticeable ill effects 

 only when the soil and its environment are unfavorable to the best develop- 

 ment of the tobacco plant and favorable to the rapid development of the 

 root-rot fungus. 



The control of this disease in the seed bed has already been discussed. 

 There is no question but that it can be completely controlled by thorough 

 sterilization either by steam or formaldehyde. The control of the disease 

 in the field is an entirely different matter, however, and it is practically 

 out of the question to attempt to eradicate it in the field by methods used 

 for its eradication in the seed bed, because of their prohibitive cost. 

 Even if the cost of material were considerably less than it is at present the 

 labor and time factor would render such methods prohibitive. It is 

 believed by some that a method of control by steam or formaldehyde 

 can be devised, and that it will not be impossible of application from the 



I Chapman, G. H. Mosaic Disease of Tobacco. Mass. Agr. Exp. Sta. Bull. No. 175 (1917) 



