TOBACCO INVESTIGATIONS. 25 



in a field, and as yet is probably not of wide distribution. It remains to 

 be proven whether the fungus is actually actively parasitic, or is capable 

 of attacking only those plants which have become weakened by some 

 other cause. As opportunity offers, work on this disease mil be con- 

 tinued. 



Root-rots, apparently induced by Fusaria or closely related forms of 

 fungi, are seemingly on the increase, if we may take the isolation of these 

 forms from diseased roots in almost pure culture as an indication of the 

 causal agent. The roots of a large number of plants from patchy fields 

 have been examined in the laboratory in the past two years, and in many 

 cases forms of Fusarium, or closely related forms, have been isolated from 

 the surface of the diseased roots. So far the few experiments in which, 

 it was attempted to infect the roots of healthy plants with pure culture 

 material in the laboratory under control conditions have failed. It is a 

 fact, however, that on many of the poorly developed plants in some fields 

 we are consistently finding this fungus. A critical study of the question of 

 Fusaria as the cause of a tobacco root-rot is being made by James Johnson 

 of the Wisconsin station and the United States Department of Agriculture. 

 In the wi'iter's opinion, however, it msiy prove a difficult matter to establish 

 the parasitism of this fungus, but it is to be hoped that something definite 

 will result in the way of control measures. It should be noted that in 

 fields where we have found the Fusarium associated with a root-rot of 

 tobacco, there is a noticeable lack of infection due to Thielavia, the ordinary 

 root-rot fungus of tobacco. This would indicate that the conditions 

 necessary for the optimum parasitism of Fusarium are distinctly unfavor- 

 able to the development of the Thielavia. If this proves true, as these 

 are both soil fungi, the question of control becomes rather perplexing. 

 If it is shown that the Fusarium under certain conditions of soil reaction 

 is actively parasitic, and these conditions are the ones that we have found 

 to be practical for the control of Thielavia root-rot, the question of the 

 finer adjustment of the soil reaction becomes an important factor, and 

 more difficult of successful application. 



Canker, noticeable in the field in 1916 as a decay and blackening of the 

 stem at the ground, sometimes extending up the stem for some distance 

 and occasionally girdling it, occurred only in isolated cases, and was of 

 no importance economically. The direct causal organism, or organisms, 

 is not well known, but the primary cause is probably due to a slight attack 

 of damping-off in the seed bed, or even mechanical injury at the soil level, 

 secondary organisms then gaining admission through the weakened tissue. 

 So far the only field found to contain any considerable amount of canker 

 is one to which very large amounts of manure were applied annually, the 

 seed bed also being treated similarly. This excessive amount of organic 

 matter furnishes a very favorable medium for the growth of bacteria and 

 fungi, especially in the seed bed. 



The leaf spots observed may be roughly divided into two classes, — 

 those caused by organisms, and those with which no organism is associ- 



