THE BROWN ROOT ROT OF TOBACCO AND OTHER PLANTS ( 



decay was much more common in the susceptible roots, this no doubt 

 accounts for the more common occurrence of Fusarium. Its fre- 

 quency on apparently immune crops is, however, significant in this 

 connection. 



No fungus other than Fusarium could be consistently cultured 

 from diseased roots. A detailed microscopical examination of dis- 

 eased tissue was therefore resorted to either by means of teasing the 

 tissues apart under the binocular, by means of free-hand razor sec- 

 tions or with stained paraffin sections. 



The examination of the roots under the binocular frequently 

 revealed a Rhizoctonialike mycelium in considerable abundance, 

 especially when the roots were torn apart. The strands often ap- 

 peared to be beneath the cortical tissue, and with needles they could 

 be separated from the host tissue and placed directly on nutrient 

 media in Petri dishes. 



The fungus, however, was not readily culturable. In a few in- 

 stances growth of Rhizoctonia was obtained, but it could not be 

 ascertained that this was the same species or strain which commonly 

 existed on the roots themselves. This strain of Rhizoctonia was 

 culturally somewhat different from the ordinary Rhizoctonia with 

 which we are acquainted. Inoculation experiments, however, again 

 failed to give positive results. 



A study of free-hand sections through lesions stained with carbol 

 fuchsin revealed a striking picture in some cases. The cells appeared 

 to be crowded with a very fine mycelium, such as would be expected 

 in the Actinomyces. This lead was followed up carefully for several 

 months. Great difficulty was experienced in isolating this fungus, 

 although apparently it was eventually accomplished. Artificial soil 

 inoculation, however, again failed to yield positive results. 



During the course of the investigation various species of bacteria 

 have been isolated. No particular organism, however, has seemed to 

 predominate, and negative results only have been obtained with a 

 number of different species which have been used in inoculation 

 experiments. 



Stained paraffin sections were found to be of little value in indi- 

 cating the presence of a causal organism, although the histological 

 picture of the disease is very well shown. (Figs. 5 and 6.) 



EVIDENCE REGARDING THE NATURE OF THE DISEASE 



Although it is not believed that the writers have definitely dem- 

 onstrated that neither Fusarium, Rhizoctonia, Actinomyces, nor 

 bacteria are the cause of brown root rot, the evidence points strongly 

 in that direction. Of the organisms with which they have been 

 concerned, a nonculturable Rhizoctonia would seem most likely to 

 be the parasite involved. There still remains the additional possi- 

 bility that an unknown parasite is concerned. 



Evidence pointing toward the parasitic nature of brown root rot 

 has been repeatedly obtained. This evidence may be summarized as 

 follows : 



(1) Soil mixtures: Diseased soil mixed with healthy soil in proportions as 

 low as 10 per cent of the former will result in the production of the disease, 

 although the extent of the disease is roughly proportional to the quantity 

 of diseased soil added. 



