THE BKOWN EOOT EOT OF TOBACCO AND OTHER PLANTS 17 



The experiment was now varied to compare drying in the ab- 

 sence of direct sunlight and in direct sunlight. To exclude direct 

 sunlight the soil was covered with paper in one case and placed in 

 a dark room with a current of air passing over the soil in the other 

 case. The soils when dried for one week, whether exposed to sun- 

 light or not, produced a growth of tobacco approximately six times 

 as great as the untreated controls. Similar experiments were re- 

 peated several times with like results, except for the fact that the 

 period of drying could be shortened to one to two days with almost 

 as good results, provided the layer of soil was made sufficiently 

 thin and a good current of air passed over it. It was very evident 

 that sunlight was not the effective agent. 



It is reasonable to assume, however, that aeration of the soil 

 rather than desiccation may have been the active agent in destroying 

 brown root rot in these experiments. Attempts at aerating the soil 

 without drying by drawing the air through cylinders of the soil 

 gave negative results, but it is not likely that all parts of the soil 

 are uniformly exposed under these conditions. 



Aeration of the soil by spreading it in a thin layer before an 

 electric fan, keeping it moist by watering two or three times daily, 

 was then tried. Parts of the soil, however, did dry for short periods. 

 Tests by growing plants in this soil showed that the aerated moist 

 soil gave equally as good control of the disease as the aerated dry 

 soil. This experiment when repeated with two soils, aerating dry 

 and moist for one week, gave an improvement of as much as 600 

 per cent with moist aeration, but dry aeration was apparently still 

 more effective. Further experiments gave conflicting results as 

 to the relation of aeration in the presence and absence of moisture. 

 A more satisfactory method for conducting such studies should be 

 developed, since it was found difficult to repeat the experiments in 

 such a way as to make certain that similar treatments were given in 

 all cases. 



The writers are therefore not prepared to state definitely, whether 

 desiccation or aeration, or both, were most concerned in the destruc- 

 tion of brown root rot in these experiments, although they are 

 inclined to believe from their prelimina^ experiments that both 

 play a part. 



FIELD-PLAT EXPERIMENTS 



It was recognized before the laboratory work on brown root rot 

 progressed very far that it was important to obtain more reliable 

 information upon the relation of different crops to the disease under 

 field conditions, with the hope that a system of rotation might be 

 found which would aid in remedying the trouble. On account of 

 unforeseen difficulties, however, a satisfactory start in rotation 

 experiments was not obtained until 1922. 



A series of plat experiments was conducted on the farm of Spenser 

 Bros., Suffield, Conn., in 1919, on land which gave practically a crop 

 failure the preceding year, due apparently to brown root rot. The 

 outstanding results from these plats were that the greenhouse tests 

 on crops affected were essentially corroborated. The excellence of 

 growth of such crops as potatoes and corn, compared with relatively 

 poor yields of tobacco, was indicative of the general productiveness 



