May 15, 1919 Influence of Soil Enviro7iment on Rootrot of Tobacco 73 



media. At temperatures of only 3 or 4 degrees above the optimum in 

 culture, however, the fungus, though making some growth, behaves 

 quite normally, and it is not difficult to conceive of no infection or 

 growth occurring on the host at temperatures above 30°. The results 

 are probably in line with the relation of temperature to infection with 

 other parasites, where it is known that the fundamental factor concerned 

 is that of spore germination. 



The following brief description of some experiments may be of inter- 

 est : The roots of tobacco plants in 7-inch pots were forced to grow out 

 through the perforation in the bottom of the pots by setting them in 

 battery jars partly filled with water. After the roots had made a good 

 start, the jars containing the plants were set in the temperature tanks, 

 at high and low temperatures, 31° to 32° C. and 17° to 18°, respec- 

 tively. They were allowed to remain there for a week to 10 days. Dur- 

 ing this time many fresh roots were formed. Two jars at each tempera- 

 ture were now removed to a temperature of 23° to 24°, and young endo- 

 conidia of T. hasicola introduced into the water surrounding the roots. 

 Other plants remaining at 31° to 32° were also inoculated in a similar 

 manner. Good visible infection occurred in 3 to 4 days in all plants at 

 23° to 24°. No difference was obsen/ed at this time or later in the 

 roots which had formed, either at a high or at a low temperature. No 

 infection occurred at 31° to 32° after 8 to 10 days, but infection did 

 occur when the pots were removed to a lower temperature without rein- 

 oculation. This showed that the fungus had not been destroyed. 



This test at least demonstrated that the increased growth of the host 

 at higher temperatures is not due to the overcoming of the effects of the 

 disease by increased root development, but is due to the inability of the 

 fungus to infect the host. It also tends to show that any resistance or 

 susceptibility at high or low temperatures which the roots have developed 

 is rapidly lost, since infection must have occurred within 24 to 36 hours 

 after changing from the extremes to the medium temperature. There is 

 room for a great deal of investigation, however, upon the intimate environ- 

 mental relations of host and parasite in this disease, and it is expected 

 that this subject v/ill be treated in more detail in another paper. 



INFLUENCE OF ORGANIC MATTER IN THE SOIL 



The content of organic matter and humus in the soil has been ascribed 

 by most investigators of tobacco rootrot as being a very influential 

 factor in determining the amount of disease. Practically all the writers 

 agree that the addition to the soil of vegetable matter in the form of green 

 manures or barnyard manures increases the extent of the disease. 

 Massee {ig) has gone so far as to state that the disease can not occur at 

 all in the total absence of organic matter, since he believes the fungus 

 must gain some stimulus v/hile living as a saprophyte before being able 

 to penetrate the host. Nearly all of these conclusions, however, have 



