26 Levi Otto Gratz 



mately as great as from naturally infected soil or from soil into which had 

 been mixed an average amount of inoculum. The soil was left in this 

 condition for about seven months. These conditions were probably 

 more unfavorable than any field conditions would be, since no plants 

 wore growing in the soil, the temperature was abnormally high in the 

 greenhouse, and the soil was practically free from moisture. 



TEMPERATURE AND MOISTURE STUDIES 



It is evident from the literature that temperature and moisture are 

 important factors in the parasitism of this fungus. Johnson (1914), 

 Peltier (1916), Sherbakoff (1917, b), and others suggest that damping-off 

 in general is favored by wet soils and humid atmospheres, relatively high 

 temperatures, and poor ventilation. Rolfs (1904) concluded from his 

 laboratory investigations that a high temperature and plenty of moisture 

 were essential for the development of Corticium vagurn. He suggested 

 that this may be the reason why heavily watered fields of potatoes may be 

 severely injured in hot weather, while those sparingly watered and well 

 cultivated are apparently not injured. Richards (1921, 1923) obtained 

 infection within a temperature range of 9° to 27° C, with the optimum 

 approximately at 18°. Considerable infection, however, was obtained at 

 from 15° to 21°, depending on slight changes in environmental conditions. 



Experimental methods and data 



Temperature-moisture relations • 



Since no report was to be found in literature of researches to determine 

 definitely the interrelationship of soil moistures, soil temperatures, and 

 damping-off by Corticium vagum, some experiments in this connection were 

 performed. The results of preliminary experiments were in accordance 

 with the conclusions of Johnson (1914) and others who found that a combi- 

 nation of high soil moisture, high humidity, and poor ventilation, is 

 favorable for the development of fungi which cause damping-off. 



This phase of the problem was studied in controlled-temperature tanks. 

 These tanks, as used by Burkholder, 11 were 4 feet square and 8 inches deep, 

 and contained water the temperature of which was automatically con- 

 trolled by electrical units. In the experiments conducted by the writer, 

 a third tank, not electrically controlled, was thoroughly insulated with 

 sawdust and cork and was used for the lower temperatures only. Several 

 changes of cold water during the day aided in maintaining the desired 

 temperature in this tank. The surface of all the tanks was covered with 

 paraffined canvas, into which sixteen holes were cut at definite intervals to 

 permit the immersion of ordinary glazed gallon jars (19 by 19 centimeters). 



11 Burkholder, Walter H. The effect of two soil temperatures on the yield and water relations of healthy 

 and diseased bean plants. Ecology 1:113-123. 1920. 



