528 Journal of Agricultural Research voi.xx, No. 7 



a warm-weather parasite, and at lower temperatures the likelihood of 

 its occurrence is diminished. (PI. 67, I and II.) 



It is significant that the optimum for the growth of the wilt Fusarium 

 in culture was also found to be between 28 to 30 C. Growth was 

 very slow at io°, and no growth was obtained at 7 and 35 °. Since no 

 growth occurred in culture at 35 , it is seemingly quite evident that no 

 infection should occur above 35 and that chances of infection probably 

 are considerably reduced before that temperature is reached. 



It may now be recalled that the Fusarium-wilt of tobacco was first 

 brought to our attention in 1916, when it apparently was assuming 

 serious proportions, although it had been previously noted by the growers 

 in lesser amounts. It will also be remembered that the summer of 1916 

 was one of the warmest seasons recorded by the Weather Bureau stations 

 throughout the country, and that the soil temperature was correspond- 

 ingly high that season, as shown, for instance, by records taken at 

 depths of 2, 4, and 8 inches at Wisconsin (5). The season of 191 9, when 

 the disease occurred in Ohio, was also relatively warm. Though the 

 evidence is scanty for the occurrence of the disease under field conditions, 

 there is no doubt a correlation with high soil temperatures. 



Soil reaction. — In experiments with Thielavia a series of soil cul- 

 tures was prepared and described (5) in which the reaction of a soil 

 of very high acidity was changed by adding varying amounts of calcium 

 carbonate so as to give different reactions ranging from high acidity 

 (9.38 tons lime required per acre) to one of high alkalinity. These soils 

 have changed somewhat in reaction during the two years in which they 

 have been used, but, as shown by the Troug color test, the same relative 

 reaction was probably maintained. The determination of the reaction 

 of these soils by the hydrogen-ion method, however, indicated that high 

 alkalinity was apparently not obtained, the P H value ranging from 5.4 to 

 7.2. These soils were sterilized, and one series in duplicate was inoculated 

 with Fusarium oxysporum var. nicotianae, the other series being left as 

 uninfested controls. Tobacco seedlings of the White Burley variety 

 were then transplanted into them. Three separate trials were run, 

 two of which gave reliable results, and one yielded unreliable results 

 because of poor infection. In one experiment all the plants died at the 

 three highest soil acidities, one died in each of the next three lower 

 reactions, and none died in the three jars at the alkaline end, although 

 finally they all became infected. In another experiment all plants 

 died in the first five grades of reaction from the acid end and one in 

 each of series 6, 8, and 9, but none in the seventh, although they were 

 both infected. The evidence seems fairly conclusive that an acid soil 

 favors the wilt disease, although it may occur in neutral or alkaline 

 soils. The crocks were, however, watered from the top, and part of the 

 soluble salts were washed downward. The soil may not have been of 

 the same reaction throughout for this reason, but the difference could 



