Dec., 1923] 
SHERWOOD — FUSARIUM WILT OF TOMATO 
539 
ion concentration and the occurrence of the common potato scab. A similar 
correlation was found to exist in certain soils of different origin and types. 
Soils found to have hydrogen-ion concentrations of pH 5.2 or lower rarely 
produce scabby potatoes. The conclusion reached was that ‘‘the limiting 
zone of the hydrogen ion exponent for the potato scab organism appears to 
be about the same for the soil as had been previously found for culture 
media”. 
General Considerations 
Besides the purely environmental conditions which may influence the 
parasitism of the tomato plant and the development of the disease, there 
must be taken into account the degree of susceptibility of the host and the 
virulence of the causal organism. Clayton did some preliminary work (3) 
with three strains of Fusarium lycopersici isolated from tomatoes which 
came respectively from Louisiana, Tennessee, and Indiana. He found 
considerable differences in the relative pathogenicity of the strains; these 
differences seemed stable and persistent. Such differences in pathogenicity 
among different strains of Fusarium lycopersici have been reported by Edger- 
ton (5). In this connection he concludes that the virulence of the organism 
is not diminished to any appreciable extent when the organism is grown on 
artificial culture media for a period of two years or even more. The writer’s 
experience accords with that of Clayton and Edgerton, namely, that, 
within reasonable limits at .least, the age of the culture does not introduce 
a factor of appreciable variability. 
Of the various conditions of soil environment which may influence ex¬ 
perimental results, Clayton (3) concluded that temperature is the most 
important single factor. His results show that soil temperature of about 
28° C. is most favorable for the development of the disease. He also de¬ 
termined that the optimum soil-moisture condition occurred at about two 
thirds of its moisture-holding capacity. 
It was assumed that whatever effect soil reaction might have on the wilt 
would be shown most conclusively under conditions otherwise favorable 
for the development of the disease. An attempt was made, therefore, to 
keep the temperature of the greenhouse at 28° C., though fluctuations 
occurred, as will be noted in connection with each experiment. The mois¬ 
ture content of the soil was not accurately controlled, but water was so 
applied as to keep the plants in good growing condition, and Clayton’s 
correlated experiments showed that this approximated two thirds of the 
moisture-holding capacity of the soil. 
The experiments were conducted with two types of soil, a sandy loam and 
a silt loam, obtained from fields in southern Wisconsin. Both were natu¬ 
rally acid soils which had been previously found by the Department of Soils 
of the University of Wisconsin to give a strongly acid reaction when tested 
by the Truog (20) method. With these as a base, various adjustments of 
