Dec., 1923] 
SHERWOOD — FUSARIUM WILT OF TOMATO 
551 
(11) experiments with barley seedlings grown in nutrient solutions showed a 
reaction of pH 8.3 to be distinctly injurious to the growth of the plants. 
The experiments with the growth of the organism in culture should be 
considered merely preliminary to a complete study of the effect of the 
hydrogen-ion concentration of culture media on the growth of the fungus. 
It is clearly understood that a complete study would require frequent testing 
during the arbitrarily selected fifteen-day period to determine the immediate 
changes in the reaction due to the growth of the fungus. Moreover, it is 
not known what further changes might have taken place after the fifteen- 
day period. The results of the two experiments as conducted, however, 
seem to justify the conclusion that the fungus is able to grow within the 
range of hydrogen-ion concentrations which most likely occur in ordinary 
soils. Furthermore, the limiting acid reaction for spore germination under 
the conditions of the experiments seems to be very well established at 
approximately pH 2.0. 
Summary 
(1) In conducting experiments to determine the effect of soil reaction on 
the development of plant diseases caused by soil-infesting organisms, it is 
necessary to know the result actually produced by materials used to bring 
about changes in the reaction. 
(2) The colorimetric measurement of the hydrogen-ion concentration 
of soil extracts affords a means of accurately determining the reaction of the 
soil before and after such materials are added. Thus, there can be arranged 
a series of soils from strongly acid to alkaline, the exact reaction of each 
member of which is known. 
(3) Experiments were conducted with naturally acid silt-loam and 
sandy-loam soils, adjusted to various degrees of acidity and alkalinity by the 
use of calcium carbonate and calcium oxide, in order to determine the effect 
of soil reaction on the development of Fusarium wilt of tomato seedlings. 
(4) The highest percentage of wilt always occurred in the most acid 
soils of the series. With very few exceptions, the percentage of wilt de¬ 
creased quite uniformly as the hydrogen-ion concentration of the soils de¬ 
creased, until approximately pH 7.4 was reached. The percentage of wilt 
which developed in soils having a more alkaline reaction than pH 7.4 was 
lower in one case and higher in the other, but the plants made a very poor 
growth in soils having a reaction more alkaline than pH 7.4. No sharp 
delimitation was apparent, neither could any limiting degree of acidity or 
alkalinity be found at which the disease would not develop. 
(5) Culture experiments were carried out with Fusarium lycopersici 
in nutrient solutions adjusted to hydrogen-ion concentrations ranging from 
pH 1.8 to 8.4. Spores of the organism exposed to the same temperatures as 
the growing tomato seedlings germinated in the solutions varying in hydro¬ 
gen-ion concentrations from pH 2.2 to 8.4. No germination occurred in the 
solutions adjusted to pH 1.8. Growth of the fungus in the solutions ad- 
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