Aug. 15, 1925 
Soil Disinfection for Potato Wart 
317 
It is interesting to note that no wart developed in the check pots for 
the 5 untreated plots. Only 2 of these 5 plots produced wart all 
three years. Another produced wart the first and third years. Two 
produced wart the third year only. While these untreated plots were 
in the garden showing the least wart, the results obtained serve to 
emphasize the necessity of continued planting of treated plots in 
order to make sure that the results obtained are dependable. 
Treatments that are sufficiently drastic to prevent or greatly 
retard the growth of potato plants might be expected to kill any 
swarm spores produced by the wart organism as long as the inhibiting 
factor remains. Unless the highly resistant resting sporangia are 
killed or die in the meantime, it is conceivable that they might cause 
wart after the factor which inhibits plant growth is eliminated from 
the soil. Plots which showed no wart during the three growing 
seasons above mentioned may be grouped according to the adequacy 
of growth of potatoes in them to give a reliable test of the presence 
of viable wart spores, as follows: Plot showing no growth, that 
treated with sodium fluoride; plots showing inadequate growth, 
those treated with sulphur; plots snowing doubtfully adequate growth, 
those treated with copper sulphate crystals, lime-sulphur solution, 
sodium carbonate, sodium chromate, a commercial weed killer; 
plots showing probably adequate growth, those treated with bichlo¬ 
ride of mercury (in part), chloride of lime, cleaning solution, formal¬ 
dehyde, kerosene; plots showing good growth, apparently adequate 
for a good test, those treated with Bordeaux mixture, bichloride of 
mercury (in part), crude oil. 
RESULTS OF POT EXPERIMENTS WITH BICHLORIDE OF MERCURY AND SODIUM 
CHLORIDE 
The efficiency of bichloride of mercury and salt solutions for killing 
the wart organism in the soil was tested by using treated wart- 
infected soil to inoculate potatoes. Two-inch glass soil tubes 8 were 
filled with thoroughly infected soil from one of the gardens in Free¬ 
land, Pa., the soil was compacted, a 1 per cent solution of mercuric 
chloride plus 5 per cent sodium chloride was made and applications 
added at the rate of 2 quarts per square foot. This treatment was 
followed almost immediately by water at the rate of 2 quarts per 
square foot. Chemical tests on part of the tubes showed the presence 
of mercury to a depth of 8 to 9 inches in the soil. At the end of three 
weeks, pots were partly filled with wart-free soil and potato seed 
pieces placed on top. Then soil was removed from the treated tubes, 
and from checks, an inch at a time, and sprinkled oyer the seed pieces 
and soil in the pots. The pots were then filled with wart-free soil. 
The potatoes made normal growth and at the end of three months 
were examined for the presence of wart. The 90 pots inoculated 
with soil from the surface to the 9-inch depth of treated tubes showed 
no wart. In 6 of the 29 pots inoculated with soil from below the 
9-inch depth of treated tubes wart developed. In 10 of the 35 pots 
inoculated with untreated wart-infected soil from check tubes wart 
developed. A duplicate set of tubes gave almost exactly the same 
amounts of wart. So little of the treated soil was used in each pot 
8 These tubes were used in the study of the penetration of soil fungicides and are described on p. 331. 
The methods used are discussed on pp. 330*334. 
