Aug. 15, 1925 
Soil Disinfection for Potato Wart 
305 
Results of Steam-Pan Treatments 
RESULTS IN 1920 
Through an error on the part of the seed house which furnished 
them, the potatoes used in planting most of the plots in 1920 were of 
a wart-immune variety and consequently very meager results were 
obtained. Nine plots were, however, planted with susceptible 
varieties. Of these, five became infected with wart, as did also two 
plots which were left untreated as controls Two plots in which 
wart developed had been treated with 1 per cent formaldehyde (34 
pint per square foot) followed by steam at 90 to 95 pounds pressure 
for 25 minutes; one with 1.5 per cent formaldehyde followed by steam 
for 40 minutes; and one treated with steam alone for 12 minutes at 
140 pounds pressure. The four plots which were free from wart at 
harvest had been treated as follows: One plot with 160 c. c. per square 
foot of a 1 to 400 solution of mercuric chloride followed by steam for 
25 minutes; one with 134 per cent formaldehyde and steamed for 
25 minutes; one with steam alone at 140 pounds pressure for 25 
minutes; and one with steam alone for 75 minutes at 90 to 95 pounds 
pressure. The results practically eliminated the possibility of using 
a short steaming period following treatment of the soil with formal¬ 
dehyde, since four of the five plots receiving such treatments and 
planted to susceptible varieties were found to have produced warted 
potatoes. 
RESULTS IN 1921 
In 1921 the plots given steam-pan treatments in 1920 were planted 
with varieties of potatoes known to be susceptible to wart. Figure 2 
presents a diagram showing the treatments given, the relationship of 
the plots to each other, and indicates the occurrence of wart in 1921. 
Table I summarizes the results obtained. 
Three of the 6 plots given 85 minutes or more of steaming at 90 
pounds pressure showed wart, as did a large part of the steam- 
iormaldenyde plots and all of the steam-mercuric chloride plots. 
Steaming for 85 minutes should heat every wart spore in the soil to 
a temperature of 90° C. or more and keep it hot for some minutes. 
These results indicate that the potato wart organism is very resistant 
to heat. It may be that only the wart spores in the upper 4 or 5 
inches of soil were killed by the treatments. Potatoes were planted 
the first year without stirring the soil to more than this depth, 
whereas the plots were spaded to a depth of 7 or 8 inches before 
planting the second season. This may explain in part at least the 
difference between the results Broadbent obtained from planting the 
same year the treatments were given, and the results cited above, 
which were derived principally from plantings made the year follow¬ 
ing treatment. This view is borne out by the fact that of the nine 
plots treated in 1920 and planted to susceptible varieties in 1920 all 
but the one given 75 minutes of steaming showed wart in 1921, 
whereas in 1920 four of these plots were wart free. 
One of the plots which was steamed for 85 minutes and was free 
from wart at harvest lay between two plots which showed over 60 
per cent of wart-infected plants. This would indicate that under 
favorable conditions this treatment is effective, even in badly infected 
soil. It will be noted that steaming at the maximum pressure 
