Aug. 15, 1925 
Soil Disinfection for Potato Wart 
327 
It has been stated that the flow of steam secured where the pressure 
was regulated to 85 to 95 pounds with the outlet closed appeared to 
be equal to that secured when the steam was regulated to about 10 
pounds gauge pressure with the outlet wide open. This opinion was 
originally based on the appearance and sound of the flowing steam. 
The table shows that the temperatures obtained were similar also. 
The temperatures obtained with 25 minutes of steaming at 10 pounds 
are abnormally high for some unknown reason, the temperature at 
the 7-inch depth being greater than was secured at this depth with 
40 minutes of steaming at the same gauge pressure. The maxima 
after 25 minutes of steaming show a discrepancy largely attributable 
to this. The maxima obtained after 40 minutes of steaming are as 
nearly identical as could be expected, the 1, 4, and 7 inch maxima 
being respectively 98° C., 74°, 38° for 95 pounds pressure closed, 
compared to 99°, 72°, 38° for 10 pounds pressure open. The number 
of minutes elapsing after steam was turned on before these maxima 
were reached are almost equally comparable, being 32, 54, and 151 
minutes for the 95-pound pressure group and 29, 62, and 155 minutes 
for the 10-pound pressure group. All three depths reached a maxi¬ 
mum of 100° where the soil was steamed 40 minutes at 20 pounds 
pressure, the lengths of time to attain the maximum being 15, 32, and 
38 minutes. The maximum sustained temperatures for these groups 
are closely correlated with the maximum temperatures. 
Wart spores have not been found deeper than 8 inches below the 
surface of the soil. It was found that steaming for 85 minutes at 90 
pounds pressure did not always prevent the occurrence of wart. It 
is thought that this was because the method of regulating the steam 
supply was inaccurate, so that the failure was due to a steam flow 
below that intended. The temperature table shows that at 10 
pounds pressure 100° C. was reached in 70 minutes at the 7-inch 
depth. At the rate of penetration shown, the 8-inch depth should 
have reached approximately 100° at the end of 85 minutes of steaming. 
The maximum sustained temperature at the above 7-inch depth was 
96°. Hence the maximum sustained temperature at 8 inches follow¬ 
ing 85 minutes of steaming at 10 pounds pressure should be near the 
boiling point also. The results obtained in wart control when cor¬ 
related with recorded temperatures for the treatments used indicate 
that a sustained temperature somewhere near 100° is necessary to 
kill the wart sporangia. On this basis only plots treated for 75 
minutes at 10 pounds or for 40 minutes at 20 pounds could be ex¬ 
pected to show freedom from the disease. In fact, the maximum 
temperatures obtained at 7 inches in the other groups show that the 
treatments they represent would be utterly ineffective against 
organisms with only a moderate tolerance of heat—unless those 
organisms occurred or remained viable only in the upper 2 or 3 inches 
of soil. Steam was used, full force (135 pounds), on one pan area in 
the steam garden, the temperature at 7 inches going up to 99° in 20 
minutes. The temperatures fell rapidly, however, so that the “ maxi¬ 
mum sustained temperature” was an average of temperatures with 
a minimum of 88°. The average for one-half hour was 93°. 
In studying Table V, it should be borne in mind that penetration 
of heat is relatively rapid as long as it is being supplied in quantity 
in the steam pan, but that as soon as the steam is shut off the pene¬ 
tration slows up. The shorter the steaming period the longer it takes 
