920 
Journal of Agricultural Research 
Vol. XXXI, No. 1 
pared as before, potato seed pieces from the same lot of potatoes were 
prepared in the manner described above, and the thermometers were 
recalibrated. The tanks were adjusted by pairs to temperatures of 
13°, 13.5°, 14°, 14.5°, 15°, and 15.5° C. The duration of the experi¬ 
ment was 300 hours, beginning at 6 p. m. March 9, 1922, and ending 
at 6 a. m. March 22, 1922. Readings and adjustments were made 
every hour. The average deviations from the desired temperatures 
are given in Table II. 
Table II .—The temperature which it was desired to maintain in each tank in the 
final experiment , together with the average deviation for the experimental period , 
expressed in degrees centigrade 
Desired temperature___ 
13.0 
13.5 
14.0 
14.5 
15.0 
15.5 
Average deviation: 
First tank___ 
+0.0133 
4-0.008 
-0.0003 
-0.0073 
-0.0053 
-0.0013 
Second tank... 
+0.004 
4-0.0003 
-0.0013 
-0.0073 
4-0.0046 
4-0.0033 
At the termination of the experiment the cans were lifted, and 
the seed pieces removed and examined for evidence of infection. All 
of the controls appeared distinctly different from the inoculated seed 
pieces held at the same temperature. The cut surfaces were clean 
and bright as if almost freshly cut. The inoculated pieces held at 
13° were more heavily calloused than the controls, and appeared a 
trifle darker, but there was no evidence of infection in any case. The 
buds of the seed pieces had germinated normally. Of the seed pieces 
inoculated with Fusarium oxysporum , none held below a temperature 
of 14.5° C. appeared to be infected. They seemed to be a little more 
heavily calloused, and very slightly shrunken about the edges. At 
14.5° and above the seed pieces were more or less shrunken at the 
edges, and the cut surfaces had developed a mottled appearance, 
frequently characteristic in this infection. The reisolation of the 
causal fungus from the tissue below the surface was depended upon 
to show whether or not infection had occurred. Mere examination 
was insufficient. The tubers were broken open toward the cut sur¬ 
face, and fragments of tissue below the surface, but not including it, 
were transferred to poured plates of potato agar. Where a, fusarium 
developed it was transferred to suitable tube media and identified. 
From the seed pieces held at 13° and 13.5° no fungus developed. 
Of those held at 14°, six pieces, or 25 per cent, yielded F. oxysporum 
in pure culture. At 14.5°, 15°, and 15.5° all seed pieces yielded 
F. oxysporum in pure culture. 
The critical temperature for infection of the potato seed piece was 
found to be approximately 14° C. under conditions such as pre¬ 
vailed during the course of these experiments. 
DISCUSSION 
The conditions under which the experiments were performed were 
approximately normal, so far as the soil was concerned. Because 
infection occurred at 14*° C. it must not be assumed that wilt or 
death of the plant would have taken place had the experiment lasted 
long enough to have produced plants. Vegetative activity offers 
resistance to the advance of seed-piece rot. Other factors, stich as 
vigor of seed and aeration, commonly prevent or retard seed-piece 
