126 MISS S. LONGMAN ON THE DRY-ROT OF POTATOES. 
IV. HEATING EXPERIMENTS. 
Since the fungus of dry-rot is of frequent occurrence in potato tubers and 
since, as shown by these investigations and by those of Smith and Swingle, 
it not only sets up pathological eonditions in the tuber but also attacks the 
aerial shoots, it was evidently useful to ascertain whether tubers could be 
sterilized with respect to the fungus by means of heat. То this end a series 
of experiments were made of which Tables I. & II. are summaries. 
In the first series (Table I.) batches of infected tubers were heated in tins 
up to certain temperatures and the effects on the tuber and on the fungus 
determined subsequently by cultivation-tests. 
A Hearson incubator was used in these experiments, and the temperature 
of the potatoes determined in each case by means of a thermometer inserted 
into the middle of a tuber. It was found that when tins containing each 
five tubers were used, it took 7 hours for the potato-thermometer to read 
the temperature recorded by the incubator-thermometer. The records in 
Table I. (2) were obtained by heating batches of tubers during one day until 
the temperature of the potato-thermometer corresponded to that of the 
incubator-thermometer ; allowing the tubers to cool for one day and then 
reheating them again on the third day. 
Table I. (1) shows that when the tubers were heated gradually till they 
attained a temperature of 53:5? C. neither potato nor fungus was damaged ; 
but that when heated up to 57:2? C. the fungus was uninjured though some 
potatoes were killed. 
Table I. (2) gives the results of intermittent heating. Under these 
circumstances the tubers failed to withstand heating up to 54:5? C. whereas 
the fungus withstood this treatment. Table Т. (3) records the results 
of heating infected tubers up to certain temperatures and of maintaining 
them for 10 hours at these temperatures. As will be seen from the Table, 
exposure for 10 hours at a temperature of 49:5? C. is fatal neither to tuber 
nor fungus; exposure for a like time to 50:5? C. suffices to kill the tuber 
but not the fungus. 
A second series of experiments (Table IL, p. 128) was made in order to 
determine the heat-resistance of the fungus in dry, moist, and wet conditions. 
As was to be expected, the fungus in a dry state (blue sclerotium) proved most 
resistant, withstanding a temperature of 64:5? C. for 10 hours. А moist 
eulture was killed when allowed twice to reach a temperature of 50? C., and 
a wet culture (in а Buchner-tube with water) if it once reached 50? С. 
Hence the heat-resistance of the fungus in the living potato is greater than 
that of a moist culture on sterilized slabs of potato. 
