250 
Journal of Agricultural Research 
Vol. XXI, No. 4 
the minimum time necessary for the fungus to become established in 
the host tissue at the different temperatures listed in order to produce 
scab subsequently. 
The interrelation of the host and parasite also demands consideration. 
Temperature, of course, influences the host as well as the parasite, and 
the lack of infection in these experiments at certain temperatures may 
have been due to special physiological conditions or states either in the 
host or in the parasite, or even in both. It may be suggested that there 
was not enough time for infection in those cases where it failed to occur. 
From the experiments in which leaves floating in water were left for a 
long time in contact with spores of the fungus at different maintained 
temperatures, it appears that, even with long periods of time, there are 
certain temperatures, not fatal to either parasite or host, at which in¬ 
fection does not take place. At some temperatures the fungus even 
grew fairly well on the surface of the leaf without producing infections 
leading to scab. 
The relation of temperature to infection by the scab fungus, as here 
brought out, seems to suggest a satisfactory explanation for some differ¬ 
ences in the results of inoculations previously recorded by the author (2). 
A set of inoculations made in August, 1906, failed to produce scab, 
while a similar set made in January, 1909, was successful. An exam¬ 
ination of the weather records for the locality where the experiments 
were made shows that the mean daily temperatures for the two weeks 
following the August experiment fluctuated between a minimum of 23 0 
and a maximum of 30° C., while the corresponding temperature range 
for the January experiment lay between a minimum of n° and a maxi¬ 
mum of 19 0 . In the former case, the daily means were above the range 
for infection of seedlings in the experiments here reported except at its 
minimum, 23 0 , while the other case shows an overlapping of the range 
of daily means with the range for infection, to 19 0 . Since the tempera¬ 
tures in the greenhouse where the 1909 experiments were performed 
were probably higher than those outside, the successful infections in 
January were almost certainly due to the fact that the temperatures 
fell for the most part entirely within the range that is favorable to 
infection. 
The temperature relation here emphasized is probably important in 
determining the occurrence of scab in the orchard and in the explana¬ 
tion of scab epidemics. But temperature is only one of a number of 
conditions, such as stage of growth, humidity, time period, etc., that 
must be fulfilled for scab to occur. 
Among the various conditions discussed by Grossenbacher (5) as favor¬ 
ing the development of scab, the following general observations seem to 
bear on this temperature relation. 
If the air is fairly dry and the weather mostly warm and bright during the develop¬ 
ment of the first spring flush, scab may fail to develop. On the other hand, the disease 
