Mar. 15, 1925 
Spore Dissemination of Venturia Inaequalis 
539 
first observation of fruit infection was 
made on June 8, when lesions were 
visible on the young fruits, calyx lobes, 
and pedicels. Daily field records 
showed that lesions continued to appear 
on the fruit throughout the period of 
observation, which ended on August 
24. The results from the bagging 
work showed that the first fruit infec¬ 
tion occurred during the rainy period 
of May 19 to 23. The most abundant 
infection and the largest lesions de¬ 
veloped in the early part of the season. 
The fact that the later lesions are 
smaller suggests that the fruit, like 
the leaf, may develop resistance with 
age. Such resistance, however, ap¬ 
pears to be developed in a much smaller 
degree, since fruit infection has been 
shown to appear late in the season or 
even in storage. 
INOCULATION EXPERIMENTS 
The life history studies were sup¬ 
plemented by some inoculation work 
in the orchard. Metal boxes, 28 by 12 
by 10 inches, with tightly fitting, felt- 
lined covers, were constructed as infec¬ 
tion chambers. A slit at one end of 
each box permitted a branch to enter 
and a tripod served as an adjustable 
support. The box was placed in posi¬ 
tion about the inoculated or control 
branch, the cover put in place, and the 
orifice through which the branch 
entered closed with plastic clay. A 
metal shelter about 4 inches above the 
box shaded it from the sun. When 
necessary the box was cooled by water. 
Three types of shoots were selected 
for inoculation: (1) Those which had 
been bagged continuously since May 
18 and were therefore free from infec¬ 
tion, (2) those which had been exposed 
to infection throughout their develop¬ 
ment, and (3) those which developed 
during July (“water sprouts”) and con¬ 
sequently bore young leaves. The 
inocula consisted of sterile-water sus¬ 
pensions of conidia from naturally 
infected leaves and from oatmeal-agar 
cultures. The experiments were per- 
performed in July and early August. 
The chief results of these experi¬ 
ments may be summarized briefly. 
Thirty-six trials on disease-free shoots 
which had been bagged prior to May 
18, and which bore no young leaves, 
gave no infection. In similar trials on 
shoots which had been exposed con¬ 
tinuously to infection, there was no 
evidence that additional infection was 
induced. However, on branches which 
bore young leaves, abundant infection 
was induced on the young leaves pro¬ 
vided the temperature was kept from 
running too high. While the exact 
maximum temperature for infection 
was not determined it appeared to be 
not much higher than 25° C. The 
maximum age of susceptible leaves is 
of course variable with conditions. In 
these tests it usually ranged between 12 
and 15 days. These results are there¬ 
fore in accord with those obtained in 
the life history and seasonal develop¬ 
ment studies. 
SUMMARY 
An effort was made to devise a 
satisfactory apparatus for determining 
the spore content of orchard air. 
Preliminary experiments were made 
with an electrical device. The re¬ 
sults were promising, but the greater 
economy and simplicity of a mechanical 
filter led to its adoption for this work. 
It is believed, however, that the elec¬ 
trical method has potentialities of 
adaptation to problems of this type. 
By wetting leaves freshly collected 
from the orchard it was shown that, 
in the vicinity of Madison, Wis., asci 
were capable of discharge under favor¬ 
able conditions on May 7, 1917. Be¬ 
cause of dry weather, however, the first 
natural discharge was delayed until May 
19. Following this date ascospore dis¬ 
charges occurred during rains through¬ 
out the spring and early summer, the 
last recorded discharge occurring on 
July 18. The very heavy discharges, 
however, were limited to the period 
May 19 to June 7. After this time 
the discharges were relatively small. 
The maximal concentration of asco- 
spores of Venturia inaequalis in the 
orchard air was observed on May 21, 
when the average rate of catch for a 
period of hours was 1 ascospore 
to 0.4 liters of air. The wind velocity 
for this period varied from 14 to 23 
miles an hour. The filter was 3 feet 
above the ground. 
It was found that, when the asci 
were in condition to eject their spores, 
the presence of an adequate supply of 
water was the most important requi¬ 
site for their discharge. In the ex¬ 
periments dew was not sufficient to 
induce discharges of consequence. 
In cases where abundant asci were 
in condition to eject their spores in 
the presence of water, heavy discharge 
started soon after rain began and con¬ 
tinued with continuous rain as long 
as the supply of ripe asci lasted. 
These periods of very heavy discharge 
lasted from 3 to 15 hours. Undoubtedly 
the duration of such periods varies 
widely with conditions. 
