Mar. 15, 1925 
Spore Dissemination of Venturia Ineaqualis 
531 
small or nil the entire surface of the ex¬ 
posed area of the membrane was ob¬ 
served. This was accomplished and 
duplication of observation avoided by 
the use of a mechanical stage and a 
micrometer. If spores were numerous, 
counts were made of at least 30 fields, 
chosen at random but without duplica¬ 
tion from all parts of the filter, and the 
total number was computed on the 
basis of this average. The results thus 
obtained agreed closely with those from 
counts over the entire exposed area. 
A Leitz microscope was used, with a 
No. 4 eyepiece, a No. 6 objective, and 
a tube length of 170 mm. 
FIELD EXPERIMENTS 
During late April and early May 
preliminary observations were made 
to determine the time at which asco- 
spores would be sufficiently mature for 
natural discharge. Over-wintered ap¬ 
ple leaves bearing numerous perithecia 
of Venturia inaequalis were brought 
into the laboratory daily, thoroughly 
moistened, and placed in moist cham¬ 
bers in such position that one could 
ascertain by miscroscopic examination 
of the glass surface below them whether 
or not ascospores had been discharged. 
No discharge from such material was 
observed until May 7. 
On May 6, just as the cluster buds 
of the apple were beginning to open, 
the apparatus was installed in the 
Turville 8 orchard, about 1^ miles 
south of Madison. This orchard, con¬ 
sisting of about 50 large trees planted 
in four rows running east and west, 
was in sod, and the ground under and 
about the trees was abundantly lit¬ 
tered with over-wintered leaves which 
bore perithecia of Venturia inaequalis 
in great profusion. The apparatus was 
placed under the outer branches of a 
Fameuse tree which stood in the south¬ 
ernmost row, second from the eastern 
end. The membrane was placed at 
elevations varying from 13dz to 3 feet 
above the ground, and was protected 
from rain by a small roof-like shelter 
which was raised sufficiently to permit 
free passage of air over the filter. The 
machine was run continuously, except 
for brief stops for changing membranes 
and caring for the motor, until June 19, 
and subsequently at intervals. The 
results are summarized in Table I in 
correlation with hourly records of rain¬ 
fall. Hourly records of temperature 
and wind velocity show valuable corre¬ 
lations but are not included in the 
table because of limitations of space. 
The field and laboratory observa¬ 
tions indicate that on May 6, when 
the field filtration tests were started, 
the ascospores were just approaching 
maturity. Apparently no discharge 
had occurred prior to this time. Dur¬ 
ing the dry period from May 6 to 18 
no Venturia spores were caught. On 
May 19, rain fell from about 2 to 5 
8 Grateful acknowledgments are made to Thomas and Wm. D. Turville for their kindness in making 
their orchard available for this work. 
