Mar. 15, 1925 
537 
Spore Dissemination of Venturia Inaequalis 
inaequalis are to be expected at about 
the time the fruit buds separate in the 
clusters just prior to the blooming 
period. Subsequent studies, however, 
have shown that, in the vicinities of 
Madison and Sturgeon Bay, Wis., 
ascospore discharge ordinarily begins 
at a much earlier stage of bud develop¬ 
ment and that the lateness of the initial 
discharge of 1917 may be attributed 
largely to the paucity of- rainfall in 
early April and the first 18 days of 
May. It should be noted that even 
in 1917 it was shown by wetting leaves 
freshly collected from the orchard that 
asci were capable of discharging spores 
as early as May 7. 
Although the air filtration experi¬ 
ments were planned primarily for the 
study of ascospore dissemination a 
careful watch was kept for conidia. 
After June 2, when the first scab 
lesions were observed, conidia began to 
appear in small numbers on the mem¬ 
brane in rainy periods, particularly 
when the rain was accompanied by 
high winds. They were never caught, 
however, except under these condi¬ 
tions. These facts led the writers to 
make certain observations upon the 
conditions which favor dissemination 
of conidia. 
Apples leaves bearing abundantly 
sporulating scab lesions were placed in 
a glass tube through which a strong 
current of air was driven. A glass 
slide smeared with glycerine was held 
opposite the outlet of the tube in 
position to catch air-borne spores. 
Microscopic examinations following 
numerous repetitions of this test 
revealed only very small numbers of 
Spores on the slides. Similar experi¬ 
ments with lesions on apple fruit gave 
like results. However, when these 
same lesions were treated with a fine 
mist of water applied by means of an 
atomizer, the droplets which accumu¬ 
lated contained conidia in great num¬ 
bers. Similarly, when sections were 
cut from lesions with a dry razor and 
observed under the microscope, the 
addition of a droplet of water led to 
swelling of the conidiophores and im¬ 
mediate detachment of conidia. Fur¬ 
thermore, drippings collected during 
rains from scabby apple trees have 
shown an abundant content of conidia 
of Venturia inaequalis. These results, 
in conjunction with those from the air 
filtration experiments, indicate that 
no important dissemination of conidia 
is to be expected in the absence of 
water, though undoubtedly some spores 
are dislodged by wind-whipping of 
leaves, fruit, or branches, by contact 
with wind-blown particles, and in 
other minor ways. It appears, there¬ 
fore, that the important agency for 
dissemination of these conidia is 
meteoric water moving under the in¬ 
fluence of wind and gravitation. 
GERMINATION TESTS 
Ascospores. —Germination tests 
with ascospores were made at frequent 
intervals throughout the period in 
which their natural discharge was 
observed in the field (May 18 to July 
18). Fragments of leaves bearing 
perithecia were moistened and placed 
in such position that the spores would 
be discharged upon droplets of sterile 
distilled water on clean sterile glass 
slides in moist chambers or on plates 
of 2 per cent agar in water. The drops 
or plates were then placed in incubators 
at 18° to 25° C. In all cases naturally 
discharged ascospores showed a high 
percentage of vigorous germination, 
usually approximating 100 per cent. 
Germination tests of ascospores caught 
on the membranes gave like results. 
Some experiments conducted at con¬ 
stant temperatures ranging from 2° to 
26° C. showed germination at all the 
temperatures tried, the optimum for 
germination and growth under these 
conditions being between 14° and 20° C. 
Conidia. —Germination tests of co¬ 
nidia from fruit and leaf lesions were 
made at frequent intervals from their 
first appearance until late fall. The 
same media and incubators used for 
ascospore germination were employed. 
Abundant viable conidia could be 
found at any time during these experi¬ 
ments, but the percentage and vigor of 
germination varied greatly in individual 
tests. This is not surprising in view of 
the fact that mature conidia are much 
less protected than are ascospores and 
not so uniformly removed from the 
parent fungus when rain follows their 
maturity. Consequently, a sample of 
conidia, even when secured by touching 
a droplet of water borne in a wire loop 
to the sporulating surface of a scab 
lesion, is likely to be less uniform than 
a sample of naturally discharged asco¬ 
spores. 
SEASONAL DEVELOPMENT OF THE 
HOST PLANT 
In relation to production and dis¬ 
semination of spores and the seasonal 
development of the disease, the follow¬ 
ing brief notes on the seasonal develop¬ 
ment of the host plant are pertinent: 
The unfolding of both vegetative and 
fruit buds was unusually late and slow 
in 1917. On May 7 the pink of the 
