538 
Journal of Agricultural Research 
Vol. XXX, No. 6 
petals of many fruit buds was evident, 
and by May 18 nearly all of the blos¬ 
soms were open. On May 22 the 
largest leaves were about 23 ^ inches 
long, and petals were falling. The last 
leaves to develop appeared from about 
May 19 to 24, some variation occurring 
with varieties and individual trees. 
Most of the petals were off by May 
25, and little leaf expansion occurred 
after June 2, when the first conidia of 
the season were observed. There was 
practically no leaf expansion after 
June 8 (except on “water sprouts”). 
SEASONAL DEVELOPMENT OF THE 
DISEASE 
The seasonal development of the dis¬ 
ease was studied by means of a series 
of bagging experiments, supplemented 
by daily observations in the orchard. 
No spray was used in this orchard. 
Each day from May 7 to June 19, six 
branches on at least two varieties were 
inclosed in large manila paper bags 
which were securely fastened in place 
with strong twine. In this way com¬ 
parative records were kept of the devel¬ 
opment of the disease on branches 
which had been exposed to different 
infection periods. Many of the bags 
were broken 9 during storms, necessi¬ 
tating discarding a considerable number 
of branches from the experiment. 
Enough were left, however, to give 
some valuable data, a brief account of 
which follows. 
None of the branches which were 
bagged before May 19 and on which 
the bags remained unbroken during the 
storms of May 30 and June 2 showed 
any evidence of infection throughout 
the period of observation, which ended 
on August 24. All branches bagged 
after May 22 developed infection, and 
with the exception of the last two leaves 
formed, which in some cases were put 
out after May 22, developed about as 
many leaf lesions as those which were 
continuously exposed to infection. 
Leaves of branches bagged after the 
rain on June 2 were infected in the 
same manner and to approximately the 
same extent as those of similar un¬ 
bagged branches. This is of especial 
interest in view of the heavy ascospore 
discharge and favorable infection period 
of June 6-7. 
The first leaf infection of the season 
was noted on June 2. This would allow 
a 14-day incubation period after the 
rain of May 19. This agrees with the 
results of field inoculation studies made 
in early spring in later years, in which 
the periods of incubation for leaf infec¬ 
tion have commonly varied from 13 to 
17 days. The records on ascospore 
dissemination and seasonal develop¬ 
ment of the disease are, therefore, in 
accord in placing the first ascospore 
discharge and infection of the season 
in the rainy period of May 19 to 22. 
The fact that no infection of signifi¬ 
cance occurred on leaves of branches 
which were bagged during the period 
May 19 to June 2 and subsequently 
exposed indicates that the leaves had 
become highly resistant to infection by 
the scab fungus by the latter date. 
A striking suggestion of the variation 
in the susceptibility of leaves in rela¬ 
tion to their stage of development was 
found in the number, distribution, and 
incubation periods of leaf lesions 
induced by natural infection. On 
June 2, when the first scab lesions of 
the season were observed, leaf expan¬ 
sion was almost finished. Lesions 
were clearly visible, in most cases, only 
on the upper surfaces of the apical 
leaves (for convenience called leaves 
No. 1) of shoots. Occasionally, early 
evidences of infection were apparent 
on the next leaf back, and within a day 
or two leaves No. 2 showed infection 
in about the same degree as No. 1. 
The development on leaves No. 3 and 
No. 4 was visible by June 8, while by 
June 12 to 15 small lesions were evident 
on many leaves No. 5 and No. 6 and 
rarely on No. 7. In late July and 
August the older leaves frequently 
showed traces of fungous growth on 
their lower surfaces but no definite 
lesions. The earlier infection thus 
appeared on the younger leaves and 
usually on the upper surfaces after 
relatively short incubation periods, 
while the latest infections to become 
evident appeared after prolonged in¬ 
cubation on the lower surfaces of the 
older leaves. Later field observations 
and field and greenhouse inoculation 
studies have confirmed the general 
outlines of these observations. They 
have shown, however, that there is a 
considerable range in the degree of 
these variations under different con¬ 
ditions. Under some conditions, for 
instance, only one or two leaves of a 
shoot will become infected from a 
given inoculation, while under others 
infection may develop on a half dozen 
or more. 
The seasonal development of the 
disease on the fruit was much more 
difficult to follow in detail than on the 
leaves because of shedding induced by 
insect injury and other causes. The 
9 In later seasons the substitution of bags made of parchment paper obviated this difficulty. 
