488 
in spring and summer and causes the 
spots described above. The spores of the 
fungus are produced in the spots on leaves 
or fruit in great abundance, and are dis- 
seminated by the wind, spreading the dis- 
ease to other leaves and fruit. One finds 
the scab spots first appearing in the spring 
on the under side of leaves on the lower 
branches. Spores produced in these spots 
are disseminated to other leaves and to 
the developing fruits. Several genera- 
tions may thus occur during a single 
season. 
The mycellum of the fungus present on 
the leaves which fall to the ground in 
the autumn does not die, but develops 
in these leaves during the winter as a 
saprophyte, and in the spring produces 
spores quite different both in manner of 
formation and in the shape and size from 
those borne in the spots on living leaves 
and fruit. The spores found in the spots 
on leaves and fruit are one-celled, rarely 
two-celled, and are borne on the ends of 
short threads, while the spores developed 
on the dead leaves in spring are always 
distinctly two-celled and are borne in lit- 
tle cylindrical sacs called asci. A con- 
siderable number of these sacs are borne 
in a hollow, more or less pear-shaped re- 
ceptacle buried in the tissues of the leaf. 
These receptacles containing the asci are 
known as perithecia, and when mature 
project as little black elevations from the 
surface of the leaf. These are barely 
visible to the naked eye. At maturity an 
opening appears in the projecting eleva- 
tions. The asci elongate and protrude 
through this opening and forcibly eject 
the spores, which, wafted by currents of 
air, may be carried to the young leaves 
of the apple, where they germinate and 
produce the first scab spots. The sexual 
spores are disseminated about the time 
the blossoms open. So far as is known, 
all primary infection of the leaves and 
fruit in the spring takes place as a con- 
sequence of disseminations of the sexual 
spores. Subsequent infections result from 
the dissemination of the conidial or sum- 
mer spores. 
The development of the scab fungus is 
found to be very much influenced by 
ENCYCLOPEDIA OF PRACTICAL HORTICULTURE 
weather conditions. Moisture on the sur- 
face of leaves and fruit is essential to the 
germination of the spores and the conse- 
quent infection of the plant. On this ac- 
count scab is found to spread most rapidly 
in the spring, early summer and late fall. 
The disease spreads little, if any, during 
the summer in dry seasons. In seasons of 
Fig. 2. Scab on Apple Leaf. . New Hampshire 
Experiment Station. 
frequent summer rains, as in 1912, scab 
may spread all summer, if the trees are 
not properly protected by spraying. 
Apple scab is not uncommonly found de- 
veloping in storage. If fruit infested with 
scab is stored the fungus may continue 
to develop around the edges of the old 
spots. New spots due to infection in stor- 
age may also occur. It is not uncommon 
to find small black spots of apple scab de- 
veloped on apples in storage, particularly 
in the hollow about the stem. This may 
occur on fruit that was perfect when 
stored. 
Treatment 
As with all fungi of this nature, treat- 
ment must be preventive rather than cura- 
tive. The method of treatment is depend- 
