Heppner gazette. [volume] (Heppner, Morrow County, Or.) 1892-1912, 1911-05-11
Life History of Apple scab.

Apple scab is caused by fungus
which attacks both foliage and fruit.
On the foliage the spots are at first
more or less circular in outline, olive
green or brown in color, becoming
darker and irregular in shape as they
mature. The leaves are frequently
more or less curled or wrinkled.
When the spots are abundant, the
leaves fall prematurely. Considerable
defoliation may take place where in
festation is abundant.
On the fruit the fungus produces
more or less circular spots of the
greenish black color. The vegetative
stage of the fungus develop under the
cuticle of the apple fruit finally rup-
turing it by the elongatioh of the
threads which bear the spores. The
ruptured cuticle may frequently be
seen clinging as whitish membranous
shreds about the edge of the recently
developed spots. AS the spots become
older all trace of the fungus may be-
come obliterated and only evidence of
the former spot is seen in large or
small rough russetted spot. Ere
quently the fruit is distorted when
mature as the result of early scab
infections. Where scab spots are
abundant the fruit may become
cracked. Scab in any degree of sever
ity on the fruit renders it unsightly
and unmarketable.
The spores of the fungus are pro-
duced in the spots on leaves or fruit in
great abundance and are disseminated
by the wind, thus spreading the dis
ease to other leaves and fruit, either
in the same or other trees. Several
generations may thus occur during
the season. 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 serve to spread the disease to
young fruit and to other leaves.
The development of the scab fun
gus is found to be very much in
fluenced by weather condition. Mois-
ture on the surface of leaves and fruit
is essential to the termination of the
spores and the consequent infection of
the plant On that account scab is
found to spread most rapidly in spring,
early summer and fall in the North
west. The disease spreads little, if
any, during the dry season. The my
celium of the fungus present on the
leaves which fall to the ground in
the fall does not die, but develops in
these leaves during the winter as a
saphrophyte and in the spring pro-
duces spores quite different from those
borne in the spots on living leaves and
fruit, both in the manner of formation
and in the shape and size. 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 little
cylindrical sacs called asci. Eight
spores are always developed in each
sac. A considerable number of these
sacs are borne in hollow more or less
pear-shaped receptacles imbedded in
the tissues of the leaf. These recep-
tacles containing the asci, known as
perithecia, project from the surface of
the leaf when mature as little black
elevations. These are barely visible
to the naked eye. At maturity an
opening appears in the projecting ele-
vations. The asci elongate through
this opening and the spores are eject
ed forcibly, and, waited by currents
of air, may be carried to the young
leaves of the apple, where they germ
mate and produce the first scab spots.
This ascus spore stage of the fungus
which develops on the fallen leaves is
sometimes called the sexual" or
"perfect" stage. The stage which
develops on the living leaves and fruit
is called the conidial or summer
spore" stage.
The ascus 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 following the dissemina-
tion of the sexual spores.