Biology Reference
In-Depth Information
Fig. 2
Fire Blight. A hold
over canker developed on
apple limb at base of
blighted twig; B bacteria
swarming through tissue
orchards in Illinois, Iowa, and other states in the
Northern Mississippi Valley. Then it devastated
pears on the Texas Gulf. Reaching California by
1910 it played havoc up the coast to Washington.
Symptoms
Blossoms and leaves of infected
twigs suddenly wilt, turn dark brown to black,
shrivel and die, but remain attached to twigs (see
Fig. 2
). The bark is shrunken, dark brown to
purplish, sometimes blistered with gum oozing
out. Brown or black blighted branches with dead
persistent leaves look as if scorched by fire. The
bacteria survive the winter in living tissue at the
edge of “holdover cankers” on limbs. These are
dead, slightly sunken areas with a definite margin
or slight crack where dead tissue has shrunk away
from living. In moist weather bacteria appear on
the surface of cankers in pearly viscid drops of
ooze, which is carried by wind-blown rain or
insects to blossoms. Infection spreads from the
blighted bloom to the young fruit, then down the
pedicel to adjacent leaves, which turn brown,
remaining hanging around the blighted blossom
cluster. Leaf and fruit blight is also possible by
direct invasion, a secondary infection via bacteria
carried from primary blossom blight by ants,
aphids, flies, wasps, fruit-tree bark beetles, and
honeybees, sometimes tarnished plant bugs, and
pear psyllids.
The tissue first appears water-soaked, then
reddish, then brown to black as the bacteria
swarm between the dying parenchyma cells.
Division may take place every half hour; so they
multiply rapidly and are usually well in advance
of discolored external tissue. A collar rot may
develop when cankers are formed near the base
of a tree. Water sprouts are common sources of
infection.
As spring changes to summer, the bacteria
gradually become less active and remain dormant
at the edge of a woody canker until the next
spring at sap flow. Ordinarily they do not winter
on branches smaller than 1/2 inch in diameter.
Control
Spraying during bloom is now
a standard means of preventing blossom blight.
Use bordeaux mixture or a fixed copper or strep-
tomycin at 60 to 100 ppm. The latter is very
effective at relatively high temperatures; at
65
F and below, copper is more satisfactory.
Start spraying when about 10 % of the blossoms
are open and repeat at 5-to 7-day intervals until
late bloom is over. A dormant spray for aphid
control helps in preventing fire blight. One or
more sprays may be needed for leafhoppers,
starting at petal fall.
Inspect trees through the season and cut or
break out infected twigs 12 inches below the
