Fireblight - Erwinia amylovora
Atthowe - Missoula County
Photo courtesy of Thomas Yule
HOSTS: Apple, pear, quince, crabapple, cotoneaster, hawthorn,
mountain ash, pyracantha, serviceberry, and other species in the rose
first appear water-soaked, then black and shriveled. Blighted twigs
curl to form a characteristic "shepherd's crook". Leaves quickly die,
blacken, and remain attached to the limb. Infected fruit is shriveled
and dried. As the disease progresses, it affects older, larger limbs.
Cankers are usually discolored, sunken, and defined by a narrow, raised
margin. Amber-colored ooze is usually present at infected buds, twigs,
blight develops in three phases:
Blossom blight occurs when flowers are open with
stigmas and petals intact during wet weather >65°F. Canker
blight occurs when the fireblight bacteria that overwinters on cankers
from the previous year renew growth and infect new healthy bark tissue
during wet weather >55° F.
Canker blight usually occurs during warm wet weather 1
– 2 weeks after apple trees stop blooming.
Shoot blight occurs when bacteria invade the tips of
new shoots (the tip and youngest 3 leaves are most susceptible) during
wet weather >55° F. Shoot blight occurs after the appearance of
blossoms and/or canker blight symptoms when shoots have 10 – 12 leaves.
Frost, wind, or hail can worsen fireblight infection.
CONTROLS: See Apples - Disease Resistant Cultivars!
Biological: Pseudomonas florescens (Blight
Ban) sprayed when blossoms begin to open has helped reduce the blossom
blight stage of fireblight. Streptomycin can be sprayed during bloom if
fireblight infection periods occur. Two applications have proven to be
more effective than one. (Listen to the Pest Alert Phone Line for information on infection period.)
Cultural: Prune out and destroy infected shoots and
branches on a hot sunny day. Temperatures above 90°F stop
fireblight infection. Remember to sterilize pruners between each cut.
Keep irrigation water off of trunks, branches and leaves. Avoid
excessive nitrogen fertilization.
Chemical: Dormant: Bordeaux mix sprayed as
buds begin to break helps prevent infection. Spray after pruning. After
bloom: Copper can be sprayed when fireblight infection periods
occur. Do not spray copper during bloom.