Notes on Nut Diseases
Bacterial blight of hazelnut
Bacterial blight disease of hazelnut is caused by the bacterium
Xanthomonas campestris pv.corylinia. Bacterial blight
should not be confused with eastern filbert blight, which is caused
by a fungus. Infection by bacterial blight, particularly to young
establishing hazelnuts, can cause serious losses in hazelnut orchards.
Healthy trees are less susceptible than trees under stress.
Period of activity
Bacterial blight overwinters in cankers on branches and inside
infected buds. New bacteria ooze from cankers during the growing
season. Bacteria are spread by rain to new infection sites. Bacteria
enter through open stomata on the leaves and tender branches and
through wounds or cracks on older branches. The bacteria first infect
the outer bud scales and then move into the inner bud. Buds may
be completely killed or partially damaged. Shoots that emerge from
infected buds generally become infected from the bud scales as they
grow past.
Temperatures above 20°C favour infection, although infection
can occur at lower temperatures if the wetting period is long enough.
Moisture must be present on the plant tissue for infection to occur.
Infection time can be as short as one hour where temperatures are
warm. Infection by bacterial blight is greater after a winter where
freezing injury has occurred.
Buds are susceptible to bacterial infection from summer, when they
are three quarters grown, to when they open the following spring.
Twigs and branches are susceptible in summer and fall. Leaves are
susceptible until they have reached their maximum size. The bacterium
does not infect root systems.
Identification
Bacterial blight infections are easiest to see in late spring.
Leaves develop small, angular spots that are less than 3 mm in diameter,
reddish brown and surrounded by a yellowish green circle of tissue.
Leaf lesions eventually coalesce at the tip of the leaf. Infections
on developing nut husks are less common but can appear as dark brown
or black spots.
On branches, early infections or cankers can be difficult to detect.
One and two-year-old branches are infected more often than older
branches. Cankered bark is slightly sunken and a reddish-purple
colour. The underlying tissue is brown. A sticky liquid containing
new bacteria oozes from infected tissue during periods of high humidity.
Cankers can completely encircle an infected branch killing the branch
beyond the canker. Leaves on dead twigs turn brown and cling to
the branches. This symptom can be confused with eastern filbert
blight disease.
Management
Infected branches should be pruned well below the cankers in late
winter to reduce the source of new inoculum. All prunings should
be removed from the orchard and burned or buried. Cooper sprays
are registered to help manage bacterial blight. Up to three applications
can be applied per year. The first application of copper spray is
late August or early September. The second application is in the
fall when three quarters of the leaves have fallen. For severe infections,
a third application of copper spray can be made the next spring
just before bud break occurs.
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