Notes on Nut Diseases
Walnut Blight
Carpathian walnut is susceptible to a destructive bacterial blight caused
by Xanthomonas campestris pv. juglans. Severe leaf infections can defoliate
trees by early August, which reduces the health of the tree and development
of the crop. The most economically important damage occurs when the developing
nut is infected.
Identification
One to several black lesions may appear on male catkin flowers in spring
Infected young nuts develop black, slightly sunken lesions at the flower
end. Additional lesions develop on the sides of the nut as it matures.
Shoots develop black lesions along the stem. Leaves show irregular-shaped
black lesions on the blade. Leaf infections can lead to defoliation when
severe.
Period of activity
Walnut blight bacteria overwinter primarily inside infected dormant
buds. Infection can occur as soon as buds break and growth begins in spring.
All green tissue is sensitive to infection. Rain is important for the
spread of bacteria to green tissue and causes new infection. In Ontario,
this disease appears to be more prevalent in seasons with rain and high
humidity and is active throughout spring and summer.
Management
Applications of copper spray in June and July, in periods of prolonged
wetting, at seven to ten day intervals can help reduce the incidence of
walnut blight infection. As a precautionary note, resistance of bacterial
blight to copper spray has become a serious problem in other countries
growing Carpathian walnuts. Use copper sprays only when necessary to control
infections.
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