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Shoot Blight
Shoot blight occurs in the spring and results from the fungus
growing through the twig into the newly expanding leaves. It
resembles frost injury and is the most destructive and energetically
costly to the tree.
Shoot blight does not require moisture to occur because
the fungus is already present in twigs in the spring.
Severity depends on the average temperature of the 2 weeks
following bud break. Temperatures between 50-55 F are most
conducive to severe outbreaks, while warmer temperatures result
in less shoot blight.
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Leaf Infection
Spores from fungal cankers on 1 year old twigs are produced
in the spring.
The spores are carried to newly expanding leaves by water,
wind, or insects and leaf infection starts.
Leaf infection begins as inconspicuous lesions on the leaf
surface.
The fungus grows into the leaf, through the veins, down
the petiole, and into the twig where it is in position to
create a canker and overwinter.
Infected trees refoliate during the summer and appear healthy
because temperatures are too warm for much extensive growth.
Healthy trees become originally infected by the leaf blight
stage.
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