Hail stones lacerate leaves, defoliate branches, remove twigs, bruise or
break the bark of twigs and small branches, and kill small trees.
In extreme cases the bark may be pounded off on the windward side of stems.
Bruises and wounds tend toward elliptical shapes and vary in length from
1/16th of and inch to four inches or more. All stem wounds occur
on the side of the tree facing the storm. Hail wounds on branches
can occur on the upper or lower sides of the branches, depending on how
strong the wind was.
All tree species in the locality display similar symptoms, although
the severity varies among species. For example, a hail storm last
summer in a young, mixed conifer plantation caused considerable damage,
but it varied by species. Hail stones simply broke the needles off
the leader and some upper branches the red pines, knocked the needles and
fascicles off the jack pines but stripped needles off the white pines and
lacerated and bruised the bark all the way down to the lowest whorl of
Bruised bark may crack after the storm as the result of drying and mechanical
stress from woundwood growth at edges of injured areas. Bruises or
wounds result in dieback of twigs and branches if tissues around
the injuries dry out. Severe hail wounds may coalesce and kill all the
bark on one side of a stem. Dieback and infection by decay and canker fungi
are more likely if injury occurs during dormancy rather than during
the growing season. Severely damaged trees may also become susceptible
to invasions by various insects.
A canker-rot of maple
Cerrena unicolor is one of several canker-rot fungi that infect
maple and many broadleafed trees weakened by fire, sunscald, dense shade,
top breakage, flooding, repeated defoliation, root damage or other environmental
factors. In central Minnesota, Cerrena it has been seen on sugar maple,
especially on smaller trees, one to three inches in diameter.
C. unicolor can gain access to trees through fresh wounds or its spores
can be carried through the bark by horntails or woodwasps. If the
tree is already stressed or weakened, it overcomes defensive responses
and kills and decays nearby sapwood. As this fungus girdles the cambium
and digests the sapwood, the tree parts above it die. It is a common secondary
factor in the decline of sugar maple, often causing relatively sudden death
of major limbs or the tree tops. Sprout clumps on a common root system
seem particularly vulnerable
Fruiting bodies are usually produced on killed bark during the summer,
the first year after the cambium dies. They are often found in clusters.
The upper surfaces of the fruiting bodies are hairy white to greenish gray
and are concentrically ridged. Their lower surfaces are tan or white
and have irregular pores.
Maple sugarbush owners who try to maintain vigorous, wound-free trees
and promptly remove infected ones should be able to reduce the incidence
of this disease.