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Hail damage

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 branches. 

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.