Identifying tree health problems

Start with what is normal

Leaves drop in autumn, and evergreens shed their oldest needles every year. Shaded lower branches die naturally. Colour change in late summer can be a response to drought and not to disease. Look at the whole crown, and compare with neighbouring trees of the same kind.

Signs on leaves

• Spots, blotches or curling: fungal leaf diseases, which are usually cosmetic on mature trees.
• Holes and chewed edges: insects, mostly caterpillars and beetles.
• Sticky, shiny coating on leaves and cars beneath: honeydew from aphids or scale insects.
• Scorched edges in summer: drought, wind or salt.
• Yellowing between green veins: often a nutrient problem tied to soil chemistry.

Signs on bark and wood

• Small D-shaped exit holes in ash bark and thinning crowns: emerald ash borer.
• Weeping sap or dark oozing patches: bacterial or fungal infection.
• Shelf-like mushrooms on the trunk or at the base: decay fungi, a warning of structural weakness.
• Vertical cracks and cankers: frost, sunscald or fungal cankers.
• Sawdust at the base or in bark crevices: wood-boring insects.

Signs in the crown

Dieback of the outer twigs and thinning from the top down point to stress at the roots or to pests. Leaves that wilt and turn brown on one branch while others stay green are a sign of vascular disease such as Dutch elm disease. Oak wilt, caused by a fungus, does the same in oaks.

Big threats to know

• Dutch elm disease: a fungus spread by elm bark beetles, which killed a huge number of American elms.
• Emerald ash borer: a beetle from Asia that kills ash trees.
• Chestnut blight: a fungus that removed the American chestnut as a forest tree.
• Hemlock woolly adelgid: a sap-feeding insect that is killing eastern hemlock.
• Oak wilt: a fungus that kills oaks, especially red oaks.
• White pine blister rust: a fungus on five-needle pines.

What to do

Take clear photographs of the whole tree, the leaves and any damage, and note when the problem began. Contact your state forestry service or university extension office, or a qualified arborist. Do not move firewood far from where it was cut, since moving it spreads wood-boring insects and disease. Report pests that you suspect are new to your area.