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Common Tree Diseases in Minnesota
Minnesota's trees face a unique set of threats — from invasive beetles and fungal infections to bark diseases and scale insects. Left untreated, many of these issues can cause permanent damage or kill a tree entirely.
Use this guide to identify what might be affecting your trees, and reach out to our ISA-certified arborists if you need a professional eye on the problem.
Needle Cast Disease
During spring or early summer, fresh needles at the ends of branches are green while older needles become brown to purple. Small black spots are visible in orderly lines on infected needles. Discolored needles drop in late summer. Harm usually begins on lower limbs and progresses upward. If most needles are infected for 3–4 consecutive years, the branch will die.
Apple Scab
Leaf spots are circular, olive-hued, up to ½ inch in diameter with a velvety texture and fringed edges. With age, spots darken to brown or black and merge. Leaves covered in numerous spots become yellow and fall by mid-summer. Infected fruit displays olive-green patches that eventually become brown and corky.
2-Lined Chestnut Borer &
Bronze Birch Borer
Metallic wood-boring beetles (Buprestid beetles) prevalent in Minnesota. Bronze birch borer (Agrilus anxius) targets birch; twolined chestnut borer (A. bilineatus) targets white and red oak. They feed in distressed, dying trees. Canopy dies back from the top down. Elevated strips visible on limbs and trunk. Maintaining tree health reduces appeal to borers. Insecticides can protect valued trees.
Deep Root Fertilization
High-pressure tools inject a nutrient and organic material mix into soil around tree roots at ~5 inches depth — where most feeder roots are located. Applied in a grid pattern across the full root zone to the dripline. Used to aid recovery of struggling or deteriorating trees. Many arborists recommend this as the preferred fertilization method for trees and shrubs.
Magnolia Scale
Female scales are ~½ inch in diameter, smooth, elliptical or spherical, pinkish-orange to brown with a waxy coating. Nymphs overwintering are dark gray with a red-brown stripe. Hosts: Magnolia. Infected branches exhibit reduced growth. Scales produce honeydew attracting ants, wasps, and other insects. Severe infestations cause branch and plant death.
Dutch Elm Disease
Dutch elm disease (DED) causes wilting and mortality in all elm species in Minnesota. Spread by elm bark beetles during feeding, and through root connections between trees. Fungicide injections can protect elms from bark beetle infection. If detected early, DED infections can be pruned out and fungicides can protect the tree. DED-resistant varieties are available.
IPS / Bark Beetle
Bark beetles infest mature red pine forests and pole-sized trees. Epidemics are common in drought years. Thinning decreases competition and reduces infestation risk. Time thinning and forestry tasks in autumn/winter when beetles are inactive. Avoid thinning during or within one year after drought. Consult a forestry expert to establish the best thinning season.
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