Guide

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Beech bark disease: white fluff and craters on beech trunks

December to April

American beech is the forest tree known for smooth, pale gray bark, but beech bark disease has left many New England beeches rough and pitted. A tiny introduced scale insect covers itself in white wool-like wax on the trunk, and fungi then get in through its feeding wounds and kill patches of bark.

Why this time of year

The scale spends the winter on the trunk as a young insect covered in white wax, so the white dots are there right through the cold months. The cankers and crater-like scars are part of the bark itself and stay on the trunk in every season.

How to notice it

  1. Find a beech: on a healthy tree the bark is smooth and light gray.
  2. Look on the trunk at rough patches, just below where big branches join, and under moss or lichen for small dots of white fluff. On a badly hit tree the white can spread over much of the trunk.
  3. Look for damage to the bark itself: sunken, crater-like scars, cracked and split strips, or rough lumpy bark where the tree has tried to heal. On bark that is still fairly smooth you may also find clusters of tiny bright red dots, the fungus's fruiting bodies, which ripen in fall.
  4. Then look for a beech nearby that is still smooth and clean. A few beeches stay free of the disease even in badly hit stands, and some of those have turned out to be resistant to the scale.

Try this with kids: Can you find the smoothest beech in the woods? Look at its trunk without touching it, then find a bumpy one and count the white fluffy dots you can see.

Word to know: A “canker” is a patch of bark the fungus has killed, often sunken or cracked, with healed rough edges around it.

Safety

  • Badly diseased beeches can break off partway up the trunk in wind (foresters call it beech snap), so look from a step away and don't climb or lean on a dead or heavily cankered beech, especially on a windy day.

Sources

  • fs.usda.govUSDA Forest Service Forest Insect and Disease Leaflet 75 (Houston and O'Brien, revised August 1983; a scanned PDF, read here by OCR). Primary source. Establishes: the disease results when bark attacked by the beech scale (Cryptococcus fagisuga) is invaded and killed by fungi (named Nectria coccinea var. faginata and N. galligena in this 1983 text, now called Neonectria). The scale's glands 'secrete a white woollike wax'; 'The white wax secreted by the beech scale is the first sign of the disease'; isolated dots of white wool appear on roughened bark, beneath mosses and lichens and below large branches, and the whole bole may become covered. LIFE CYCLE (carries the months): eggs from midsummer, hatching late summer to early winter; the settled insect becomes a second-stage nymph 'covered with woollike wax. The insect overwinters in this stage and, in the spring, molts to become an adult female.' Fruiting bodies (perithecia) are tiny, bright red, lemon shaped, mature in the fall, and on dead bark keep producing spores the next year. Cankers: bark becomes roughened around callus; Figure 9 caption: 'craterlike scars indicate where small, isolated nectria cankers were walled off by callus tissue.' History: scale brought to Nova Scotia around 1890; by 1932 isolated infestations in eastern Massachusetts; since spread throughout New England. Larger trees (over about 8 inches diameter) succumb more readily. Beech snap: trees weakened by decay under killed bark are broken by wind. 'Vigorous trees free of the disease are often found in heavily affected areas' and trials showed such trees resistant to the scale.
  • umass.eduUMass Extension fact sheet. Massachusetts extension source. Establishes the current name Neonectria and the scale-then-fungus sequence that produces a perennial bark canker that slowly kills the tree; 'The scales appear as small, white-colored flecks on the bark'; 'As more bark is killed, trees take on a rough and cankered appearance'; red, oval perithecia appear and 'sporulate during the autumn season' (photo dated mid-October); BBD 'has killed millions of American beech (Fagus grandifolia) throughout New England'; a photo of a disease-resistant American beech beside a badly infected one. Gives no resistance percentage and no month-by-month life cycle.
  • gobotany.nativeplanttrust.orgNative Plant Trust Go Botany, American beech. Establishes the healthy baseline: 'Its exceptionally smooth, light gray bark makes the American beech stand out in any forest'; habitat forests; present throughout Massachusetts (S5); notes mature trees 'are falling victim to a Nectria fungus that blisters the bark'.
  • research.fs.usda.govUSDA Forest Service Treesearch record, Koch, Mason and Carey 2012, 'Screening for resistance to beech bark disease: Improvements and results from seedlings and grafted field selections'. Primary research source. Establishes: 'It is currently estimated that between 1 and 5 percent of the native American beech are resistant to beech bark disease.' This is why the card says only 'a few' smooth beeches resist, and never calls a given smooth tree resistant.

Checked against these sources on November 6, 2026.

In the guide for December, January, February, March, April

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