treeblog - Pests, diseases, disorders, competing growth and unfavourable conditions (a field trip): the pests and disorders

On Thursday the 21st of January my arboriculture class set out from college on a field trip to see a smörgåsbord of pests, diseases, disorders, competing growths and unfavourable conditions afflicting a variety of trees in the vicinity of York and Malton. Some of them were new to me, most I was already aware of, but it made for a very interesting way to spend a day and we got to see some cracking trees. Here’s a quick run-through of the pests and disorders that we saw:

PESTS
January isn’t a very good time to see pests in Britain. I think the scene is a lot more banging in the summer months.

European rabbit (Oryctolagus cuniculus) - The field trip took place in a particularly harsh winter and the poor old bunny rabbits had resorted to stripping bark from trees. The damaged trees we saw formed part of a rough hedge along a field and were growing right beside a few rabbit burrows. The photo shows damage to an ash but we also saw damage to a tiny hawthorn (Crataegus). Bark stripping weakens trees (when trees are ring-barked / girdled all material above that point dies) and opens them to infection. [Other animals such as hares, deer and squirrels will also strip bark.]

Humans (Homo sapiens) – The example we saw was disfiguration of the bark by people carving initials or symbols. Graffiti has a negative impact on the aesthetic appeal of the tree and can open it up to infection. [Other examples of direct human damage: vandalism (breaking off branches), vehicle damage (usually inflicted by tall vans and lorries), and butchery by D.I.Y. / cowboy tree surgeons.]

Witch’s broom (Taphrina betulina) – T. betulina is a fungus that causes dense balls of twigs (that look like birds nests from a distance in winter) – witch’s brooms - to form on the branches of silver and downy birches (Betula pendula and Betula pubescens respectively). As far as I was aware* the fungus doesn’t have any significant effect on the health of host tree, although as witch’s brooms grow larger every year, I assume that they eventually become so heavy that the supporting branch will break.

[ * I found this abstract to a scientific paper by Spanos & Woodward (1994) ¹ online: The impact of infection by Taphrina betulina on the growth of Betula pubescens was studied in naturally regenerated stands in the North East of Scotland. Infection by T. betulina was associated with a significant reduction in height growth and poorer developmental tendency, vigour, and stem quality of B. pubescens, whereas diameter at breast height was little affected. Tree height was reduced by an average of 25% over all diameter classes, with the greatest effect in smaller trees. For brooms of 100 mm diameter and over, numbers and sizes were strongly correlated with diameter at breast height and tree age. Reductions in height and vigour were not correlated with the number and size of brooms present, but were more pronounced in younger, smaller trees.]

Included bark (to the left of the red line) in the crotch where two main stems meet on a beech.

Included bark - “Included bark forms when the bark of the branch and trunk squeeze together” (Shigo, 1991) ². “In crotches that have very narrow angles of attachment the branch bark ridge [a ridge of bark in the crotch] sometimes fails to expand outward and is swallowed by the growth of the branch and trunk. Each year thereafter, more bark is enclosed within the crotch. This condition is referred to as included bark… Included bark has long been associated with weakness in tree crotches since it is frequently seen in failed tree forks. It is easy to assume that the included bark prevents the formation of connecting wood between two stems and therefore reduces crotch strength. …branch attachments with included bark are inherently weak and should be removed” (Farrell, undated) ³.

Sphaeroblasts - Disappointingly, I don’t have a photo of these intriguing fellas. You ever seen a Malteser- to fist-sized ball in the bark of a tree? Those are sphaeroblasts, described by Strouts & Winter (2000) ⁴ as “Bark-coloured spheroid lumps, small or large… woody, bark-covered structures, being an abnormal development of a bud which has produced annually a woody sheath without ever producing a shoot. Harmless.”

Burrs or burls - The burr we visited on our field trip – seen in the photo above – was a huge burr on a good-sized oak. It formed in a similar way to how sphaeroblasts form – the difference I think is that the buds in a burr are on the outside of the bark, not beneath it. The buds seem to multiply like crazy and sometimes produce tiny little shoots that never amount to anything. As far as I know, burrs are harmless to trees. They’re supposed to be highly valued by craftsmen because of the spectacular grain of the wood inside.

A pair of oak marble galls (one not fully developed) [photo taken 15 January 2009].

Galls - We saw a few old marble galls on the oak with the massive burr. I mentioned these in a post in January 2009: oak marble galls [are] caused by asexual Andricus kollari larvae. A. kollari is a member of the family Cynipidae, “whose members are of special interest because most of them induce gall formation on plants and many of them display a marked alteration between sexual and parthenogenetic generations. They are called gall wasps… Most of the European species occur on oaks, although some species attack roses and certain herbaceous plants. There are about 90 British species.” - from Chinery’s Insects of Britain & Northern Europe (Collins Field Guide, 1993). Again, as far as I’m aware, galls don’t significantly affect the health of a tree in normal conditions.

¹ Spanos, Y. A. & Woodward, S. (1994). The effects of Taphrina betulina infection on growth of Betula pubescens. European Journal of Forest Pathology, 24 (5), 277-286.
² Shigo, Alex L. (1991). Modern Arboriculture. Shigo and Trees, Associates.
³ Farrell, Robert, W. (undated) Structural Features Related to Tree Crotch Strength (Thesis submitted to the Faculty of Virginia Polytechnic Institute and State University for the degree of Master of Science in Forestry). Available in PDF format from: http://www.treeworld.info/manualuploads/crotchstrength.pdf [Accessed 11th February 2010].
⁴ Strouts, R. G. and Winter, T. G. (2000). Diagnosis of ill-health in trees. The Stationary Office.

5 comments for Pests, diseases, disorders, competing growth and unfavourable conditions (a field trip): the pests and disorders

Wow - I've never seen or even considered the possibility of rabbit damage to a tree! Again, great photos - thanks for sharing and great to hear how you are learning :)

I just saw on the twitter thing that you've seen catkins on the trees.
I've seen:
lambs.
catkins (willow and hazel, mainly).
a few of my maple seeds are growing.
I have currant bush seedlings coming through.
Cherry seeds growing. Yesterday, I had to bury one that I'd found on the top of the soil.
Etc

Cheers Katy!
---
Reece - It's so good to see the signs that we're coming into the beginning of the end of winter. Spring can't come soon enough!

People carving their initials into trees are one of the few things that really annoy me. It's horrible to be in woods completely away from humans, only to be reminded of their existance by coming across a poor graffitied tree. Anytime I'm travelling on the bus from central to KB I always notice how tree branches are sheered off at the level of the upper deck (and some of the branches have even started growing in the direction of the prevailing traffic) although they've maybe been trimmed deliberately.

I've been sat on the top deck of buses when they've rammed into branches! And only today at work, we saw a lorry driver intentionally drive up a verge and into a youngish tree to get around a skip-wagon that was temporarily blocking the road! Street trees are in harm's way all too often when it comes to bad driving.

Pests, diseases, disorders, competing growth and unfavourable conditions (a field trip): the pests and disorders

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