Galls on Campus 7: Walnut

Before getting back to galls that can be seen around the Whiteknights campus, I ought to mention that at the end of last year another gall book appeared, making three in one year!  This one, Britain’s Plant Galls, a photographic guide, by Michael Chinery (WildGuides, 2011, £ 14-95) is a short, 96p, paperback and an excellent introduction to galls, concentrating on 200 or so common galls (about 20% of the British gall fauna (or is it flora?)) that can be seen, with some good-quality photos of the galls.

Chinery: Britain's Plant Galls

This up-to-date book really should be the starting point for anyone interested in galls, before tackling the AIDGAP key or the New Naturalist Plant Galls book, both mentioned in earlier posts.

What on earth is this tree?

Whilst waiting for galls to form this year (and many start very early: on trees as soon as flowers or buds are produced) there are still a number of galls seen last year to report,  such as these two galls found on the small walnut, Juglans regia, outside the Food Biosciences building on 22 July last year.  I would not have recognised this tree without the fruit and Alastair’s confirmation that this was a walnut, albeit a cut-leaved cultivar, cv laciniata.

Ah, its a walnut

After humans have fiddled with the leaf shape of this tree (just to confuse entomologists!) we can hardly begrudge gallers in further modifying the leaf structure; and these two have made a good job of this tree.  Both galls are erinea caused by mites.

Aceria erinea on walnut

The larger brown elongate one bulging between veins is caused by Aceria erinea.  The erineum is up to 1 cm long with the underside lined with a felt of pale hairs.  Such is the shape of these leaves and their weeping arrangement that I think the mites have become confused, as they have made some galls up the wrong way, with the hairs on the upper surface of the leaf, instead of the lower.  This gall is common, but the other one, caused by Aceria tristriata, the black walnut blister mite, is much less common.

Aceria tristriata on walnut

This gall is a 1-2 mm rounded pustule starting green and then becoming brown, and is often found along the veins.  Like many similar galls I have mentioned previously on sycamore and lime, it is open on the underside.  The fact that this mite has a common name should sound warning bells – why?  And sure enough a Google search will find that this is quite a pest species (in Europe and North America), not least because it can transmit the bacterium Xanthomonas juglandis, the cause of walnut blight which can cause significant yield loss.


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One Response to Galls on Campus 7: Walnut

  1. I’m amazed that anything can feed on walnut given both the high tannin content and the presence of juglone, an astringent naphthaquinone, that makes the leaves taste very bitter.

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