Friday, July 24, 2009

Oak Leaves Under Attack

Several tree species - most notably oaks - put on a new flush of growth in summer, sending out shoots with fresh green foliage (bottom photograph) to supplement the older leaves of spring that have suffered from insect attack and general wear-and-tear. The new shoots are known as Lammas growth, because they’re well developed by the ancient Celtic harvest festival of Lammas day - 1st. August. Lammas growth is most prominent in younger trees during this 'second spring', but sometimes the freshness of this new foliage doesn’t last very long. Take a look at the new shoots and you’ll find that many will be distorted and coated with a greyish-white powder (second photo from bottom). This is the parasitic oak powdery mildew Erysiphe alphitoides that thrives in the warm, humid weather that we’ve been experiencing lately. Under the microscope you can see a mass of transparent fungal hyphae covering the leaf surface (third image from bottom) visible in the microscope photo (x400) in the clear areas between the blocks of green tissue. The hyphae draw their nutrition from the delicate new leaf tissue and send up short aerial hyphae that bud-off powdery spores (fourth and top images, x100 and x400 respectively), that blow away in the wind and infect another leaf.


  1. Hi Phil. I've never come across the term Lammas growth before but that's clearly what I was looking at when I was up on the west coast of Scotland a month or so back. I was struck by the beautiful reddish colours here and there in the tips of new light green growth in the atlantic coastal oak woods and how this contrasted with the darker greens of the old leaves. Its a pity that there is no way to post related photographs within the Blogger comments system.
    I also hadn't realised that this was a distinct second phase of growth so I continue to live and learn!

  2. Hi Nyctalus,Some trees - like horse chestnut, for example - don't make any Lammas growth at all. One year's entire growth on a horse chestnut twig is basically what was already formed in the terminal shoot bud in spring, when it burst...


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