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.