Lammas growth and powdery mildew

Several tree species - most notably oaks - put on a new flush of growth in summer, sending out shoots with new  foliage to supplement the older leaves of spring that have suffered from insect attack and general wear-and-tear. The new shoots, which are often tinged with red pigments, are known as Lammas growth because they’re well developed by the time of the ancient Celtic harvest festival of Lammas day - 1st. August.

In oak trees the beauty of this 'second spring' does not last very long. In a few weeks the new shoots will probably be distorted and coated with a greyish-white powder. This is the parasitic oak powdery mildew Erysiphe alphitoides that thrives in the warm, wet weather that is a typical English summer. The origins of the disease are a mystery but it probably arrived from overseas with consignments of plants. It first appeared in mainland Europe in 1907 and in England in 1908.

Under the microscope at x400 you can see a mass of transparent fungal hyphae covering the leaf surface 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 ( x100 above, x400 below), that blow away in the wind and infect another leaf.