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.

Long term project ...

There is a saying amongst foresters that 'you should plant as though you are going to live forever'. Sound advice, since most forest trees have natural life spans that far exceed ours - provided that the trees can survive hazards like rabbits, fungal diseases, deer, climate change, gales or developers building more out of town shopping malls.

For any tree planting enthusiast who takes a really long-term perspective and wants to leave a natural legacy, now is the perfect time to plant an oak tree........ 

.......... because acorns germinate almost as soon as they fall from the tree, producing a sturdy white root that spears down through the leaf litter and continues to grow through the milder days of winter. Then, in spring, the first leafy shoot appear. So pot up a few germinating acorns now and a successful start for the seedlings is almost guaranteed. 

It's a 'mast year' for oaks here, with thousands of acorns on the ground. Most will be eaten by pigeons (I once found a dead one with 23 in its crop), deer and mice but the lucky ones might be carried away by a jay from the shade of the parent tree and buried for future use when food is scarce. The jay will probably forget where it cached its winter food supply - and all the while the buried acorns will be sending down those roots, ready for the start of their first growing season. Come to think of it, jays probably give more oak trees a good start in life than we humans do.

Exploding Acorns

Knopper galls (above), uninfected normal acorn below 

These weird objects are knopper galls, caused by a tiny wasp called Andricus quercuscalicis, that I found on oaks growing beside the River Tyne at Wylam this morning. The wasp lays its eggs in flower buds which, instead of developing into acorns, grow into these popcorn-like galls that provide a home for the wasp larvae and pupae through the winter. The young galls are bright green and covered in sticky sap, but as they age they turn brown before dropping off. If you collect a few and keep then in a dry container over winter you can watch the minute wasps emerge through the pore in the gall in spring. The causative gall wasp first arrived in Devon sometime in the early 1960s, having slowly spread across Europe from Turkey, and colonised rapidly once it arrived in England, so it’s now well established throughout most of Britain and well into Scotland. It first arrived here in the North East in the 1980s. The conspicuous damage that it does to acorns led to widespread speculation that it would destroy the acorn crop and would be a major threat to the future of oaks in Britain, but infestation levels vary a lot from year to year and it’s unlikely that this minute wasp poses a long-term threat to our national tree. Whenever you see a knopper gall (usually on pedunculate oak Quercus robur), there’s sure to be a Turkey oak Quercus cerris (identifiable by its hairy acorn cups) somewhere nearby, because the wasp spends half of its life cycle on this introduced tree, which is widely grown in parks, arboreta and large gardens, and the remainder on native pedunculate oak.

Turkey oak acorns