Dear Readers, whilst admiring the crack willow for Wednesday’s post, I noticed this rather strange growth on one of the twigs. So splendid was it that I had to get my husband to hold the branch down so that I could get a photograph. It looks rather as if the catkins, normally so long and elegant, have gone completely berserk, and so I contacted my friends over at the British Plant Galls Association to see what they thought.
Apparently, this is a Babylon willow witches broom. Who ever guessed that there was such a thing? And the mystery doesn’t stop there. I had alway thought of galls as being caused by tiny insects such as the larvae of moths or wasps, or by fungi, but this is caused by something very different.
There are many different types of ‘witches broom’. What they have in common is that the growth of the plant is distorted, and is often multiplied or mutated – in the echinacea in the photo below, the leaves have been ‘persuaded’ to behave like flowers.
The cause of the distortion is a tiny bacterium known as a phytoplasma. These mysterious creatures were only discovered in 1967 – they are tiny even by the standards of bacteria, have no cell wall, and have proved to be more or less impossible to culture. They are spread from plant to plant by sap-sucking insects such as leafhoppers, and once inside a plant they live in the phloem, which transports both the sugars and nutrients that the plant needs, and the phytoplasmids which it definitely doesn’t.
Our witches broom is probably not good for the crack willow, but there is only one gall that I can see. Other variants can be much more harmful – there is one kind of witches broom that can devastate cocoa plantations, and some others that can affect trees grown for timber. Furthermore, all kinds of organisms can cause this kind of growth – viruses and fungi can affect normal shoot and leaf development, as can various insects. All you need is something that affects auxins, which are chemicals at the shoot or bud tip – normally, these tell the plant to stop growing, but if they are somehow turned off, the plant continues to make catkins, or leaves, until you get a matted, nest-like mass. In fact, some animals actually take advantage of this nest-like appearance by using it as a place to sleep, like the Northern Flying Squirrel (North America’s only gliding rodent).
I am especially impressed by the effect of phytoplasma on the bamboo below, though I wouldn’t be quite so happy if I was a giant panda, as although the bacterium can induce a huge mass of distorted flowers, these are usually sterile.
So, here I am with a whole new selection of things to think about. As is usual, I imagine that I’m about to see galls everywhere. If you have a favourite plant gall (hasn’t everyone 🙂 ) do share. The natural world is full of all sorts of strange and wonderful things, for sure.
Photo One By Estreya – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4608429
Photo Two byBy PJTurgeon – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=23282295
Photo Three By Amityadav8 – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=62620401