I found this strangely flattened stem when I was pruning the Forsythia bush in our garden (see below for an example of a normal Forsythia shoot for comparison). It's suffering from fasciation - a sideways proliferation of the growing tip of the plant that results in a broad, flat stem. This was most probably caused by infection with the leafy gall bacterium called Rhodococcus fascians (formerly known as Corynebacterium fascians) which upsets the balance of hormones that control the plant's growth.
Fasciation occurs in a wide range of plants and you can see some further examples by clicking here.
Posts
