21st August 2025

I like a mystery. In this case a number of Creeping Thistle (Cirsium arvense) plants that have rather bleached looking tops. No other plant species around them seem affected. I've seen it in previous years but never got around to finding out why. It turns out that the colour loss is caused by a strain of a bacterium called Pseudomonas syringae. It was first identified in the UK in Kent in 2008. The bacterium attacks the chloroplasts (which produce the green pigment chlorophyll) causing the bleaching effect. The bacterium is being investigated as a possible organic control method for creeping thistle, which can be a persistent weed on agricultural land.