I’ve noticed in previous years that the top shoots of many Creeping Thistle plants (Cirsium arvense) sometimes turn white at this time of year and I’ve only recently found out that this is caused by Pseudomonas syringae, a bacteria that produces a chemical called tagetitoxin, which poison chloroplasts and causes chlorosis.

There’s an excellent article on the Llanelli Naturalists website, entitled ‘The Mystery of the “White” Thistles’, which includes the following explanatory paragraph (nb this article was uploaded in January 2009 so the situation has probably changed since it was written).

This bacterial disease was recorded in the UK for the first time by a retired plant pathologist, Dr John Fletcher, in the vicinity of his home in Canterbury, Kent, only six years ago. The bacterium causing the disease is now thought to be a pathological variety of Pseudomonas syringiae – a complex of bacterial types that affects over 180 species of fruit, vegetable, forage and horticultural plants. The infection on creeping thistle was found for the first time in Canada in 2003. Scientists from Alberta (Zhang et al, 2004) recorded the disease at several locations across Canada and have named it the “White‐colour disease of Canadian Thistle”. Their diseased plants showed apical chlorosis and these symptoms were associated with stunted growth, fewer shoots, inhibition of flowering and/or sterility. These are exactly the same symptoms found on the local populations of Creeping Thistle at Burry Port.