The Mystery of the "White" Thistles PDF Print E-mail

Figure 1. Creeping Thistles where the tops of a large proportion of plants have turned white. Photograph R.N. Stringer

In August 2008 RNS and RHD were carrying out a rust survey of the vegetation in the vicinity of the Ashpits Ponds, Burry Port (SN465013). During the investigation we came across numerous stands of Creeping Thistle (Cirsium arvense). A majority of the plants within these stands were perfectly normal and flowered profusely. However, the tops other plants were completely devoid of chlorophyll and appeared bleached, thus making them very conspicuous (figure 1).

At first the cause of this apical discolouration was thought to be due to a rust disease as many of the affected plants were infected with Puccinia punctiformis. This rust is commonly found on creeping thistle and its life history is quite interesting. In the spring the rust permeates the host and affected plants can be recognised immediately by their pale‐green colour and spindly appearance. We thought that the discoloration of the plants at the Ashpits ponds was a consequence of a rather severe rust infection. We had found evidence of rust infection on the plants that we had examined and thought that this was the end of the story.

By chance, the following day, RNS received a package from Dr Tom Preece (Shropshire) which contained a specimen of creeping thistle collected by Dr Richard Shattock (ex University College of Wales Bangor) from Anglesey. This specimen was identical in appearance to the plants we had examined at Burry Port. Dr Shattock also informed Dr Preece that the disease symptoms were caused by a bacterium and not a rust as we had originally thought!

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 (Figure 2).

Biochemical, nutritional and genetic analysis of the bacterium indicated that it was an isolate not previously found on Creeping Thistle and has provisionally been allocated a code (CT99B016C) rather than a name at this stage. In Canada this strain can infect the sow thistles Sonchus oleraceus and S. asper as well as Dandelion (Taraxacum officinale).

Dr John Fletcher has been collating the records of sighting of this disease throughout the UK. To date the disease has been found in the Isle of Wight (David Biggs), southeast England (John Fletcher), Anglesey (Richard Shattock), Pembroke‐shire (Sam Bosanquet, Michael Carpaty) and Glamorgan (Sam Bosanquet). In Carmarthenshire detailed studies by RNS and RHD have shown that the disease does extend from Bynea to Kidwelly. The infection is sporadic and we have only found the diseased plants associated with “waste ground”, amenity areas in the Millennium Coastal Park), the verges of main roads, etc. Interestingly there are no records to date from agricultural land.

Dr Tom Preece has not recorded any sightings from Shropshire or Montgomeryshire – both counties associated with intensive agriculture. The disease is absent from inland Monmouthshire (Sam Bosanquet) but Sam has recorded white thistles from Chichester (West Sussex). Indeed, all the records to date are noticeably closely associated with coastal areas.

Nobody has discovered as yet how the disease got to Britain or the mechanism whereby the disease is spread. Dr John Fletcher has artificially spread the disease by initially topping infected plants with hand shears and then immediately moving on to prune uninfected plants. These uninfected plants soon succumb to the disease. It is thought that this mechanical method could be the mechanism by which the disease is spread along motorway verges.

It is quite unusual to be in a position where we record the presence of a new pathogen disease so early when it reaches our shores. We are therefore in an ideal position whereby we can monitor the spread of the disease over time. The symptoms of white‐colour disease of thistle are very characteristic and affected plants can be identified easily by eye from quite a distance away.

This new disease is not only of scientific interest but also has a commercial value. Creeping thistle is a weed of badly managed pastures and its eradication can be time consuming and costly. It will therefore comes as no surprise to learn that this strain of Pseudomonas has already been patented with a view to promoting and marketing the organism as a “green” biological control agent against Cirsium arvense.

We would welcome any records of white‐thistle disease from Llanelli Naturalists members as this will help with epidemiological studies relating to the impact of this organism in Britain.

The authors wish to thank all those mentioned in the article for their contributions to this study.

 

Reference

Zhang, W., Sulz, M., Mykitiek, T., Li, X., Yanke, L., Kong, H.N., Buyer, J.S., Lydon, J. (2004). A Canadian strain of Pseudomonas syringae causes white‐colour disease of Cirsium arvense (Canada thistle). In: Cullen, J.M., Briese, D.T., Kriticos, D.J., Lonsdale, W.M., Morin, L. and J.K. Scott, editors. Proceedings of the XI International Symposium on Biological Control of Weeds, April 27 – May, Canberra, Australia. pp 215‐220.