By Molly Shaw
Californian thistle (also called Creeping thistle, Cirsium arvense) is a tough weed to control once it gets a foothold. Strong underground roots spread the plant aggressively, making it a problem weed in many regions from the UK to North America and here in New Zealand.
Walking through an infested field or hedgerow in North America and Europe where Californian thistle originates you sometimes see pale yellow thistle leaves, almost as if a chlorophyll-inhibiting herbicide like Command or Solicam had been sprayed. It turns out that this dramatic tissue bleaching is caused by a fungal infection, white tip disease (Phoma macrostoma). The fungus infects shoots as they emerge from the soil, causing bleaching and plant death.
The fungus has recently been formulated and registered for use in Canada and the U.S. as a broadleaf bioherbicide / mycoherbicide. A recent study in the United Kingdom  also found white tip to be present there, and it is the same type used in the Canadian mycoherbicide. This means that the disease is already present in the UK environment, the registration process can potentially be streamlined.
A biological control for Californian thistle would be very useful for many New Zealand farmers. However, with NZ’s strict phytosanitary requirements, the potential for white tip to be approved for use here would be considerably higher were it to be already present in our environment. So, as you are out and about in your farm, please keep a look out for any bleached thistle shoots that look like they may be white tip (see photo below). If you do see any, please take a photo and note the exact location and email the information to [email protected] Finding the disease in NZ does not guarantee that the Canadian manufactures will release the herbicide here, but it will increase the attractiveness of NZ as a market.
Photo credit: Karen Bailey, Agriculture and Agri-Food Canada
The bleaching caused by white tip disease in thistle is quite distinct on the newest leaves. Often just the plant tips are white, with leaves below the normal healthy green. This is not to be confused with Californian thistle rust (Puccinia punctiformis) which while it can cause the leaves to become pale, the underside is covered by rust coloured pustules.
If you notice this symptom on your farm, please email [email protected] and ideally include a photo and the exact location (e.g., a GPS or Google Earth reference). Charles will then collate the information and communicate it to the researchers in Canada.
 Evans, H.C., Seier, M.K., Derby, J.A., Falk, S., and Bailey, K.L., Tracing the origins of White Tip disease of Cirsium arvense and its causal agent, Phoma macrostoma. Weed Research, 2013. 53(1): p. 42-52. http://dx.doi.org/10.1111/j.1365-3180.2012.00951.x