Puccinia punctiformis
How does it damage californian thistle?
You will commonly see this rust fungus on Californian thistle, as it is widespread throughout the country. Affected plants are usually covered in brightly coloured spores (yellow, orange or brown), and become chlorotic and stunted looking. Infected stems do not usually flower and root buds may also be affected.
How effective is it?
The rust only attacks Californian thistle and appears to be limited in its ability to infect plants – usually you will only find some diseased plants in a patch of thistles, but occasionally more widespread outbreaks occur.
A weevil (Ceratapion onopordi) is being released here that may be able to vector the rust and spread it around more widely. See Californian thistle stem miner.
