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Cirsium arvense, aka creeping thistle, and black fly

Mother Tree
Posts: 11893
Location: Portugal
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My friend Adeline, from the UK, just sent me these photos which I thought were interesting and should be shared.

She said that "like many grassland farms, we have a creeping thistle problem. Usually we top once or twice a year to stop them seeding, but it doesn't do a lot toward stopping them coming back. This year I am seriously thinking we should leave them alone, as per photos below."

"A very sorry looking stand of thistles."

"Seems whatever is killing them is carried by black fly, which are very abundant this year."

"The black fly seem to come and go and leave them looking like this."

"I am wondering what is going on to the root system, because this one has not had black fly. Healthy one next door."

"End result. I don't want to top and get in the way of this process. Would like to bottle it for another year."

I asked her if she though it was a disease *carried* by the blackfly rather than the blackfly themselves?

"I think so, at first I thought just blackfly, but it seems to damage where there are no blackfly and also eventually neighbouring plants. It also seems to affect spear thistles, but not so much, and they don't usually get blackfly like this. It looks like maybe a septoria or even a mixed infection with rust."

Has anyone seen anything like this before? Any experiences or knowledge to share?

Posts: 99
Location: zone 6a, north america
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we got the similar symptoms on many of our thistles here. was also thinking septoria -- it's spreading across our land like crazy. mints/basils/comfrey/mugwort/hibiscus/salvias/garlic mustard all have similar symptoms. was initially thinking it was due to the moisture in the air/soil (we had a very wet June/early July). whatever it is has had no effect on the strong thistles which are good 6 ft + at this point.

does any of her other plants have this or just thistles?

i've been hacking them and throwing in the pile to char to try and minimize spread of the whatever it is (fungus?).
the black fly hypothesis is interesting. maybe they are carrying the whatever it is from plant to plant on their "feet".
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