Looks like Diodia.

Thank you tyrlych! After googling "diodia", I can now say with certainty that I have Diodia virginiana or "Virginia buttonweed". I'm not the only one having trouble getting rid of it, either. It spreads from seeds or rhizomes, so pre-emergent doesn't work. The only things that offer decent control in St. Augustine cause discoloration / injury. One guy said the only way to get rid of it is to use a sod cutter and replace with new sod.

Sigh.

I have major himalayan black berries covering our area,after "fighting " it for the last 20 years +, I have given up.
I would cut it back to ground level as far as our property line but the absetee land lords do not care about the problem so I am constantly having the black berry plant reinfest our pasture.
The plants in the neibours fall over the fence and take root again also the plant pops up in other areas by growning under ground and spreading, 15 feet into our property every year, a continual fight to cut them back.
I researched this problem and found in Australia there is 8.8 milliom Ha. of black berry weed, their Government has a programme in place to irradicate the black berry weed, they use leaf rust, it does not effect anything but the black berry.
I tried both levels of Government to obtain information on leaf rust, neither had any interest in helping other then suggesting cutting them down and using herbicide.
This is only a short term solution as the neibours wont make any effort and it is back to square one again.
Has anyone got a " recipe" for leaf rust ?????

barry berry, I will probably be kicked off for this. But what are you thinking of. If you infect your blackberries you could infect any member of the blackberry family. Not just in your small field but all accross the country. Rather than killing everything try to find a different solution.

Since you mention you have a pasture get a herd of goats.