Q.Osteospurmum Jucundum Failing to Thrive
I had about 18 planted and slowly they all wilted, rotted and died. I ordered more and the same thing happened. A few of them have lasted three years but now the same is happening. They are not overwatered and I deadhead down to the base of the stem as needed. They only seem to really thrive in pots. In some areas I have dug up the soil and replanted in new soil but same is happening. Any suggestions?

Certified GKH Gardening Expert
There are many infections that are specific to certain plants. This looks like the case here.
When you plant in this spot, then try to move, it will take the infection with it on the roots. This will mean that the new area will be infected as well.
I would plant elsewhere in the meantime, and begin correcting the issue.
Start by conditioning the soil. Use DOLOMITIC LIME and WETTABLE SULFUR cast into the soil and scattered around. Using these according to the instructions on package will usually solve the issue within a few weeks.
If this doesn't respond to the treatment, and after a few weeks, still dies out, then you will want to turn to a stronger fungicide. This article will give you more information on the subject: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
It is important to note that they prefer drier soils. This will drastically reduce the instances of infection. For more information on the care of these plants, please read this article: https://www.gardeningknowhow.com/ornamental/flowers/african-daisy/african-daisies-osteospermum.htm

Thank you. So do I dig up the plants and move them to pots, treat the soil as you recommended and then put them back? Some of the plants look like they are rooted in deeply?