Certified GKH Gardening Expert
Definitely a fungal infection, and usually caused by overwatering. Especially in container, it is important to let the top two inches of soil dry out completely before you water again. It is also very important to have proper drainage holes in the container.
You may be able to correct the issue with wettable sulfur and dolomitic lime
This will not correct the burning caused by planting directly into compost, though. This is meant as an amendment to garden, or potting soil. Using alone would not be advised. I would recommend mixing 1/3 of that mix into a potting mix, and replant into that.