🦠 Plum Fungal Issues

So first a disclaimer, I have a few plums from a volunteer in the yard.  When it was in the ground it was great but I’m having trouble with it in the pot.
I think you’re right with the fungus.  

• The browning is on the tips and the lower margin.  That always makes me think water or roots.  

• Although you can dilute it, no one I know uses lime sulfur apart from the dormant period.  Is your 3336 a granule or the liquid.  If spray can do both the top and the soil.  The granule is good but usually takes a bit longer.
  Most fungal issues can be solved with less water, and maybe more sun.