I'm new to passiflora but something I noticed with my plants is that they don't like a lot of organic matter in their soil. I kept having plants do this too. I changed to mostly about 2/3 sandy loam and 1/3 potting soil and they are doing very good. They seem to be very susceptible to root rot. In most places the organic matter is a thin layer on top of the soil. I figure when adding things like fertilizer or mulch it should be left towards the top to keep it off the roots. The stuff will break down and go into the soil slowly without a lot of organic matter being able to rot at the roots. Old leaves don't die off like the one in the picture nor do the caterpillar bites that I get do this. I would bet it is a small portion of rot at the roots. Mine started like the picture and then progressed to the whole plants killing some. When I took them out and put them in less organic matter they cleared up and grew vigorously.