I have several platyceriums which I keep in a greenhouse in the winter and under a tree in the summer. One is an upright growing plant with blunt tips. I have had to spray it several times this past year with Safer soap to control mealy bugs. I try to remember to wash the leaves off after an hour or so and also try to remember never to fertilize when the medium is dry, but this plant continually has tip burn - the tips turning dry and brown and curling up. Now the common bifurcatums are also showing brown tips and my huge andinum has brown-black spots on the shield. We had a cold, wet spell last month and I suspect fungus. I have dusted with maneb and sprayed with daconil and sulphur but the spot keeps growing on the andinum and tips keep turning on the bifurcatums and the unknown plant.

Does anyone have an idea what I am doing wrong and what I can do to stop the fungus - if it is fungus? And stop the tips from shrivelling?

Thank you for your assistance.