Hi, I'm wondering if anyone can help with my gardenias. They have what looks like a black mould or soot all over the leaves. Am I correct in assuming it is scale or is it a fungus. We have had very hot summers and the plants probably haven't had enough water and fertilisation. I have recently learnt that they do need a bit of feeding. Thanks
Gardenias require acidic, fast-draining soil that is always moist but never really wet. When I planted my gardenia, I mixed a generous amount of peat moss into the soil. Once a year, I sprinkle some soil sulfur around it. About twice a year, I add enough gypsum to hide the soil, which is heavy adobe clay. The gypsum keeps the clay porous and improves drainage.
They are heavy feeders. I feed my gardenia whenever I feed my dwarf citrus, lightly twice a month from early April through mid October. I alternate, using citrus food (yes on the gardenia) and ammonium sulfate. Each time I feed, I also add a large pinch of zinc sulfate.
The black stuff is most probably sooty mold is growing on the excrement of aphids.
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