I have 4 gardenias and 3 different issues, that im trying to identify. Please help, any feedback or how to bring back the health of the plant is greatly appreciated.
I live in central florida and its been pretty cold lately, with some rain.
1) yellow leaves (im assuming low acidity, use fertilizer)
2) black leaves, looks like mold
3) yellow spots (i cant figure this one out at all)
You've got a scale or whitefly infestation, and that has led to sooty mould infection. You don't need to worry about the sooty mould, it will disappear eventually on its own, its only there because of honeydew produced by the insect infestation, but you can wash it off gently with horticultural soap and water if you want. You do need to treat the scale or whitefly though; you can tell which of these pests it is by the simple exercise of moving the leaves - if it's whitefly, they'll fly up for a few moments before settling back on the plant. If nothing moves, it's scale insect. Adult scales don't respond that well to insecticides, it's best to use those while they're still at the nymph stage, but some recommendations for various treatments/management here http://www.clemson.edu/extension/hgic/pests/plant_pests/shrubs/hgic2059.html
The likelihood is that they all have the same problem, even if some have not yet developed sooty mould or obvious 'yellow spots', so inspect the plant/s that don't seem to have sooty mould thoroughly and treat for scale/whitefly if you see any evidence at all.
Some of the leaves are showing iron deficiency - this is often the case during cooler weather when iron uptake is slowed, but treatment with powdered iron chelate in spring (now, maybe, where you are?) should sort that out, further info here https://www.gardeningknowhow.com/edible/helping-a-gardenia-bush-with-yellow-leaves.htm
Here are the things to begin to do and probably on a yearly basis. Get rid of the non decomposed bark, seriously. It is causing iron to NOT be able to be available to your gardenias. As well as low nitrogen content to your soil because the decomposers working on that non decomposed bark use up all the available nitrogen.
The second thing is these plants probably should not have been planted along your concrete foundation. Concrete puts out lime which raises the pH and your acid loving gardenias need a more acidic soil to uptake nitrogen and iron.
Because of this chemical imbalance your plants are susceptible to insects, primarily, looks like, aphids or scale. Can't really tell. The black is called HONEY DEW. Aphids/scale, insects in particular go poo and pee. This excrement is then decomposed by bacteria and that by product is called honey dew.
Lower the pH of your soil using sulfate, SLOWLY. There are answered questions right here in Gardening in Stack Exchange on how to lower the pH into the right range for your gardenias. Plan on testing the pH now and once per year. Use the sulfate according to the test of your soil's pH. But not before you do some tests. Then test after a treatment. Plan to lower the pH in stages, not all at once.
Fertilize with good ole Osmocote 14-14-14 or a similar formulation where the N P and K are equal or the N is a lower number. Any higher and you'll get great vegetation with no flowering. There are also fertilizers that are made for acid loving plants that would help lower the pH. Do not apply any more than the directions tell you or you could kill these plants very quickly.
The lime from the concrete raises the pH and this chemistry causes your acid loving gardenias to be unable to get the chemicals (Nitrogen and iron at the moment) they need to make their own food through photosynthesis.
