Probably too dry. Are they newly planted?

We don't get much rust here in NJ, unless it come in imported on a plant via mail order. Rust looks like the orange dust from Cheetoes/Jax corn curls. We are more likely to have blackspot or powdery mildew. Your plants have blackspot. Age of plant doesn't matter, some varieties are less resistant to disease. By "Bayer rose fertilizer" do you mean one of the Bayer Advanced combination products (fertilizer/insecticide/fungicide)? These are not effective for blackspot control. The fertilizer isn't enough, the insecticide kills worms, and the fungicide can't be effective in time to prevent an outbreak of blackspot. For effective blackspot control, try Bayer Advanced Disease Control for Roses, Flowers, and Shrubs. It is a fungicide only. This can be started now, applied once a week until the disease is under control, then every 10 - 14 days. You can use a small spray bottle, and measure the product accordingly. Next year, start your fungicide treatment about a week after pruning in late March/early April. Make sure that the plants have been watered well before applying the fungicide. For fertilizing, try Espoma Rose-tone or Espoma Holly-tone. These are organic, and they also provide amendments to the soil. I also apply a layer of composted manure in the fall. Also, check the amount of water that the plant is receiving. Roses like to be watered deeply, and less frequently. The top of the soil may look moist enough, but the roots may not be getting enough moisture (even with the rain we've had this spring).

Probably stress from the heat and drought. If the cambium layer - the layer just under the bark - is still green, - check by lightly scratching the bark in several places, and it stays green over the next few months - then the bush should come back OK in the spring. You may not get much in the way of flowers, though. If you can spare the water, watering it widely and deeply, or if it rains well, mulching as widely as you can - no deeper than 4", and putting as little mulch as you can directly around the stems, to limit rotting - will help keep whatever moisture is in the ground, in the ground and available to the roots. If you can't do that, then you can't. Forsythia are pretty tough plants - it should live next year, even if you don't get the spring flower display you would like. Of course, if next summer repeats this one......

I very strongly doubt that your tree has a phosphorous deficiency. I have NEVER heard of a phosphorous deficiency in container citrus, when it is regularly fertilized with a NPK fertilizer. Phosphorous does not leach from a container, as does nitrogen and potassium. Nitrogen and potassium are both VERY water soluble and readily leach with every watering, but not phosphorus. All of the leaves that are turning yellow, and dropping are the older leaves on the tree. Your tree is withdrawing the nitrogen from the leaves (thus they are yellowing) to reuse in other part of the tree before discarding them. When a leaf no longer is capable of supplying its expected quota of energy to the tree, it is quickly discarded. There is no welfare program for citrus leaves. However, your tree is also under fed. If the tree's problem was other than aged leaves, you would be seeing yellow dropped leaves from the new leaves also. Also it is not wise to add this and that chemical in hopes of coming across a cure. First ascertain what the problem actually is then correct it. I'm sure it was also not helpful to the tree to let it remain in the same container and "soil" for 5 years. Fertilize the tree using a high nitrogen fertilizer. You can add vinegar if you wish, but there are much better ways to acidify. Organic acids such as vinegar, have an extremely short life.