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Please be more specific. What kind of plant? What shape is black, and how much of the leaf is affected? How much of the plant has the black coloration? Do you see pests of any kind?

This article will help you with that:
https://extension.psu.edu/plants/gardening/fphg/stone/diseases/cytospora-canker-of-stone-fruits

You can do this, and many people do (the filling in diapers is commonly used for this, though official water retaining gels are used as well). There is some concern from some people about how these chemically created gels can affect plants that are used as edibles. You may want to take this into account when growing edibles with these gels, but in terms of does it work, yes it does and helps keep water in the soil for the plant without danger of root rot.

As long as the fruit is not coming in contact with any dog droppings, it should be fine. I would have the soil tested for pH, just in case. If it is a smaller yard, the urine from the dogs may have affected the pH.

These articles may help: https://www.gardeningknowhow.com/edible/fruits/fegen/dwarf-fruit-trees-a-planting-guide-for-fruit-trees-in-containers.htm, https://www.gardeningknowhow.com/garden-how-to/projects/espalier-instructions.htm