Most gardeners will remove and dispose of diseased garden plants. That is the safest way to deal with them. Tree diseases are not usually a problem for the garden plants, so the leaves from the trees should not be a problem. Even if you remove diseased plants from the garden, be sure to rotate your crops the next year. This is good advice even if you didn't have diseased plants.
Crop rotation helps prevent the build-up of disease organisms in the soil.
Crop rotation means that you move the crop you grew in one spot to another place the next year. The reason that the material turned into the garden doesn't decompose over the winter is probably due to the fact that the soil was too dry that winter, or the plant debris was too woody. In the spring when you rototil the garden, rake out any uncomposted material and finish composting in a compost pile, or use it as mulch between the rows in the garden. Adding manure as you turn the debris under in the fall and making sure the soil is moistened in dry winters will maximize the in-situ composting. A friend gave me a leaf from one of her beautiful African violets and told me how to grow new plants from it.