If the tree is dying, there is likely nothing one can do to stop that. Fruit trees typically succumb to a number of factors such as wet soil, cold winters, mice damage* or other damage and heatwaves with infectious diseases often playing a role as well (in plum that is black knot https://www.youtube.com/watch?v=ATDG8...)
The following text is helpful (from http://web.extension.illinois.edu/coo...)
But it doesn't suggest solutions to stop the decline.
"Examining the Tree
Foliage. Examine the leaves for signs of disease or insect damage for size, color, and vigor. Fungus diseases and insects can be controlled by adequate pest control treatments. If the leaves are small, yellowish and weak, this could be a symptom of nitrogen deficiency, but more likely is a symptom of problems with the trunk or roots and these areas should be examined. If one branch shows symptoms of weakness but the remainder of the tree looks healthy, examine the bark on that branch.
Bark. The outer layer of the bark is composed of dead cells whose function is to protect the inner-bark from mechanical injury and dehydration. The phloem is adjacent to the outer-bark, consists of live tissue, and is the primary conduit for the movement of sugars, amino acids, and other substances manufactured by the various parts of the tree. The cambium lies between the phloem and the xylem (the woody part of the branch or trunk). Its function is to produce new live cells for both the phloem and xylem. The main parts of the bark, thus, are the dead outer-bark (epidermis) and the live inner-bark composed of the xylem and phloem. The live inner-bark is the critical part to examine. Unhealthy inner-bark frequently gives a different appearance to the outer-bark – sunken, different color, different textures. These different appearing areas are a good place to start examining the inner-bark.
Using a sharp knife, remove a small portion of the outer-bark to look at the inner-bark. Healthy inner-bark is pale yellow to cream-colored to greenish-yellow. Dead inner-bark will be dark brown, and unhealthy inner-bark will be losing its natural color. If the inner-bark is healthy in that spot, try a few more spots, but always make the cuts small and shallow so the small wound will heal rapidly. If the inner-bark is dead, keep checking all around that spot to see where the dead inner-bark ends and the live inner-bark starts.
Branches. Examine the appearance of the bark on both the large and small branches. Look for discolored areas, disease lesions, bark splits, borer activity and injury caused by storms, hail, machinery, sun scald, and cold. Examine the inner-bark in several places to see if it is healthy.
Trunk. Look for discolored or sunken areas, disease lesions, bark splits, borer activity and injury caused by rabbits, mice, storms, sun scald, cold and fluctuating winter temperatures. The south and southwest sides of the trunk frequently show the most cold injury. Examine the inner-bark. Long, narrow, vertical, dead areas of the inner-bark are less likely to cause death than horizontal areas that girdle half or more of the trunk.
Crown and Roots. The crown is the trunk area near, at and below ground level and is especially vulnerable to injury from wet soils, standing water, mice, machinery, winter cold diseases, and borers. Remove soil from around the trunk to enable examining both the outer-bark and inner-bark. If the inner-bark of the crown is healthy, you may want to look at the inner-bark of the major roots. "
*Mice are serious pests of apple trees, and frequently damage other types of fruit trees as well. All ages of trees may be attacked. Meadow and prairie mice eat the bark (including the inner-bark) from the trunk and roots, both above and below the ground level.