Then the branch dies and eventually the whole plant dies. Several diseases and boring insects also can cause the symptoms you describe. Check the stems for sunken or discolored areas called cankers or discoloration inside the cut stems. Prune out infected canes several inches below the canker or where the stem joins a healthy branch. Disinfect tools between cuts to avoid spreading the disease.
Borers feed inside the cane causing it to swell.
Prune out the infested canes beneath the damaged area. Sanitation and proper care is usually enough to keep these problems in check. My mother grew two fruit trees from seed for me. The apple tree is 20 years old, and I finally transplanted it from an indoor pot to the garden. The other is a grapefruit tree that is 18 years old and is in a pot that I put out each year and take in for the winter.