20 year old lemon tree

Question

Our 20 year old lemon tree appears to be dying. The leaves are dropping off and are yellowish, and some of the branches are dead. The fruit still seems ok. We have never had any problems with this tree before and we have not done anything different. Our ornamental grapevine is also looking not good, leaves yellowish and no healthy growth, which is unusual for this time of year. There is a little splitting of the lemon tree’s bark near ground level, and one branch in particular. Its bark is cracking. Noticed some mealy bugs for the first time ever on lemon tree. Our concern is that our neighbour has several trees that appear dead or dying!!

Answer 1

Since both your lemon tree and grapevine may be affected, this could be a soil nutrient deficiency. Nitrogen, iron, potassium, or magnesium deficiency can cause yellow leaves and poor growth. You may want to have a soil test done.
http://www.aces.uiuc.edu/vista/html_pubs/hydro/symptoms.html
https://www.gardeningknowhow.com/plant-problems/environmental/leaf-chlorosis-and-iron.htm

Or, there could be a disease present. This article is about oranges, but most of the advice applies to lemons as well:
https://www.gardeningknowhow.com/edible/fruits/oranges/yellowing-orange-tree-leaves.htm