Magnesium deficiency is caused by either a lack of magnesium in the soil, or in some cases by excessive use of fertilizers which inhibit the availability of magnesium to the tree.
It can affect most types of fruit tree, but is most common in apple and pear trees. The leaves develop light brown blotches, sometimes with a slight yellow tinge.
Magnesium deficiency can look quite similar to general (and more common) wind / scorch / frost damage - but it tends to be a lighter shade of brown than these other brown-leaf problems.
The photo shows a typical case of magnesium deficiency on an Ashmead's Kernel apple tree.
The problem is easily resolved by applying 2-3 applications of Epsom Salts at 14 day intervals. This should be diluted in water and sprayed on the leaves in the evening.