Not sure if you have an evergreen or deciduous plant. If deciduous, shedding of leaves starts with resorption, during which the plant scavenges mobile nutrients and other valuable biocompounds and stores them for future use. One of the mobile nutrients the plant removes from its leaves for recycling is magnesium. Magnesium is central to the chlorophyll molecule, a green pigment that plays a major roll in the process of photosynthesis. When the magnesium is removed from chlorophyll, other pigments become expressive, which accounts for the yellow blotches.
If you've had excessive amounts of rain, that might be playing a part, and if you haven't fertilized this year, that might also be at issue.
Al
