Any Idea What This Is?

[Mojave]

This has been showing up here and there around my pepper garden. The yellowing will continue until 1/2 of the leaf is yellow and then the leaf falls off. Any idea what it could be, and why only 1/2 the leaf turns yellow?

[RayR]

Are these the oldest leaves or does it happen in middle or upper growth?

[Mojave]

Originally Posted by RayR

Are these the oldest leaves or does it happen in middle or upper growth?

95% or more is on the bottom leaves. Very occasionally it will appear about mid plant.

[RayR]

The only times I see yellowing leaves falling off a pepper plant is when there is a nutrient deficiency (generally Nitrogen, PH can also be a big factor if it's too low or too high) or when the roots are too wet.
If it's a N deficiency, the whole plant may also have a light green color instead of a deeper shade of green.
More info needed I think and clear pictures of plants that are showing the symptoms.
In-ground or in containers (potting soil used)?
Fertilizers used, organic, synthetic?
Moisture level in soil?
Variety(s) of peppers showing these symptoms?