Trouble shooting: Oldest leaves become pale and drop off

When older leaves grow pale and drop off, but new growth continues...

...suspect a light problem or a lack of nutrients.

If the newer foliage is shading the older foliage and the plant's in low light, the plant will usually let the older leaves go in favor of the leaves in more sun.

If the green and pale leaves are equally lit, the plant is favoring young growth over old and some fertilizer may arrest the leaf loss.

In either case the loss of leaf is a living process. The plant is withdrawing the chlorophyll from the leaf, nd other mobile nutrients, and re-using them in the new growth.

This Sedum cauticola cutting is growing. It's slow, mid-winter growth but undeniable. The original leaf has given its all to get the new leaves started, but there simply was not enough nutrient in the cutting's tissues to sustain this so that leaf may be lost. Dilute liquid fertilizer is the remedy, and then watch the plant's remaining leaves to re-apply if those start to pale.