With nothing to go on other than the image, I would guess a high level of dissolved solids (salts) in the soil solution. This can occur over time as a result of watering by way of some sort of wick set-up, or by fertilizing too frequently or with a solution too strong.

It might be telling to have a little background info with regard to your care regimen and to see some images of the leaves, stem, and pot the plant is in.

Al