Answer: Myoporum is an Australian native, generally not prone to diseases so I wonder if what you are seeing is a residue from your water? Minerals can build up on plant stems (and other things in the garden). It is usually white and hard and can indicate the flood or saturation level on plant stems and tree trunks. Look for an even ring around stems and trunks. Myoporum is extremely drought tolerant so apply very little water to your plants. If they begin to appear wilted or droopy, you can water them deeply, then withhold water for weeks or months at a time.
Now is not the time to fertilize; wait until early spring for that. Don't spray a fungicide or insecticide, either, until you've positively identified the problem. You may want to take a close-up photo or cut a sample of the problem from the plant and take it in to your local nursery for diagnosis.
Best wishes with your plants!
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