Answer: Plumbago is generally disease free so I suspect that the soil pH isn't quite right. Alkaline soils can cause yellowing, then browning of leaves because nutrients are tied up in the soil and not available to the roots. Try feeding your plants with an acidified fertilizer to help lower the soil pH.
Plumbagos like loose, rich, neutral soils and a deep soaking once or twice a week.
Hope this information helps!
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