|We have two small (3') potted Bay Trees which are situated next to each other, and have been treated exactly the same. Recently the one has started to die from the inside out, with the leaves turning brown and dying. Whilst the other continues to flourish and grow well.
Do you know what this could be?
There are several possibilities.
These plants are often subject to infestation by scale, and this can cause the symptoms you described. So the first thing I would do is inspect it very carefully in good light. Scale can be treated with insecticidal soap, applied and reapplied according to the lable instructions.
Another possibility is that a fungal infection is at work; if this is the case, knowledgeable nursery personnel should be able to help you identify the problem and suggest how to treat it. Generally speaking, increasing air circulation, cleaning up and removing any fallen leaves, and avoiding wetting the foliage would help as well.
If you do not find any scale or fungus, then I would suspect some sort of root problem, perhaps encircling roots or a clogged drainage hole, or perhaps a pest has taken up residence in the roots and is causing a disturbance. Since the plant is not doing well in any case, you might want to gently unpot it and take a good close look at the roots.
I hope this helps you troubleshoot.