My lemon myrtle sapling (backhousia citriodora) has developed dark brown/black spots on yellowing leaves (please refer photos). I bought the plant late last year and have had it potted outside in full sun in Adelaide. It is planted in a good potting mix (cannot remember which one, around $8 a bag).
When first planted I watered in twice with seasol a few weeks apart and through the winter I did not water at all - my moisture meter never dropped below half in this time.
A number of times I have plucked off quite a few of the leaves which were suffering to try and stop the spread(?), but that doesn't seem to have worked.
I have heard about myrtle rust, but I don't believe this is what it is (perhaps someone with more experience can correct me?), also I don't think myrtle rust has made it to South Australia. To me, it doesn't look like it matches the description.
Does anyone have any advice on how I can get this plant healthy again?
Thanks,
Harry.