Thanks for the response blckrose! :)

It is a Backhousia citriodora. I've got it in the pot for the winter to give it some shelter, and because the spot I've allocated for it doesn't get much sun at all at this time of year. It's interesting that you mentioned the mulch. It's eucalyptus mulch ("eucy mulch" on the bag). I just Googled it and the first thing that came up is an article about its toxicity to plants... Could that be the culprit? Maybe I should use something else or ditch mulching altogether? The foliage is quite sparse at the moment, so circulation should be pretty good. I really hope it's not a fungus. I've gotsome mancozeb in my cupboards...

Yeah, I'd try mancozeb; I've not previously used it, I use triforine on my roses, though only when 'blackspot' becomes fairly obvious - perhaps a week mix of manco. And I think I would get rid of that mulch, and at this time of year, winter, with no growth taking place, I'd let it dry out a bit to just moist about a finger tip below the soil. Also, it's been a bit of a wet winter along the coast, so humidity might be a contributor. I use to live and garden at Allambie Heights, in Sydney, so I'm familiar with the climate there.

Does your mulch bag say on what you should & should not be putting the eucy mulch, Sophie? Lots of Eucalyptus litter does contain toxins. In gardens you'll often see relatively weed free areas under many Eucalypts because the toxins in the litter suppress weed growth. Remove the Eucalypts & the weeds will grow. In natural bushland Eucalyptus seedlings will often germinate after the parent tree dies & the toxins in the litter have been diluted or leached out by natural rainfall. Backhousia citriodora grows in coastal rainforests from MacKay to Brisbane where I suspect there aren't a lot of Eucalypts. So your idea of trying a different mulch or (even not mulching) has merit, I think.

Have your plant checked out to see if it's Myrtle Rust, that new disease that's come over from South America (sweeping the world actually). It's got into the east coast and right up to Cape York. It attacks plants in the Myrtaceae family, Eucalyptus, Melaleuca, Corymbia, etc., including Backhousia and is expected to have a devastating effect on Australia. Some of the forests in northern NSW have lost most of their canopies from it. The symptoms are the same as what you describe. It can be treated locally by fungicides but country-wide it's virtually impossible.

That's interesting, tropicbreezent. I haven't heard of Myrtle Rust. Our natural environment is in for another huge attack it seems.

Woo! Thanks for the help everybody!

@blckrose & Woko: - Taking the mulch out today. The pack is totally unhelpful, but it's from a landscaping company and I think it is for exactly what Woko says; stopping weeds. Massive noob mistake, because I've also got it around a mintbush, a holy basil bush and a lemon verbena bush and they've all got leaf-curling, yellowing and leaf-drop going on. I had assumed it was a winter thing. I'll give the mancozeb a go as well, just to be sure.

@tropicbreezent: I did come across that when I was Googling diseases, but the photos show some pretty prominent yellow pustules, and I haven't seen any of that. There are a few acne-like spots with halos visible in my photos, but the majority of it seems quite different. I'm really hoping that it's something else. Where can I ask?

Not so much a mistake as a learning experience, Sophie. Gardening is very much an experiment. Observe & learn.

I'm not sure how far south in NSW it's gone but if you contact your Agricultural Department they'll be interested to check it out for you. At this stage I'm not all that familiar with it but we've been given the heads-up on it. They're saying by the end of this coming wet season/summer we're going to be finding out what it's all about.

Sophie, try the suggestions above and if you're still concerned - and really, I don't think it's a big problem, after all if it does die you can always go buy another, they're readily available - take it, or a leaf, to the Royal Botanical Gardens in Sydney; they have a herbarium-cum-lab there and they'll readily tell you exactly what, and if there really is, a problem and how to overcome it.

K. I'll give those outlets a call, and post what they say in case somebody else comes here looking for answers. Thanks so much everybody for the help! :)

In case anyone is interested: I asked about this on Don Burke's website before posting here, and he has put forth the suggestion that it might be salt spray from being near to the beach (which I am, and can't do anything about). He also implies that the pot is probably too big, since the moisture doesn't seem to be going anywhere.

So... this cleared up pretty well. A lot of the leaves that had the spots have fallen off by now, but some of them are still there—without the spots! Maybe there's some very light scarring, but otherwise they're perfectly green again.

I still don't know what it was - I never got a diagnosis (the herbarium wanted money, you see). I'm also sorry to say that I never sprayed with fungicide (no excuses there, just never got around to it), never repotted and it's still in the same spot. I did, however, remove the mulch and ease up on the watering immediately, as was suggested. I also think I may have applied a bit of slow-release fertiliser, and just last week I did get around to spraying it with a liquid copper solution but it had already cleared up by then.

Perhaps something of note is that a couple of my parsley plants are now showing similar symptoms.

I've grown an extensive rainforest of over 400 local area species, including Lemon Myrtle and other Backhousia, under eucalypts - ironbark, gums, etc. and used several large truckloads of fresh tree lopper mulch, all with maximum benefit! Only downside is that eucalypts will take all the moisture and fertiliser they can steal from your intended use for your plants. And it can inhibit growth in some planted species such as fruit trees. Again, a greedy need for nourishment they don't really need. I never had myrtle rust, but did have the brownish colouring on older leaves under shade. No leaf drop, no treatment either.

Please work with your natural flora and don't chop it down! If you are concerned about fire risk, that's a different matter, but growing dry rainforest (scrub) species around the house will give some protection because of the high moisture content and less oil in the foliage.

Marjorie, SE Qld.

Oops - I meant to emphasise that most of my mulch has been eucalypt, preferably ironbark as it lasts longest of course. Just don't use it too close to timber framed buildings because as with any mulch except cypress and tea tree, termites love it.