I have had this plant for 6 weeks and it has been growing tremendously fast. It sits in a south facing window, receives direct sun in the morning for a few hours and then sits in bright, indirect light all day. I water and mist the plant a couple of times a week.

However, I have noticed that the underside of the older leaves have a rusty, orange tinge and rough feeling texture to them. On one leaf this is progressively worse and is now showing on the upside. The others remain bright green on top but have a lot of this discolouration underneath. The last two new leaves do not have this (yet).

Is it a result of sunburn, transplantation or perhaps disease? Any help would be greatly appreciated as I don't want this beauty to die!!!

I am not an expert on growing Basjoo in the house...mine are outside in the ground, but that looks like a pest infestation to me. The way it is centered a lot around the midrib.

How strange.. I haven't seen any pests on them and I mist/wipe their leaves almost daily. Perhaps it is disease instead?

Thanks for your suggestion!

Misting really isn't necessary or recommended. If you want to raise the humidity use a humidifier or a tray of water underneath. Just don't put the pot in the water!

It's a tough job raising one of these plants in the house. They just love being outside. Could you put it outside during the summers?

Oh really? I will switch to using a pebble tray in that case. Thanks!

Unfortunately I don't have a garden so all of my plants are inside Just hope it doesn't die on me!

Have you tried wiping that orange stuff off? Does it come off?

I have tried and it doesn't budge! A brownish residue comes off onto the towel but the rust colour on the leaves stays the same. The new leaves do not have this problem and growth has not been stunted at all so I'm really unsure how to treat this issue. If it is a infestation, I wouldn't know what bug either.

Well, my first thought was mites. Overall your plant looks droopy, like it is losing energy. And the distribution of the 'residue' is what mites do, they hug the undersides of leaves on the mid-rib and string themselves out along the veins of the leaves so they can suck out the sap. And after they are removed, the damage does not correct itself, a damaged leaf will always look damaged until it dies off and is removed.
@plantmanager does that look like mites to you?

The leaves have always had a slight droop since I purchased it from the garden centre - especially the bottom ones but I assumed this was fairly normal. I will attach a photo of when I first bought it. It has definitely become more droopy since the discolouration of the leaves but is still shooting up a new leaf every week.

If I apply a pesticide tomorrow, will I still need to remove the leaves?

I've had only limited experience with them, but yes, it sure is possible. I'd get a magnifying glass and take a good look at those areas.

If you can. cutting off the ones that seem most affected might help a lot of it is mites. Usually, here, they are red insects that spin webs. They spin a very tiny white webbing in the same sort of distribution on the undersides of the leaves as yours.
Do you have a magnifier? Can you see anything really tiny crawling about on there?

Be careful with pesticides. Start with some soapy water or alcohol diluted half and half with water. Spray those on the weird areas.

Thanks for your advice, both of you. I don't have a magnifier but I removed the worst affected leaf and looked closely - it looks as though on the mid-rib there is a tiny web running the entire length with tiny white things on it and then on the rest of the leaf lots of black bits. Is this spider mites as you suggest? @gina1960

@plantmanager I have sprayed the whole plant (and wiped the leaves) with 1/4 apple cider vinegar, 1 tbsp vegetable oil, few drops of natural dish soap and 2 cups of water. Hopefully this will resolve the problem...

Those actually look more like thrips.(The white ones), They do the same thing. Spider mites, thrips and scale are all sap suckers and will feed off a plant in the same manner. Thrips can be easier to get rid if than mites, because mites are actually arachnid-like and some things do not work on them. Once a mature scale forms its shell it can also be hard to get rid of them. Don;t stop with just one treatment. You should take the entire plant outside and spray every nook and cranny, every leaf surface adaxial and abaxial, and the trunk to dripping, at least 3 applications a week apart to get new hatchlings

Do thrips produce webs? The underside of this leaf had a very fine web running along it. Thanks for the advice, I will continue to spray it and wipe it down, and I will put it on my balcony this afternoon.

It could be a combination of 2 pests. Sometimes when a plant is weakened by one, another will also move in