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We have this large rubber plant for some years now and it’s growing constantly. complete rubber plant
Large Version

Except from two issues:

  1. The first was overwatering two years ago, but we fixed it as we quickly re-potted the plant. We used a soil mix of 1 shovel sand and 4 shovels soil (more or less poor in nutrients). Because I had read, that the soil should not need to be that nutritious and some kind of sandy soil.
  2. Reallocation three month ago by temperatures not much above 0.

Each time the plant has dropped many leaves, but also recovered (slowly).
There are always some fresh new leaves growing, but they constantly got these kind of white coat on the new leaves.

fresh leaveswhite coat
white coat II

Large versions of the pictures above: 1 2 3

This white coat comes after a few weeks. We did not know how to handle this. I thought about some kind of fungal infection, but trying to rub it off with the fingers does not work.

Looking at these two pictures, I also thought about Thrips. Since a neighboring plant had some. The neighbor got quickly isolated as I found the Thrips. I found a few of them (5 to 10) on the rubber plant and removed them. Since then, I am looking from time to time for Thrips, but found no individuals.

leave topleave bottom
Large Version from these pictures: 1 2

So my question is:

What could cause this white/silvery mottling on the Rubber Plant leaves and what can I do to handle this?

I also appreciate some advices to increase the overall condition of the rubber plant.

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  • Check for spider mite - inspect closely all over for any signs of faint webbing, they can cause this mottled appearance.
    – Bamboo
    Commented Mar 15, 2022 at 15:41
  • Have you added anything alkaline to the soil, like calcium or rockdusts? Manganese-deficiency (often caused by a high pH) sometimes can produce speckles, I believe.
    – Brōtsyorfuzthrāx
    Commented Mar 16, 2022 at 2:35
  • @Bamboo: Thanks for the advise, a look at a few leaves did not show any webbing, but I will take a complete look later.
    – Martin Backasch
    Commented Mar 16, 2022 at 8:10
  • @Brōtsyorfuzthrāx: I had added sand from the sandbox of my children to the sole (1 shovel sand 4 shovel sole), when I was fixing the overwatering two years ago. Because i read that the sole does not need to be that heavy. It may corelate with the increase of the white spots, since they are heavily on the new leaves instead of the old ones. Would it be harmful to dissolve some Manganese pills/carbon tablet from the drugstore and water the rubber plant with it?
    – Martin Backasch
    Commented Mar 16, 2022 at 8:22

3 Answers 3

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The silvery mottling is a sign of thrip larvae. They live inside the leaf and tunnel through it. If you had thrip on your plants you probably still do as the adults fly when disturbed and they are hard to eradicate.

Seeing as most ficus will bud out easily from old wood I recommend cutting it back hard and moving it towards more lights. Cut it back to leave a stem of 30 cm or 12 inches from the base and reduce watering until new growth appears. If the new growth shows the same silvery mottling you know you still have thrip on this or other plants that are nearby.

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  • That's NOT what I want to read, but I know, I asked for it! :) I augur ill, that you could be right. When searching for thrips and rubber plant I will found pictures of leaves looking exactly like those on my rubber plant. I will take a closer look later. Did you think a health cure with Neemoil could work?
    – Martin Backasch
    Commented Mar 16, 2022 at 14:25
  • @MartinBackasch Neem oil has some residual effectiveness but will not touch the thrip larvae inside the leaf who will emerge later and become adults. I was never able to control thrip without resorting to toxic systemic pesticides
    – kevinskio ♦
    Commented Mar 17, 2022 at 11:41
  • I will isolate the rubber plant and will give the Neemoil a try. I had read that watering with some Neemoil in the water, will stop the larvae from emerging. Cutting back is still an option, but not the preferred one.
    – Martin Backasch
    Commented Mar 17, 2022 at 11:50
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It might be manganese-deficiency. It could be caused by depleted soil, or the manganese could be present, but unavailable, due to a high soil pH. Rubber figs are said to like acidic soil (so, they might get manganese-deficiency symptoms in alkaline soil more easily than other plants).

Thrips, scale insects, leafhoppers, spider mites (as Bamboo mentioned in the comments), and such, also can make leaves speckled like this.

I might recommend fertilizing it regularly (with 24-8-16 Miracle Gro or something else with magnanese and NPK in it), whether or not you add magnanese sulfate by itself or take measures to reduce the soil pH. Nitrogen (N), phosphorus (P) and potassium (K) are pretty important if the pH is high. The 24-8-16 stands for 24% nitrogen 8% phosphorus and 16% potassium.

You said a fourth of your potting mix is sand. Sand has a pH of 7, which is neutral, and supposedly in the range of what rubber figs like according to some people (but I have my doubts that it's ideal). I'm not sure what sole is, or its pH (maybe you meant soil). If it's outdoor garden soil, it's more likely to be a problem than a potting mix for indoor plants. I don't suggest using outdoor garden soil for indoor plants, if you have an alternative. It can bring insects and diseases, and it may not be as friable.

I haven't tried giving plants manganese pills. I'm only familiar with manganese sulfate, and manganese from fertilizers with many nutrients. There's also manganese EDTA.

Peat moss is acidic (if you want to use some of it in the mix). Adding the right kind of sulfur to reduce the pH is possible (but it takes time).

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  • 1
    Yes I mean soil, sorry for that. Your answers sounds reasonable to me, since I used some soil which was rather poor in nutrients then the indoor mix i usually use. And I also did not fertilize the soil. As I mentioned before I had read, that rubber plants do not need such a nutrient soil. Since most of my plants get a fresh soil and pot every year, I just forget, that a fertilizer could be needed. I will look, what I can get and come back later.
    – Martin Backasch
    Commented Mar 16, 2022 at 11:20
  • 1
    Remember, even though I recommended to fertilize regularly, the plant might needs breaks from fertilization every so often; and it'll probably want less fertilizer in the winter (not as soon as it gets cold, but in my area, our houseplants seem to want less around Christmas or so).
    – Brōtsyorfuzthrāx
    Commented Mar 16, 2022 at 15:52
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I had a closer look at my leaves and found these little guys:

Thrips
Large Version

So as @kevinskio said in their answer:

The silvery mottling is a sign of thrip larvae.

But before cutting it back hard, I will isolate the rubber plant and try to get rid of the thrips by using Neemoil. In addition to that, I will try to strengthen the rubber plant by adding some fertilizer as suggest by @Brōtsyorfuzthrāx in their answer, since the plant is in its soil for two years now without any additional nutrition.

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