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A few months ago I had scale bugs on a different houseplant in a different room and asked about them in a different SE site:

Looking under a low power microscope I also found some flat waxy like white deposits on the undersides of leaves that seemed to show other stages in the lifecycle, something that appeared to be hibernating/transforming, and much smaller, very tiny little black crawlers only visible in the low power microscope.

Today I found these irregular but mostly circular white, waxy-looking spots on the TOP sides only of a few leaves of this houseplant.

I occasionally give this one a direct dose of sun (as shown in the last photo) but mostly keep it in a moister, indirectly lit area. I also often water it using a plastic squirt bottle from a distance of a meter or two, so it frequently gets splashed on the top leaves by tap water.

So I'm wondering if maybe my tap water and/or plastic squirt bottle have stuff in it that forms these white spots when the splashed water droplets dry, but still I'm panicking, so I'd like to ask:

Question: What are these whitish deposits on the top surfaces of these houseplant leaves and should I do something about them?


Click any image for full size:

Plant #1

my houseplant with white waxy spots on the tops of some leavesmy houseplant with white waxy spots on the tops of some leaves

my houseplant with white waxy spots on the tops of some leaves my houseplant with white waxy spots on the tops of some leaves

UPDATE Here are new photos

since three more plants have suddenly exhibited the same whitish spots overnight, and squirt bottle I've been using for the last few months

my squirt bottle my houseplant with white waxy spots on the tops of some leaves

my houseplant with white waxy spots on the tops of some leaves my houseplant with white waxy spots on the tops of some leaves

Plant #2

my houseplant with white waxy spots on the tops of some leaves my houseplant with white waxy spots on the tops of some leaves

my houseplant with white waxy spots on the tops of some leaves

Plant #3

my houseplant with white waxy spots on the tops of some leaves my houseplant with white waxy spots on the tops of some leaves

my houseplant with white waxy spots on the tops of some leaves my houseplant with white waxy spots on the tops of some leaves

my houseplant with white waxy spots on the tops of some leaves my houseplant with white waxy spots on the tops of some leaves

Plant #4

my houseplant with white waxy spots on the tops of some leaves my houseplant with white waxy spots on the tops of some leaves

my houseplant with white waxy spots on the tops of some leaves my houseplant with white waxy spots on the tops of some leaves

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  • added the fungus tag out of anticipation that this might be mold growing in the water droplets
    – uhoh
    Jul 7, 2022 at 0:53
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    Determine is residue is soluble.Place a drop of water on a few of the spots, leave an hour, remove,and see if they are still there when dry. The first picture may show wax produced by the scale insects. Pattern doesn’t look like disease. Jul 7, 2022 at 3:35
  • @PolypipeWrangler thanks, I will investigate that today.
    – uhoh
    Jul 7, 2022 at 21:55
  • @PolypipeWrangler it does not dissolve, but there are probably some types of suspended solids that won't redissolve once dried out so I don't think that's definitive. However I've added images of three more plants that suddenly show similar white spots. It looks more like a fungus than a deposit to me.
    – uhoh
    Jul 10, 2022 at 2:33
  • 1
    It doesn't look like insect pests. Suggest moving the plants to better light
    – kevinskio
    Jul 10, 2022 at 12:25

1 Answer 1

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The water you are misting the plants with also contains dissolved minerals. The composition and amount depends on where the water is from. My area has a lot of limestone so the tap water contains dissolved compounds of calcium.

After the water evaporates the minerals are left. They dry to form the white residue you see.

You can resolve this by:

  • not misting the leaves. This is not a significant source of humidity and you can provide the same effect by letting the pot sit on top of a dish of water with some pebbles
  • or use distilled water which has much lower levels of minerals

The residue can be removed with a clean cloth and gently rubbing the leaf. Difficult spots can be removed by using the cloth with a slightly acid solution like milk or watered down vinegar followed by a rinse to remove all residues

Edit: the fuzzy white stuff you asked about in another question are mealy bugs that can be controlled with soap and water and persistent control (every 5 to 6 days for at least three times maybe more)

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  • Thank you for this! I guess "squirt bottle" is ambiguous; I'll add a photo to my question but it basically looks like this i.stack.imgur.com/3MCjA.png and probably should be called a "squeeze bottle". Rather than mist, it applies a gentle stream of water with which I can water even a small pot about 1.5 meters away from where I'm standing. As it leaves drops of water on the leaves, it probably results in exactly the same problem as a mister would, but only because I'm a lousy aim and let the top of the plant stop the stream so that it runs down to the pot (analogous to rain in my mind)
    – uhoh
    Jul 7, 2022 at 21:31
  • About the linked question, I managed to take some photos through a microscope with my cell phone. I'll go find them and update that question by adding a few later today. Actually if you think the question about species identification and life cycle is on-topic and answerable here I could ask there to have it migrated here; it's been six weeks and there are so far no answers.
    – uhoh
    Jul 7, 2022 at 21:34
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    @uhoh you can migrate it here or ask it again: either way you will find out what the pest is
    – kevinskio
    Jul 8, 2022 at 10:36
  • I've updated the question with the aforementioned squirt bottle photo and with photos of three more plants that just "sprouted" white spots as well. I'm currently doing experiments to see if I can produce any white residue due to dissolved minerals, but this stuff really looks like fungus to me - especially the dense patches on plant #3. Starting today I will always water the soil directly and not use short-cuts like letting the stream of water hit the leaves and letting it drip down. My "solarium" (bathroom) gets quite hot and humid; and these plants were on the floor. I'm making changes...
    – uhoh
    Jul 10, 2022 at 2:30
  • The new pictures look like disease (powdery mildew?) .Also, to be scientific, spray some cleaned plastic and metal surfaces and let them dry. Examine. Also, could we have the names of the plants to look up? Jul 18, 2022 at 11:49

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