This is due to microscopic insects called thrips. Since they can be difficult to find, pull the plant apart, and be careful not to let any leaves or other plants touch it. Then, take a piece of white paper and shake the leaves over it; this should reveal the insect.
After it has been found, thoroughly wash the plant’s foliage under the tap, getting rid of as many insects as you can. In the second step, apply a solution of lukewarm water and neem oil to the plant. After 8 to 10 minutes, wipe the leaves. Make careful to apply this in the evening when the plant won’t be in direct sunlight, as this could scorch the leaves.
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