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Consider the attached pictures of some leaves of my pepper plant.

There are white dots on the backside of the leaf which correspond to regions of mild translucency on the face of the leaf. Further, there are small indentations on the face corresponding to the dots.

The plant in question is quite large and is currently producing many peppers. Hence, I suspect that it's due to a nutrient problem and is more likely to be a deficiency than overfertilizing. My smalle pepper plants, which get the same amount of fertilizer, are showing less/no decoloration on their leaves.

Please help me identify what nutrient is in short supply, if any, and how to remedy the deficiency. I have checked that the white dots are not scale, it's really the leaf itself that's deformed not something sitting on top. It is still plausible that it could be a fungal problem.

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It looks like edema. The leaves should be bumpy underneath, if so. I'd increase ventilation, keep the leaves dry (don't get them wet when you water), maybe lessen the temperature and humidity. Adding extra potassium can also help.

Edema isn't a pest or disease. It's just a condition caused by growing conditions (common when growing indoors under artificial lights). Mites and tiny sucking pests can contribute to it, however.

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This appears to be a scale infestation. They are small sap-sucking bugs that become immobile once they stick on to a part of the plant and start feeding off of it. I recommend using 2 parts rubbing alcohol and 1 part water on a cotton ball to rub the insects off the underside of the leaf. Go over the plant every two days and remove all bugs you see. A q-tip will help you get bugs stuck in harder to reach spots. If you are persistent the plant should be fine after three or four treatments but check regularly for any more outbreaks. I like to spray the top of the soil with the diluted rubbing alcohol too to get any nymphs that are hatching and crawling out of the soil.

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    Thank you for the suggestion. I agree that it looks like a scale infestation on first sight but I do not think this is the case. Would you agree that it should be possible to rub off scale insects? Even considerable force, damaging the leaf, does not do so. – A. Van Werde May 19 at 5:06
  • Hard scale (which are generally white or off white) are really difficult to scrub off. You may have to go with a systemic insecticide if they don't fall off a few days after treating with rubbing alcohol. – Camrin LaFond Jun 1 at 17:36

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