Answer: Several things can cause yellowing leaves on pepper transplants. If they are new transplants, the problem could be cool soil, soggy conditions, root loss prior to or during transplanting, and root decay organisms. You may need to carefully unearth a plant to examine its roots for signs of decay (brown, watersoaked roots rather than creamy-white).
When established transplants suddenly begin to exhibit yellow leaves on new growth, causes could include iron deficiency, soggy soils or herbicide damage (can come from a sprayer that was previously used for herbicide, drift from nearby lawn applications, contaminated clippings used as a mulch, or manure from animals that grazed on treated pastures...yes, that really can happen!).
Yellowing on older leaves of established transplants may be due to nitrogen deficiency (may also be due to excessive amounts of undecomposed organic matter tying up nitrogen in the soil), temporary drought, or root damage from soil insects or nematodes.
If the plants are not already too wet, give them a good feeding with a liquid plant food (compost "tea", fish emulsion or seaweed works great for this) and see if they don't perk up in a few days.
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