Yellow leaves are one of the most common things a home gardener asks about, and one of the least specific: on their own, they can mean almost anything from completely normal ageing to a genuine problem worth acting on. Across vegetables generally, four causes account for most cases, and which leaves are affected, and how, usually narrows it down fast.
Natural ageing is the most common and least concerning cause. As a plant matures and channels energy into flowering or fruiting, its oldest, lowest leaves often yellow and drop one at a time while the rest of the plant stays vigorous and green. If it's just the bottom leaves, on an otherwise healthy plant that's actively growing or fruiting, this is normal and needs no response.
Water stress, in either direction, is the next most common cause and the two can look surprisingly alike. Overwatered plants often show yellowing that spreads across whole leaves starting low on the plant, sometimes with a slightly wilted look despite wet soil, since waterlogged roots can't actually take up water properly. Underwatered plants show drier, more brittle yellowing, often starting at the leaf edges, with soil that's visibly dry a few centimetres down. Checking the soil itself, not just the leaves, is the fastest way to tell these apart.
Water stress, in either direction, is the next most common cause and the two can look surprisingly alike.
Nitrogen deficiency shows as an overall pale, yellow-green colour rather than a healthy dark green, starting with the older leaves first, and it's usually uniform across each leaf rather than blotchy or edge-first. Growth typically looks slow or stunted alongside it. A balanced feed usually greens a plant back up within one to two weeks if this is the cause.
Pest or disease damage is the fourth cause, and the one most likely to come with other symptoms beyond just yellowing: fine webbing and stippled, dusty-looking leaves point to spider mites, distinct spots or blotches point toward a fungal or bacterial disease, and sudden yellowing on just part of a plant, rather than gradually across it, is more consistent with pest or disease damage than with any of the other three causes.
Working out which of the four is affecting a specific plant means checking it against where it actually is in its own growth: a young seedling turning yellow means something very different to a mature, fruiting plant doing the same thing. That's the exact judgement premium check-ins on this site are built around, guidance timed to a plant's real stage, not a generic checklist applied the same way regardless of what the plant is actually doing.