Why Are My Tomato Leaves Turning Yellow? 7 Causes and How to Fix Each

Yellow leaves on a tomato plant. It's one of the most common worries in any garden or field, and it can mean anything from "completely normal" to "this plant won't make it to harvest." The good news is that the leaf usually tells you which one it is, if you know where to look. So let's go through seven causes, how to tell them apart, and what to do about each one.

Before we start, three questions do most of the work. First: where on the plant is the yellowing? On the old, lower leaves, or on the new growth at the top? Second: what's the pattern? The whole leaf evenly, the spaces between the veins, or spots? And third: is it the whole plant, or just one side, or one stem? Keep those three in mind as we go.

Cause number one is plain old age. As a tomato grows taller, it shades its own bottom leaves, and it moves nutrients up to the new growth and the fruit. So a few of the oldest, lowest leaves slowly turning yellow, while the rest of the plant is green, growing, and setting fruit, is normal.

What to do: nothing dramatic. Snip off the yellow lower leaves with clean pruners. That also lets air move near the soil, which helps prevent the leaf diseases we'll get to later.

Cause two: nitrogen deficiency. Nitrogen can move inside the plant, so when it runs short, the plant pulls it out of the older leaves to feed the new ones. You'll see the lower leaves turn evenly pale, then yellow, the whole leaf, veins included, and the yellowing creeps upward. The whole plant looks pale, with thin stems and slow growth. It's common in sandy soil, after heavy rain, and in pots, where frequent watering washes nutrients out. The fix is to feed. Work in compost or a balanced fertiliser, and side-dress when the first fruit sets, because that's when demand climbs. But go easy. Too much nitrogen gives you a big, leafy plant with very few tomatoes. A soil test tells you exactly what's missing.

Cause three isn't a nutrient at all. It's water, and usually too much of it. Roots need air as well as water. In soggy, compacted, or poorly drained soil, or a pot with no drainage holes, the roots are starved of oxygen. They stop taking up nutrients, and the leaves turn yellow, often with wilting, even though the soil is wet. Long dry spells stress the plant too, and big swings between wet and dry lead to problems like blossom-end rot on the fruit. The fix: check the soil with your finger before you water. If it's still moist a few centimetres down, wait. Water deeply and less often, at the base of the plant, ideally in the morning. Make sure pots can drain, and add a layer of mulch to keep moisture even.

Cause four: magnesium deficiency. This one has a signature look. The older leaves turn yellow between the veins, while the veins themselves stay green, a bit like a stained-glass window. It often appears once the plant is carrying a heavy load of fruit, and it's more likely on sandy or acidic soils, or where a lot of potassium fertiliser has been used, because potassium competes with magnesium. One tip: if that same pattern shows up on the newest leaves at the top instead, that points to a different nutrient, often iron.

The fix: confirm with a soil test first, because other problems can look similar. If magnesium really is low, a magnesium fertiliser, such as magnesium sulphate, or dolomitic lime where the soil is also acidic, will correct it. Follow the label rate. More is not better.

Now the diseases. Cause five is early blight, caused by the fungus Alternaria solani. Look for brown spots on the lower leaves with rings inside them, like a target, often with a yellow halo around them. The leaf around the spots turns yellow and drops, and the disease climbs up the plant, especially in warm, wet weather. The fungus survives in old plant debris and soil, and it splashes up onto the leaves when it rains, or when you water from above. What to do: remove infected lower leaves as soon as you see them, and bin them, don't compost them. Mulch the soil to stop the splashing, water at the base, and give plants space and support so the leaves dry quickly. Don't grow tomatoes, potatoes, peppers, or eggplants in the same spot for at least two to three years. And if the disease is severe, a protectant fungicide labelled for early blight on tomatoes can help. Ask a licensed local adviser, follow the label exactly, and rotate between fungicide groups.

Cause six looks similar, but it isn't the same: Septoria leaf spot. Here the spots are smaller and far more numerous, only a few millimetres across. They're round, with a dark edge and a grey or tan centre, and if you look closely you may see tiny black dots in the middle. Those are where the fungus makes its spores. There are no target rings. It also starts low and moves up, and badly affected leaves turn yellow, then brown, and fall, which can leave the fruit exposed to sunscald. It loves warm, humid, wet weather. The fix uses the same toolkit as early blight: remove infected leaves, mulch, keep the foliage dry, clear away plant debris at the end of the season, rotate your crops, and if it's needed, use a labelled protectant fungicide, exactly as the label says.

Cause seven is the most serious: Fusarium and Verticillium wilt. These fungi live in the soil, get into the roots, and block the plant's water-carrying tissue. The clue is the pattern. Yellowing starts on the lower leaves, often on just one side of the plant, one stem, or even one half of a leaf, with wilting in the heat of the day, even when the soil is moist. Fusarium tends to strike in warm soils. Verticillium prefers cooler conditions, and often causes V-shaped yellow patches at the leaf edge. To check, cut a lower stem lengthwise. Brown streaks running up the inside point to a wilt. There's no spray that cures it. Remove and destroy affected plants, and don't plant tomatoes in that soil again for several years, because these fungi persist for a long time. Next season, choose resistant varieties. Look for letters like V and F on the seed packet or plant label. Grafting onto a resistant rootstock is another option.

A quick word on what else it could be. If the new leaves at the top are curling upward and yellowing at the edges, and tiny white insects fly up when you brush the plant, think of whitefly-spread viruses, like tomato yellow leaf curl. If leaves are mottled light and dark green, think mosaic viruses. No spray cures a virus, so the focus is on managing the insects that spread them and removing infected plants. And remember, more than one problem can be happening at the same time.

If you'd like this in one place, Laisha has a free tomato grow guide that walks through the season stage by stage: choosing your land, raising seedlings, feeding, watering, pests, and diseases. It's free to read, with no sign-up, at laisha.ai/guides/tomato, and there are guides for more than two hundred other crops there too.

The full troubleshooting section, the "if you see this, it likely means this, do this" part, continues inside the Laisha app, which you can try free for ten days.

Quick recap. A few old leaves at the bottom, on a healthy plant: that's natural ageing.

Evenly pale, and creeping upward: nitrogen. Yellow and wilting in wet soil: water and roots. Yellow between green veins: magnesium. Spots with target rings: early blight. Lots of tiny spots: Septoria. One-sided yellowing and wilting: Fusarium or Verticillium. If this helped, subscribe for more practical crop guides, and tell me in the comments what you're growing this season. Happy growing.