Why Are Your Tomato Leaves Turning Yellow? A Diagnosis Guide

Start By Noting Where the Yellowing Shows Up

Before you reach for fertilizer or a fungicide, take thirty seconds to answer one question: where on the plant is the yellowing happening? The pattern tells you almost everything.

• Bottom leaves only, plant otherwise healthy — usually a nutrient issue, most often nitrogen.
• Yellowing between green veins, especially on older or middle leaves — a magnesium or iron shortage.
• Yellow leaves with brown or black spots, often starting low and climbing — a fungal disease like early blight or septoria leaf spot.
• Whole branches or one side of the plant wilting and yellowing — a soilborne wilt disease.
• New growth curling, mottled, or streaked with yellow — watering stress, heat, or in rarer cases a virus.

Walk your tomato patch with this list in mind, and you can usually narrow the cause before you finish your coffee. From there, the sections below go in rough order of likelihood, so start at the top and work down.

Bottom Leaves Yellowing First: Nitrogen Deficiency

This is the most common cause of yellow tomato leaves, especially in containers, raised beds, or gardens that haven’t been fed in a few weeks. Tomatoes are heavy nitrogen feeders, and when the soil runs short, the plant pulls nitrogen out of its oldest leaves to feed new growth. The result is uniform yellowing on the lowest, oldest leaves first, while the top of the plant stays green.

The leaves usually yellow evenly across the whole leaf, not in a blotchy or spotted pattern, and they may eventually turn pale green to yellow before dropping off entirely. Growth often slows down at the same time, and the whole plant can look a bit washed out.

The fix is straightforward: side-dress with a balanced or nitrogen-leaning fertilizer, or work in a nitrogen source like composted manure, and water it in well. Container-grown tomatoes need feeding more often than in-ground plants because nutrients wash out with every watering. If you’re not sure whether it’s nitrogen, a soil test will confirm it, but in practice, if the bottom leaves are yellowing evenly and the plant hasn’t been fed recently, nitrogen is the safe first guess.

Yellowing Between the Veins: Magnesium or Iron Deficiency

If the leaves are yellowing but the veins themselves stay green, creating a marbled or lacy pattern, you’re likely looking at a magnesium or iron shortage rather than nitrogen.

Magnesium deficiency shows up on older, lower-to-middle leaves first, with the classic green-veined, yellow-between pattern. It’s common in gardens with naturally acidic or sandy soil, or where potassium-heavy fertilizers have been used repeatedly, since excess potassium can block magnesium uptake. A light application of Epsom salt dissolved in water, applied as a soil drench or foliar spray, usually resolves it within a couple of weeks.

Iron deficiency looks similar but tends to show up on the newest, youngest leaves instead of the old ones, because iron doesn’t move around the plant once it’s used. It’s most common in soil that’s too alkaline for the plant to absorb iron efficiently, even if iron is present in the soil. If a soil test shows a high pH, working in an acidifying amendment or switching to an acidifying fertilizer over time will help more than iron supplements alone.

The quickest way to tell these two apart from nitrogen deficiency: nitrogen yellows the whole leaf uniformly, while magnesium and iron leave the veins green.

Yellow Leaves With Dark Spots: Early Blight and Septoria Leaf Spot

Once you see actual spots on the yellowing leaves, you’ve moved from a nutrient problem into fungal disease territory. Two look-alikes cause most of this damage.

Early blight starts on the lower, older leaves as small brown spots with a target-like series of concentric rings, often ringed by a yellow halo. As the disease spreads, the yellowing takes over more of the leaf until it browns and drops. It’s worse in warm, humid weather and spreads fastest when leaves stay wet overnight or get splashed with soil during watering.

Septoria leaf spot looks similar but the spots are smaller, more numerous, and tend to have gray or tan centers with dark borders rather than concentric rings. It also starts low and climbs the plant, and heavily infected leaves turn yellow, then brown, then fall.

For both, the management approach is the same: remove and dispose of infected leaves as soon as you spot them, avoid overhead watering, mulch to keep soil from splashing onto lower leaves, and space plants for good airflow. A copper-based or other labeled fungicide can slow the spread if you catch it early, but sanitation and airflow do most of the real work. Neither disease is usually fatal to the plant in a single season, but both will reduce yield if left unchecked.

Whole Branches or Sides Wilting and Yellowing: Fusarium and Verticillium Wilt

If yellowing shows up alongside wilting, particularly wilting that’s worse on one side of the plant or on individual branches while the rest looks fine, you’re dealing with a vascular wilt disease rather than a leaf-surface fungus.

Both fusarium wilt and verticillium wilt live in the soil and enter through the roots, clogging the plant’s water-conducting tissue. Leaves yellow, often starting low, then wilt during the heat of the day even when soil moisture is adequate, sometimes recovering overnight before wilting again the next afternoon. If you cut a stem near the base and see brown discoloration inside, that’s a strong sign of vascular wilt rather than a nutrient or leaf-spot issue.

Unfortunately, there’s no cure once a plant is infected — the fungus lives in the soil and can persist for years. Remove and discard infected plants rather than composting them, avoid planting tomatoes (or related crops like peppers and eggplant) in that spot for several years, and next season, choose varieties bred with resistance, often labeled with a “V” or “F” on the tag or seed packet. This is one case where prevention matters far more than treatment.

Curling or Mottled Yellow New Growth: Watering Stress or Viruses

If the yellowing shows up on new growth as mottling, streaking, or an odd puckered curl rather than a clean pattern, first rule out the boring explanations: inconsistent watering and heat stress. Tomatoes that swing between bone-dry and soaked, or that sit in intense heat, often show yellowing and curling on newer leaves as a stress response. Consistent, deep watering and light shade during extreme heat usually clears this up within a week or two.

If watering and heat aren’t the culprit and the mottling looks distinctly blotchy or the leaves are visibly distorted and stunted, a virus is possible, often spread by aphids or other sap-feeding insects. There’s no treatment for a viral infection — the only real response is to remove the affected plant to protect the rest of your garden and to manage the insects that spread it.

Quick Recap

• Bottom leaves yellowing evenly, whole plant pale: likely nitrogen deficiency — feed and water in.
• Yellow between green veins: magnesium (older leaves) or iron (newer leaves) deficiency.
• Yellow leaves with dark or ringed spots: early blight or septoria leaf spot — remove leaves, improve airflow, avoid wetting foliage.
• Wilting plus yellowing, especially one-sided: vascular wilt disease — remove the plant, rotate crops, plant resistant varieties next time.
• Mottled or curling new growth: check watering and heat first, then consider a virus spread by insects.