Why Are Your Tomato Leaves Turning Yellow? A Diagnosis Guide

Start With Where the Yellowing Is

Tomato leaves turn yellow for four main reasons: a nutrient shortage, a leaf spot disease, a wilt disease in the soil, or the normal aging of shaded bottom leaves. You can usually tell which by answering two questions. Are the yellow leaves old ones low on the plant, or new ones at the top? And is the yellow even, between the veins, or surrounding dark spots? Evenly yellow lower leaves point to nitrogen. Yellow between green veins points to magnesium on old leaves and iron on new ones. Yellow around brown spots is early blight or Septoria leaf spot.

Position matters because of how nutrients move inside the plant. University of Georgia Extension explains that nitrogen is mobile in the plant. When the supply runs short it is quickly moved to new growth, which depletes the mature foliage, so the old leaves show the shortage first. The same bulletin places potassium and magnesium symptoms in the older foliage first, and iron and sulfur symptoms on new leaves near the top of the plant.

What you see	Where on the plant	Likely cause
Even yellowing, no spots	Older, lower leaves, sometimes the whole plant	Nitrogen shortage
Yellow leaves low inside a large, dense plant	Shaded bottom leaves	Normal aging
Yellow between green veins	Older, lower leaves	Magnesium shortage
Yellow between green veins	New growth	Iron shortage, soil pH over 7.0
Brown spots with rings and a yellow halo	Base of the plant first	Early blight
Many small pale spots with dark borders	Base of the plant first	Septoria leaf spot
Yellowing on one side, with wilting	One branch or one side of the stem	Fusarium or Verticillium wilt
Curled, mottled or stunted leaves	New growth or scattered plants	Virus, or heat and water stress

Lower Leaves Yellow Evenly: Nitrogen or Normal Aging

Nitrogen shortage is the classic cause of plain yellow leaves with no spots. The Georgia bulletin describes it as yellowing of leaves, typically in the older, mature foliage of the lower limbs. Mississippi State University Extension describes the more advanced picture: a general yellowing of the entire plant that affects both old and new leaves. If only the bottom of the plant is pale, you are seeing it early.

Before feeding, rule out age. Mississippi State points out that as plants mature, the bottom leaves become heavily shaded and naturally turn yellow as they age. On a large plant in midsummer, a few yellow leaves at the very bottom, under a healthy dark green canopy, are not a deficiency. This shows up most on tall indeterminate varieties, which keep adding canopy above the oldest leaves all season.

If the plant as a whole is pale and growth has slowed, nitrogen is the likely answer. Side-dress with a fertilizer that contains nitrogen at the rate on its label and water it in. A soil test through your state extension service takes the guesswork out of how much to apply. Sulfur shortage can look similar, with pale green to yellowish leaves, but Georgia notes that it appears on younger leaves near the top of the plant, the opposite end from nitrogen.

Yellow Between the Veins: Magnesium or Iron

When the veins stay green and the tissue between them turns yellow, the cause is one of two nutrients, and leaf age tells them apart.

Magnesium shows on the older leaves. Rutgers Cooperative Extension describes yellowing of the leaf tissue between the veins of older leaves, and Georgia notes that it starts in the older, lower foliage first. Rutgers reports that magnesium deficiency can occur especially in sandy soils, when soil is overwatered or after heavy rainfall, and that it can appear when calcium and magnesium are out of balance even though a soil test shows adequate levels of both. Left alone, Rutgers adds, it lowers yield and leaves the plant less energy for fruit production. The quick correction from Mississippi State is a foliar spray of Epsom salt, which is magnesium sulfate, at 2 tablespoons per gallon of water.

Iron shows on the new growth. The pattern is the same, yellow between veins that remain green, but on the youngest leaves at the top. Georgia identifies high soil pH, over 7.0, as the most common cause, and adds that overwatering or underwatering can bring it on too. Its advice is to water evenly, check the soil pH, and lower it where needed with elemental sulfur or another acidifying agent as a soil analysis recommends. Chelated iron products are the other correction it names. The page on soil pH for the vegetable garden covers how to adjust it.

One more pattern is worth knowing so you do not confuse it with these. Potassium shortage shows at the leaf edge. Georgia describes a brown outer margin on mature leaves, and Mississippi State describes bright yellow margins on otherwise green leaves.

Yellow Leaves With Spots: Early Blight and Septoria Leaf Spot

Spots change the diagnosis from nutrition to fungal disease. Two common ones are covered here, and University of Wisconsin Extension notes that both first appear at the base of the plant. A third disease, late blight, is covered in the comparison of early blight and late blight on tomatoes.

Early blight makes roughly circular brown spots on leaves and stems. As the spots enlarge, concentric rings appear and give them a target-like look. Wisconsin describes a yellow halo around the spots, and Clemson’s Home and Garden Information Center describes the same bull’s-eye pattern on small brown lesions, mostly on the older foliage, with the surrounding tissue turning yellow.

Septoria leaf spot makes many more, smaller spots. Wisconsin gives the size as approximately 1/4 inch in diameter, whitish with dark borders. Clemson describes numerous small, circular spots with dark borders around a beige center on the older leaves, with tiny black specks visible in the centers. There are no rings.

Management is the same for both:

1. Mulch. Wisconsin recommends approximately one inch of a high quality mulch, without over-mulching.
2. Give plants more space. Dense foliage holds humidity and keeps leaves wet longer, which favors both diseases.
3. Keep water off the leaves. Clemson advises against overhead watering.
4. Thin the plant. Wisconsin notes that removing selected branches may slow the disease.
5. Rotate. Clemson’s rule is that tomatoes and related vegetables should not be planted on the same land more than once in three years.

Fungicides are a supplement to those steps. Wisconsin notes that a copper or chlorothalonil fungicide labeled for vegetables may provide control of early blight if applied very early, and lists the same two ingredients for Septoria. If the leaves have holes as well as spots, the leaf damage identification chart covers chewing pests.

One Side Yellow and Wilting: Fusarium and Verticillium Wilt

Yellowing that comes with wilting, and that affects one side of the plant more than the other, points to a wilt fungus in the soil. Clemson describes Fusarium wilt this way: often leaves on only one side of the stem turn golden yellow at first. Wisconsin describes Verticillium wilt as a sudden yellowing of foliage, typically first appearing on one side of a plant.

Both diseases mark the water-conducting tissue inside the stem. Clemson notes dark brown discoloration of the water-conducting vessels in plants with Fusarium wilt. For Verticillium, Wisconsin says that peeling away the outer layers of an affected branch may show brown streaking in the vascular tissue. Clemson calls Fusarium wilt a warm-weather disease.

There is no spray for this. Clemson states that no chemical control is available for Fusarium wilt. Prevention is by variety and location. Wisconsin advises looking for the letter V after a variety name for Verticillium resistance. For Fusarium, Clemson recommends cultivars marked at least FF, resistant to races 1 and 2, and notes that some newer ones marked FFF resist races 1 through 3. It adds that raising soil pH to 6.5 to 7.0 and feeding with nitrate nitrogen in place of ammoniacal nitrogen will slow the disease. Wisconsin also advises against growing highly susceptible crops in the same area of the garden year after year.

Curled, Mottled or Stunted Leaves: Stress or Virus

Yellowing combined with curling or odd leaf shapes has two possible explanations, and the harmless one is more common.

The harmless one is physiological leaf roll. Clemson lists its causes as high temperatures, prolonged periods of wet soil, and drought. It shows mostly on older leaves, whose leaflets curl upward, and Clemson notes that it does not significantly damage the crop. Even watering and a layer of mulch reduce the swings between wet and dry soil.

Viruses look different. Clemson’s descriptions give the main ones:

• Tomato yellow leaf curl virus, spread by whiteflies: upward curling of leaves, yellow leaf margins, smaller leaves than normal, stunting and flower drop.
• Tomato spotted wilt virus, spread by thrips: scattered plants that are stunted, with bronze or dark spots or prominent purple veins on younger leaves.
• Mosaic viruses: light and dark green mottling of the leaves, sometimes with malformed leaflets.

Clemson states that there are no chemical controls for viruses. Its instruction is to remove and destroy infected plant material.