Tomato leaves turning yellow
There are 11 common causes, and they need completely different responses — so working out which one you have matters more than treating quickly.
4 of them can kill the plant. Rule those out first.
Check these two things first
- Push a finger into the soil, two knuckles deep. Wet soil plus a wilting plant means too much water, not too little — and it is the commonest reason a plant is dying. Dry soil plus crisp leaves means the opposite. The leaves look nearly identical either way, so the soil is what tells you.
- Turn a damaged leaf over. Half of all diagnoses live on the underside: fuzz means downy mildew, fine webbing means spider mites, clusters of insects mean aphids. Nothing on the top of a leaf is as informative.
Most garden problems are cosmetic and resolve without treatment. Reach for a spray last, not first.
The 11 causes, and how to tell them apart
1.Fusarium Wilt
Act nowFungal disease
Yellowing and wilting that often affects one side of the plant, or one branch, while the rest looks fine. Slice the main stem lengthwise near the base and the tissue just under the skin is stained chocolate brown instead of pale green.
How to know it is this one
Cut the stem lengthwise: brown staining in the ring just under the skin confirms it.
2.Root Rot
Act nowFungal disease
Above ground, yellowing and wilting that looks like drought but does not respond to water. Below ground, roots that are brown or black, soft, and smell sour — healthy roots are firm and pale, and the outer sheath of a rotted root slides off leaving a thread.
How to know it is this one
Wilting that does not improve after watering, and often gets worse.
3.Spider Mites
Act nowPest
Fine pale stippling across the leaf, as though it had been sprayed with a very fine dust of pinpricks. Leaves go bronze or grey, and fine webbing appears between leaves and along stems in bad infestations. The mites themselves are just visible as moving specks.
How to know it is this one
Fine even stippling rather than discrete spots or holes.
4.Verticillium Wilt
Act nowFungal disease
Yellow wedges spreading inward from the leaf edges, often forming a V shape pointing at the midrib. Lower leaves go first. Cutting the stem shows brown discolouration in the vascular ring, usually lighter than fusarium's.
How to know it is this one
V-shaped yellow wedges at the leaf margins.
5.Aphids
TreatablePest
Clusters of small soft-bodied insects, usually green but also black, grey, pink or white, packed along new shoots and the undersides of young leaves. Leaves curl and pucker, and everything below becomes sticky with honeydew.
How to know it is this one
Dense clusters on new shoots and flower buds rather than scattered individuals.
6.Bacterial Leaf Spot
TreatableBacterial disease
Small dark spots that look wet or greasy, often with angular edges because they stop at the leaf veins, sometimes surrounded by a yellow halo. In humid weather the spots look water-soaked; in dry weather the centres fall out leaving small holes.
How to know it is this one
Greasy, water-soaked appearance, especially in the morning.
7.Early Blight
TreatableFungal disease
Brown spots about the size of a pencil eraser with concentric rings inside them, like a tiny archery target. The tissue around each spot turns yellow, and affected leaves eventually go fully yellow and drop.
How to know it is this one
Concentric rings inside each brown spot — the bullseye is the giveaway.
8.Nitrogen Deficiency
TreatableNutrient problem
Uniform pale yellowing of the whole leaf, including the veins, starting with the oldest lowest leaves and working upward. Growth is slow and the plant looks generally thin and pale.
How to know it is this one
Whole leaf yellows evenly, veins included.
9.Septoria Leaf Spot
TreatableFungal disease
Many small round spots, a sixteenth to a quarter of an inch across, with dark brown borders and pale tan or grey centres. Under a hand lens the centres show tiny black pinprick dots. Leaves yellow and drop from the bottom up.
How to know it is this one
Very many small spots rather than a few large ones.
10.Underwatering
TreatableGrowing conditions
Drooping leaves that feel dry and papery, crisp brown edges and tips, and soil that has shrunk away from the sides of the pot. The whole plant looks deflated, and lower leaves go crisp and yellow before dropping.
How to know it is this one
Leaves dry and crisp rather than soft and limp.
11.Whiteflies
TreatablePest
Tiny white moth-like insects, about a sixteenth of an inch, that rise in a cloud when the plant is disturbed and settle again immediately. Undersides of leaves carry scale-like immature stages and a scatter of white specks.
How to know it is this one
A cloud of tiny white insects that flies up when you shake the plant.
Still not sure?
The diagnostic key adds two more questions — which part is affected and where the trouble started — and those usually settle it.
Run the diagnostic key