Yellow leaves on a tomato plant is a symptom, not a diagnosis. That distinction matters, because the same yellow can mean five different things and only two of them are diseases. Treat the wrong one and you either waste a fortnight on a spray that was never needed, or you leave a plant in the ground that is going to infect everything beside it.
The pattern tells you which one you are dealing with. Where the yellow starts matters. So does the shape. Uniformly pale and yellow-between-green-veins are different problems, and telling them apart is most of the diagnosis.
Start with what normal looks like
Tomato plants lose their oldest leaves as they grow. The lower leaves, the ones near the soil, yellow and drop as the plant moves its resources upward. On a mature plant in August that is completely ordinary.
So the first question is not whether there are yellow leaves. It is whether the yellow is spreading. Is it climbing upward? Is it on new growth? Is it moving faster than the plant is growing? Those are the cases worth acting on.
Yellow between the veins: magnesium
This is the single most common non-disease cause, and the most reliably diagnosable. Magnesium is mobile in the plant, so when it is short the plant moves it out of the oldest leaves and into new growth. The result is a lower leaf that is yellow across the blade with the veins still clearly green. The effect is striking once you have seen it.
It is common in containers and in sandy soils, because both leach magnesium. A foliar feed of Epsom salts is the usual quick response. It greens the leaf up within a week or so. But that leaf is already lost. The point is to stop it moving to the next one.
The lasting fix is the soil. Compost, and a test to check where your pH sits, does more than any spray. Clemson’s guide to soil testing covers how to sample properly, which is the step most people skip.
Uniformly pale and small: nitrogen
Nitrogen deficiency looks different. The whole leaf goes pale, including the veins, and the plant’s growth slows and thins out. It usually starts low and works up, because nitrogen is mobile too.
In open ground this is rare unless the soil is genuinely poor. Or the plant is waterlogged, which brings us to the next point. Nitrogen washes out of the root zone with heavy watering. So a hungry-looking plant in a wet season is often an overwatering problem wearing a nutrient costume.
The watering problem, which is the most commonly misdiagnosed
Overwatering and underwatering both produce yellow leaves, and the symptoms genuinely overlap. Both cause the plant to wilt, and both cause yellowing that starts low.
The way to tell them apart is not the leaf. It is the soil. Push a finger in to the second knuckle. Soggy, and you are overwatering. Bone dry, and you are under. That test settles it in five seconds. Do it before you conclude anything else.
Overwatered tomatoes are the more common problem in containers. The roots cannot take up oxygen. They rot. And a rotting root cannot deliver nutrients. So the plant goes yellow in a way that looks exactly like starvation.
I grow mine in raised beds partly for this reason. Free drainage removes most of the ambiguity.
The diseases that actually kill a tomato plant
Three of them matter, and the difference between these and the manageable ones is that you cannot cure these. You can only remove the plant.
Late blight is the one that destroyed the Irish potato crop. It does not spare tomatoes. It appears as greasy grey-green patches that turn brown and black, and it moves fast. In humid weather a healthy plant can be dead in a week. There is no treatment once it is established. None.
Fusarium wilt and Verticillium wilt are soil-borne and work from the inside. The plant wilts in the heat and recovers overnight, for a while, and then does not recover. If you cut the stem of a fusarium-infected plant near the base, you find brown discolouration in the vascular tissue.
Clemson maintains a guide to tomato diseases with the symptom distinctions, and it is worth reading before you pull anything up, because wilt can also simply be dry soil.
Do not compost any of these. Bin it or burn it. A home compost heap does not reliably reach the temperature that kills blight spores. Compost it and you reinfect the bed next season.
The manageable diseases
Early blight and the leaf spots are a different proposition. They damage yield rather than destroying the plant, and you can slow them down.
Early blight gives concentric brown rings in the leaf, like a target. It usually starts on the oldest leaves. Then it spreads upward as the season goes on.
Septoria leaf spot produces many small circular spots with pale centres and dark margins. It comes up from the soil on splashing water.
Both overwinter in plant debris. That is why cleanup matters more than spraying. Remove affected lower leaves. Do not compost them. UC IPM’s note on powdery mildew and foliar disease is a useful adjacent reference for how these spread, because airflow and leaf wetness drive all of them.
Prevention is genuinely the treatment
None of this is complicated. It is just dull, consistent, and it works.
Space the plants properly. A tomato plant needs air moving through it. Crowded plants stay humid inside the canopy. That is where blight starts.
Water at the base, in the morning. Splash from the soil carries spores up onto the lower leaves, which is the whole mechanism of septoria. A drip line or a soaker hose under mulch removes the splash entirely.
Prune the lower leaves. Anything within about 30 cm of the soil is going to get splashed. Removing it opens the airflow and removes the first rung of the ladder.
Mulch. A layer under the plant does two things: it blocks the splashing and it keeps soil moisture even, which heads off the split fruit that follows a heavy watering after a dry spell.
Rotate. If you had fusarium in a bed, do not put tomatoes there for at least four years. That is the only defence against a soil-borne wilt. There is no other.
Keep your tools clean. Pruning an infected plant and then the next one spreads disease mechanically. Wipe the blades.
Grow resistant varieties. The letters after a tomato variety name are not decoration. An F or V in the code means resistance to fusarium or verticillium, and choosing those is the cheapest insurance available. RHS’s guide to growing tomatoes covers variety selection and the growing conditions that keep plants healthy in the first place.
What I do when I am not sure
Check the soil first. That is free, and it settles the most likely cause. Then check which leaves are affected. Old ones means a mobile nutrient or a root problem. New ones means something else entirely.
If the plant wilts in the heat and recovers at night, and the soil is moist, I take it out. Wilt that behaves like that is generally soil-borne. Waiting to be sure costs you the whole bed.