When a tomato plant tells you something is wrong, it almost always names the cause itself. Curled leaves, a black sunken base, a split skin, hard green shoulders: each marking points at one specific fault, and most of those faults trace back to watering and heat rather than to any pest or disease. Reading the symptom correctly carries the whole job, because a wrong reading sends people reaching for a spray bottle when the real fix waits in the watering can.
What follows is a symptom-first guide to tomato plant problems, sorted by what you actually see when you look at the plant. Find the picture that matches yours, read the cause, then apply the fix. Carry one headline through the whole piece: the great majority of greenhouse tomato troubles are physiological disorders, swings in water and temperature, and they clear up when you steady the routine. Only one entry below describes a true disease, and you will know it by its speed.
Leaves curling or rolling upward
Upward leaf roll, where the edges curl in and the leaf turns leathery, usually has a physiological cause. It follows hot, dry spells, swings in soil moisture or heavy pruning, and it tends to show on the lower leaves first. The plant stays green and keeps growing normally, so you need no treatment at all.
No symptom triggers more pointless spraying than this one, and it deserves a moment of calm. According to Washington State University's HortSense fact sheet, physiological leaf roll "causes no apparent damage to the plants and does not reduce yield," and "no corrective actions are necessary." A rolled leaf that is still firmly green is a plant managing its own water budget, nothing more.
Colour and vigour give you the tell. If the rolled leaves still look green and the plant reaches the size you would expect, leave it alone and steady your watering. If the curl comes with yellowing, brown patches or stunting, you face a different problem, and the sections below will sort it out.
Yellowing between the veins, on the older leaves
Yellowing between the leaf veins while the veins themselves stay green is interveinal chlorosis, and on older lower leaves it points to magnesium deficiency. Whole-leaf yellowing means something else entirely. The pattern tells you more than the colour does: green veins on a yellow background, starting low on the plant, give you the magnesium signature.
Why the feed you bought caused it
One twist catches careful growers out. Magnesium shortage is often self-inflicted through generous use of high-potassium tomato feed. The RHS guidance on nutrient deficiencies explains that plants take up potassium in preference to magnesium, so heavy tomato feed can lock magnesium out even when the soil holds plenty. The harder you fed, the yellower the lower leaves went.
The fix, with rates
Magnesium sulphate, sold as Epsom salts, puts it back. The RHS gives two routes:
- To the soil: apply Epsom salts at 30 g per square metre.
- As a foliar spray: dilute 20 g of Epsom salts per litre of water, and repeat two or three times across the 14 days of a fortnight.
One caution on the spray. Apply it only when the weather is dull and overcast, because magnesium sulphate on leaves in bright sun risks scorch. A cloudy evening is your window.
Brown or black blotches that spread fast
This is the exception to everything above. Brown or black blotches that spread quickly across leaves, with a watery rot that makes them collapse and shrivel, often with fine white growth on the underside in warm wet weather, is tomato blight. Blight counts as a genuine disease, and unlike the watering faults it will not wait.
Speed gives you the diagnostic. The RHS profile of tomato blight describes a "rapidly spreading, watery rot of leaves, which soon collapse, shrivel and turn brown," most common in warm, wet weather. A nutrient problem creeps over a week. Blight can wreck a plant in days, and it loves humid, still air, which is exactly the climate a closed greenhouse creates on a muggy afternoon.
Greenhouse look-alike: leaf mould
Under glass there is a near-twin worth knowing. Leaf mould produces very similar spotting, and the way to tell them apart is the colour of the growth underneath the leaf. Blight shows white growth on the leaf underside; leaf mould shows greyish-brown growth instead. Turn a spotted leaf over and check the colour before you decide what you are dealing with. For the full picture on the disease that genuinely threatens a crop, our guide to recognising and slowing tomato blight goes deeper than this entry can.
Fruit splitting or cracking
Split, cracked or russeted fruit ranks as a physiological disorder, not a disease. Variable water supply and fluctuating temperature drive it, where a dry spell followed by a flood of water makes the inside of the fruit swell faster than the skin can stretch. The skin gives way along a ring or a line, often on the green fruit just before it colours.
Steadiness cures it. The RHS advice on splitting and cracking tells you to water so as to maintain a constant level of soil moisture, and notes this matters most when growing in growing bags, pots or other containers, where the small root volume swings between bone-dry and sodden. A skin already split cannot reseal, so the whole game lies in prevention. Pick the routine, not the rescue.
Black sunken patch on the base of the fruit
A leathery, sunken black patch on the bottom of the fruit, the end furthest from the stalk, signals blossom end rot. A lack of calcium reaching the fruit causes it, and it counts as a physiological problem, not a pest or disease. No spray will touch it, because nothing alive lives there to kill.
The counterintuitive part lies in the fix. Your compost usually holds the calcium already; the trouble is getting it to travel. The RHS account of blossom end rot puts it plainly: for calcium to reach the parts of the plant furthest from the roots "there needs to be a good flow of water through the plant," and the compost "should be kept consistently moist… and must never be allowed to dry out." This is a watering problem wearing a deficiency costume.
How to water against it
In hot weather a greenhouse or container tomato may need watering 2 or more times a day, and split watering beats one heavy dose. The RHS is specific: "It is better to water twice a day than once with a double volume." Plants in pots and growing bags sit most at risk, because their limited root space cannot buffer an irregular supply. If you grow in bags, treat the watering can as your calcium delivery system.
Hard green shoulders that will not ripen
Hard, green or yellow shoulders that stay firm while the rest of the fruit colours up are greenback and blotchy ripening, and they belong mainly to greenhouse tomatoes. The cause is excess light, high temperature and insufficient potassium. Call this one a heat and feeding fault rather than a watering one, though it shares the same root: a greenhouse running too hot.
The RHS notes on ripening problems list the trigger as "excess light, high temperatures, and/or insufficient feeding," and adds that "adequate potassium nutrition is required to reduce greenback." So two levers help. Keep the house cooler in the fierce part of summer, and make sure the plants are getting enough potassium through the fruiting period. Shade and airflow do the heavy work here; our guide to keeping a greenhouse cool in a UK summer covers vents and shading, and the order to open things up.
Flowers dropping without setting fruit
Flowers that open, yellow and fall without forming fruit are usually failing to pollinate in heat. When the greenhouse runs above roughly 30°C by day or above 21°C at night, tomato pollen turns tacky and non-viable, so the flower cannot set and drops. The fix is ventilation, not feed or fungicide.
University of Missouri's IPM team puts numbers on it: blossom drop follows daytime temperatures above 85°F and nighttime temperatures above 70°F, with the explanation of tomato fruit set noting that pollen becomes "tacky and non-viable" in that range. Cold has the same effect from the other side, with night temperatures below 55°F, around 13°C, also causing flowers to drop. A greenhouse that bakes past 30°C by midday sits well over the pollen threshold, and the cure is air moving through it. We cover the wider causes of fruit failing to set under glass in a companion piece.
Tiny white flies and sticky leaves
Clouds of tiny white insects that lift off when you brush the plant, sticky honeydew on the foliage and a black sooty mould growing on the stickiness, are glasshouse whitefly. This one counts as a pest rather than a physiological fault, yet the response runs gentler than most people expect, and reaching straight for a pesticide tends to backfire.
The RHS page on glasshouse whitefly describes the trio of symptoms, sticky honeydew, black sooty moulds and small white-winged insects, and recommends that you do not use pesticides. For biological control, growers turn to the parasitic wasp Encarsia formosa, introduced while numbers are still low. Yellow sticky traps help you monitor the population and knock back the adults. Whitefly behave a lot like the other sap-sucking greenhouse regulars, and the same calm, no-spray logic runs through our note on cucumber leaves turning yellow, which often comes down to the same handful of greenhouse stresses.
None of this needs a particular greenhouse. The growing advice works whatever structure you have, glass, polycarbonate, old or new. What does help is a house you can cool and air without a fight, because most of the faults above start with heat and stale air. The NORDIC pine and polycarbonate greenhouse is built with a rear opening window opposite the door, so cross-ventilation comes as standard and you can pull a through-draught on the hot afternoons that cook pollen and bring on greenback. If your current greenhouse can do the same, you already have what these tomatoes need.
The pattern behind nearly all of it
Step back and the same lever keeps reappearing. Even, consistent soil moisture forms the single biggest defence against both splitting and blossom end rot, because both trace to the same swing between dry and drenched. Heat sits right behind it, driving greenback, blossom drop and most leaf roll. Sort the watering rhythm and the ventilation, and you have addressed the cause of the majority of entries on this page in two habits.
So before you buy a spray, do three things this week: water to keep the soil evenly moist rather than letting it dry and flood, splitting it into 2 doses across the day in heat; open the vents early on warm mornings so the house never tips past 30°C with the doors shut; and ease off the high-potassium feed if the lower leaves are yellowing between the veins. If a holiday is coming, our guide to keeping a greenhouse watered while you are away keeps that moisture steady when you cannot. Reach for the wasp Encarsia formosa or a fungicide only once you have ruled the routine out, because far more often than not the plant was asking for water, not medicine.
