Something has gone wrong with your tomatoes and you want to know whether to panic. The honest answer is that most of what goes wrong with home-grown tomatoes is not a disease at all. Black bottoms, split skins, curled leaves and hard green shoulders are what the RHS calls physiological disorders: the plant reacting to uneven water, heat, light or feeding. There's nothing to spray, and the next trusses usually come good once you fix the cause. The one problem that really does need you to move quickly is blight.
01At a glance
- Black, sunken patch on the base of the fruit: blossom end rot. Uneven watering, not a lack of calcium in your compost.
- Split or cracked skins: uneven water supply or big temperature swings. Pick promptly after heavy rain.
- Brown, collapsing leaves and stems in a warm, wet spell: blight. Remove it and don't compost it.
- Curled leaves in early summer: usually cold nights after hot days. It tends to pass on its own.
- Hard green shoulders that never ripen: greenback. Too much heat and light, or too little potash.
02Blossom end rot: a watering problem, not a soil problem
You'll see a circular patch at the end of the fruit opposite the stalk, greenish brown to black, which sinks as it grows. The RHS is clear that it's caused by a lack of calcium in the fruit, not in the compost. Soils and composts rarely run short of calcium. The problem is getting it to the fruit, and that depends on a steady flow of water.
- Water consistently and never let the compost dry out. In hot weather pots and growbags can need watering twice a day, and the RHS says two waterings beat one big soak.
- If you can't keep up, use bigger containers, grow in the border, or set up automatic watering.
- Only feed onto moist compost. Fertiliser on dry compost makes it worse.
Affected fruit can't be saved, so pick it off. The RHS says calcium sprays don't work, because fruit absorbs calcium poorly. Container plants are hit hardest.
03Split and cracked fruit
After a dry spell, a soaking makes the flesh swell faster than the skin can stretch. Wide temperature swings add corky scars. There's no chemical fix.
- Keep moisture even, especially in pots and growbags, where supply swings most.
- Pick ripening fruit promptly after heavy rain so fewer of them split.
- Use split fruit quickly. Splits let grey mould in.
- Late in the season, cooler nights make some splitting hard to avoid. Pick the fruit and ripen it indoors somewhere warm.
04Blight: the one to act on today
Late blight is the same pathogen that attacks potatoes, and in the UK outbreaks can start from June, usually earliest in the South West. It arrives with warm, wet weather.
Is it blight? Look for a rapidly spreading, watery rot on the leaves, which then collapse, shrivel and turn brown. In humid weather you may see white growth on the undersides. Brown patches appear on stems, green fruit gets water-soaked patches that turn brown, and ripening fruit rots fast. Outdoor tomatoes are at most risk. Greenhouse plants are less likely to catch it, but once it gets in it spreads very quickly.
- Pick off affected leaves. It only slows blight slightly, but buys time.
- Use sound fruit from affected plants quickly, in sauces or chutney.
- Don't compost blighted plants. Use the council green waste collection, burn them, or bury them deeply. The RHS warns that spores may survive home compost heaps, while commercial systems run hot enough to kill them.
- Clean canes, tools and greenhouse glass, and don't reuse compost from a blighted greenhouse crop.
- Rotate. Keep outdoor tomatoes off the same ground for at least four years, and keep potatoes off it during that time too.
There are no fungicides available to home gardeners for blight. Prevention comes down to choosing varieties. The RHS reports that 'Crimson Crush' and 'Mountain Magic' did very well at Wisley in the bad blight year of 2021, but warns that even resistant varieties may succumb in prolonged wet weather.
05Leaves and ripening: less serious than they look
- Curled leaves: usually cold nights after warm days in early summer. It looks alarming, but the RHS says it isn't a serious cause of crop loss and normally clears up as the nights warm. Aphids can also cause curling, so look under the leaves.
- Yellow between the veins of older leaves: usually magnesium shortage, often from overdoing high-potash feed. If it stays on the older leaves and the plant is vigorous, the RHS says it's no cause for concern.
- Greenback and blotchy ripening: too much heat and light, or too little potash. Shade the greenhouse, feed regularly at the label rate, and don't strip leaves above the lowest truss. 'Alicante', 'Shirley' and 'Craigella' resist greenback.
Before handling the plants or growing them where children or pets play, check the safety flags on our tomato profile and potato profile.
How we checked this
Every care and safety claim on this page is verified against horticultural and veterinary sources.
Sources: RHS — Blossom end rot · RHS — Tomato blight · RHS — Potato and tomato blight · RHS — Tomatoes: fruit splitting and cracking · RHS — Tomatoes: leaf problems · RHS — Tomatoes: fruit ripening problems · RHS — Nutrient deficiencies
Checked 8 October 2026. If our advice ever conflicts with the RHS, believe them and tell us.