When a vegetable plant looks wrong, the symptom is rarely the whole story. Yellow leaves can mean too much water, too little water, a nutrient shortage, or a disease. A tomato plant with no fruit can be too cold, too hot, over-fed, or simply not yet pollinated. The skill of troubleshooting is not knowing every disease by name — it is working through the likely causes in order, from the simplest and most common to the rarest, and fixing the thing that is actually wrong rather than the thing you guessed first.
This guide gives you that order. For each common problem it lists the likely causes, how to tell them apart, and what to do. Use the pest identifier helper and the plant disease checker to narrow down pests and diseases, and check your local extension service if a problem does not match anything here — they can diagnose from a sample or a photo, and they know which problems are common in your area this season.
Yellow leaves: water, nutrients or disease
Yellow leaves are the most common vegetable complaint, and the cause is usually water or food, not disease. If the lower leaves yellow first and the plant is growing in wet soil, the roots are probably waterlogged — roots need air as well as water, and in soggy soil they suffocate. Improve drainage, stop watering until the surface dries, and mulch to keep the soil structure open. If the whole plant is pale yellow and growth is slow, it is probably short of nitrogen — feed with a balanced liquid feed or a nitrogen-rich organic feed such as fish emulsion, and see our fish emulsion fertilizer guide. If the yellowing is between the veins while the veins stay green, the plant is likely short of iron or magnesium, which often happens in alkaline soil — our guide to soil pH testing explains how to check.
Disease is less common but possible. If yellow leaves come with spots, wilting, or a pattern that spreads from one plant to its neighbours, suspect a disease: early blight tomatoes and fusarium wilt both yellow tomato leaves, and downy mildew yellows cucumbers and brassicas. Remove affected leaves, improve airflow by spacing plants properly, avoid wetting the leaves when you water, and do not compost diseased material. If you cannot tell whether it is a nutrient problem or a disease, a photo and a sample taken to your local extension service will settle it faster than guessing.
Plants not growing, or growing slowly
A plant that sits still for weeks is usually cold, hungry, or root-bound. Cold soil is the most common cause in spring: tomatoes, peppers and cucumbers planted out too early into cold soil simply stop growing, and they may never catch up with plants set out later into warm soil. There is no fertiliser that fixes cold — wait for warmth, or protect the plants with a cloche. Hunger is the second cause: a bed that has grown vegetables for years without compost will be low in nutrients and organic matter, and plants in it will be small and pale. Fork in compost, feed, and test the soil through your local extension service to find out what is actually missing.
Other causes are worth checking. A plant in a small pot may be root-bound, with roots circling the pot and no room to grow — pot it on into a larger container. A plant in compacted soil cannot push its roots out — loosen the soil with a fork and add compost, and see our guide to clay soil improvement. And check the light: a vegetable in shade grows slowly no matter how well it is fed. If a plant is slow and its neighbours are fine, look at what is different about its spot — the light, the drainage, or the soil.
No fruit, flowers dropping, or poor pollination
A healthy-looking plant that flowers but sets no fruit is frustrating, and the causes are usually weather or feeding. Tomatoes drop their flowers when nights are too cold or days are too hot, and they set little fruit in either extreme — there is nothing to do but wait for the weather to settle, and to keep watering steady. Too much nitrogen fertiliser gives lush, leafy plants with few flowers: if your tomato plant is a jungle of leaves with no fruit, stop feeding nitrogen and switch to a high-potassium feed once flowers appear — see our fertilizing vegetables guide. Cucumbers and zucchini produce male flowers first, and female flowers — the ones with a tiny fruit behind them — appear later, so a plant with flowers but no fruit in its first weeks is normal.
Pollination is the other cause. Squashes, cucumbers and sweetcorn need insects or wind to move pollen, and in a garden with few bees, or in a greenhouse with the doors shut, fruit may fail to set. Open greenhouse doors and vents during the day, plant flowers nearby to attract pollinators, and for squashes you can hand-pollinate: pick a male flower, strip the petals, and touch its centre to the centre of a female flower in the morning. If beans drop their flowers, heat or dry soil at the roots is usually the cause — water deeply and mulch.
Bolting, bitter leaves and splitting roots
Bolting — a plant sending up a flower stalk and turning bitter — is the plant's response to heat or stress. Lettuce, spinach, radishes and cilantro bolt when temperatures rise, and once a plant has bolted there is no way to reverse it. Prevention is everything: sow early, choose bolt-resistant varieties, give afternoon shade, mulch to keep the soil cool, and sow a little every two weeks so there is always a fresh sowing coming. Bitter leaves in lettuce and cucumbers also come from dry soil — keep watering steady, and pick cucumbers young, before bitterness develops.
Splitting is a water problem. Carrots, radishes and tomatoes split when dry soil is suddenly soaked — the inside grows faster than the skin can stretch. Water evenly, mulch to hold moisture, and do not let the beds dry out completely and then flood them. Blossom end rot on tomatoes is the same kind of problem: it is caused by uneven watering interfering with calcium uptake, not by a lack of calcium in the soil, and steady watering is the cure. For more on watering, see our watering calculator.
Wilting, rotting and sudden collapse
A plant that wilts in the heat of the day and recovers by evening is usually just thirsty — water deeply and mulch. A plant that wilts and does not recover is a different problem: it may have root damage, a stem borer, or a wilt disease. Check the stem at soil level for damage — cutworms chew through young stems at night, and our cutworms control guide covers them. Check for squash vine borer in zucchini and pumpkins: the stem swells and sawdust-like frass appears at the base. If the plant wilts from one side, or the leaves yellow and wilt from the bottom up, suspect fusarium wilt or bacterial wilt — both are soil-borne, there is no cure, and the plant should be removed and disposed of in household waste, not composted.
Rotting at the base of the stem, or seedlings that collapse at soil level, is usually damping off or root rot, both caused by wet, cold, poorly drained soil. Improve drainage, do not overwater, and sow into fresh compost. Our guides to damping-off seedlings and root rot treatment cover both in detail. If a whole bed collapses suddenly, check the drainage first and the watering second — most sudden collapses in vegetables are water problems, not pests.
Pests and holes: who is eating your vegetables
Holes in leaves have many authors, and the pattern tells you who. Small round holes in cabbage and radishes are usually flea beetles — see our flea beetles guide. Large ragged holes and slime trails are slugs and snails, worst in damp weather — see slugs and snails control. Caterpillars leave large holes and dark droppings, and you can usually find the culprit on the underside of the leaf — cabbage worms are the classic example. Tiny stippled leaves with fine webbing underneath are spider mites, worst in hot, dry weather — see spider mites on plants. And if whole seedlings vanish overnight, suspect cutworms or birds, and protect young plants with collars or netting.
Do not spray first and diagnose later. Identify the pest, check whether the damage is actually serious — a few holes in a kale leaf do not matter, but a caterpillar infestation on young cabbage does — and start with the least harmful control: hand-picking, a jet of water, barriers, and encouraging predators. Our pest identifier helper can help you work out what you are dealing with, and your local extension service can confirm an identification from a sample.
Common mistakes
The biggest troubleshooting mistake is treating the symptom instead of the cause — spraying a plant whose real problem is wet soil, or feeding a plant whose real problem is cold. The second is changing three things at once, so you never learn which one fixed it: change one thing, wait a week, and see. The third is ignoring the pattern: a problem on one plant is likely a local issue, while the same problem on every plant in the bed is likely water, soil or weather. The fourth is composting diseased material, which spreads the disease to next year's beds. And the fifth is waiting too long — most vegetable problems are easy to fix in the first week and hard to fix in the fourth, so check your plants every few days, keep notes on what you see, and ask your local extension service early rather than after the crop is lost.
| Symptom | Most likely cause | First thing to check |
|---|---|---|
| Yellow lower leaves | Waterlogged soil | Drainage; let soil dry before watering again |
| Whole plant pale and slow | Nitrogen shortage | Feed with balanced liquid feed; test soil |
| Flowers but no fruit | Heat, cold or excess nitrogen | Weather; stop nitrogen, switch to potassium feed |
| Bolting, bitter leaves | Heat and dry soil | Shade, mulch, steady water, earlier sowing |
| Splitting fruit and roots | Uneven watering | Water deeply and evenly; mulch |
| Wilting that does not recover | Root damage or wilt disease | Stem at soil level; remove diseased plants |
| Holes in leaves | Slugs, caterpillars, flea beetles | Look under leaves; identify before spraying |
Frequently asked questions
Why are my vegetable leaves turning yellow?
Yellow leaves are usually a water or nutrient problem: waterlogged soil suffocates roots, while pale, slow growth suggests a nitrogen shortage. Yellowing between green veins often points to iron or magnesium shortage in alkaline soil. If yellowing comes with spots or spreading wilting, suspect a disease and check with your local extension service.
Why is my tomato plant not producing fruit?
Tomatoes drop flowers when nights are too cold or days are too hot, and too much nitrogen fertiliser gives leafy plants with few flowers. Keep watering steady, stop nitrogen feeding once flowers appear, switch to a high-potassium feed, and wait for the weather to settle.
Why did my lettuce bolt?
Lettuce bolts in heat — it sends up a flower stalk and turns bitter, and it cannot be reversed. Sow early, choose bolt-resistant varieties, give afternoon shade, mulch to keep the soil cool, and sow a little every two weeks.
What causes blossom end rot on tomatoes?
Blossom end rot is caused by uneven watering interfering with calcium uptake, not by a lack of calcium in the soil. Water deeply and evenly, mulch to hold moisture, and avoid letting the soil dry out completely and then flooding it.
My seedlings keep collapsing at soil level — why?
That is usually damping off, a fungal disease of cold, damp, poorly drained compost. Sow into fresh compost, do not overwater, give seedlings light and airflow, and use clean pots and trays. See our damping-off seedlings guide for the full prevention steps.