Common plant diseases: how to spot and stop fungal and bacterial problems

Fungus, water mould or bacteria?

A disease needs three things at once: a susceptible plant, the organism that causes it and the right weather. Take away one and the disease stalls. You can't change the weather, but you can choose tougher varieties and make the conditions around the leaves less inviting.

Powdery mildew

How to recognise it: a white to grey, flour-like dusting on the upper side of leaves, later on stems and buds. Old leaves turn yellow and crisp. It is very common on courgettes, pumpkins, cucumbers, roses and peas, especially in late summer.

What favours it: unlike most diseases, it does not need wet leaves. Dry soil, humid air at night and crowded plants are enough.

Gentle control:

• Remove the worst leaves as soon as you see them, but don't strip more than about a third of the plant at once.
• Keep the soil evenly moist; drought-stressed plants suffer more.
• Thin crowded growth and avoid heavy nitrogen feeding, which produces soft, prone shoots.
• Late in the season on squash, a light coating is often tolerable; the fruit usually ripens anyway.

Downy mildew

How to recognise it: pale yellow or angular patches on the upper leaf surface, bounded by the veins, with grey-purple fuzz on the underside. Leaves later brown and die. Cucumbers, melons, basil, lettuce, onions and grapevines are frequent victims.

What favours it: cool nights, dew and leaves that stay wet for hours. It is a water mould, not a true fungus, and can move very quickly in a wet spell.

Gentle control:

• Remove and bag infected leaves or whole plants promptly, ideally on a dry day.
• Water at the base in the morning so foliage dries before evening.
• Space plants widely and grow basil in a sunny, breezy spot.
• Choose varieties described as tolerant or resistant to downy mildew; this is the most effective step for cucumbers and basil.

Grey mould (Botrytis)

How to recognise it: soft brown rot covered with fluffy grey fur that releases a puff of dust when touched. It attacks flowers, fruit (strawberries, tomatoes, grapes), seedlings and damaged tissue, and is common in greenhouses and on overwintering plants.

What favours it: cool, damp, still air and dead or wounded plant parts, where it gets a foothold before moving into healthy tissue.

Gentle control:

• Pick off fading flowers, damaged fruit and dead leaves regularly; this is the single most useful habit.
• Ventilate greenhouses and cold frames on mild days, even in winter.
• Mulch strawberries with straw so fruit doesn't sit on wet soil.
• Make clean cuts just above a bud; ragged stubs die back and invite infection.

Leaf spots and rust

Leaf spots are a large group of fungal and bacterial diseases. Typical signs are round or irregular brown, black or purple spots, sometimes with a pale centre or a yellow ring. Rose black spot is a well-known example. Spots usually begin on lower leaves, where splashed soil lands first.

Rust shows as orange, yellow or brown powdery pustules, usually on the leaf underside, with pale flecks on top. It is common on roses, mint, beans, garlic, leeks and hollyhocks.

Gentle control:

• Remove spotted leaves early and clear fallen ones from the ground; many of these diseases overwinter on leaf litter.
• Mulch around plants to stop soil splashing onto foliage.
• For mint and other perennials badly hit by rust, cut down to the ground and dispose of the foliage; new growth is often clean.
• If a bacterial spot is suspected, avoid working among wet plants and disinfect tools between plants.

Early and late blight

Both hit tomatoes and potatoes but behave differently.

• Early blight (a fungus): brown spots with concentric rings like a target, often with a yellow edge, starting on the lowest, oldest leaves and climbing upward. It builds slowly over the summer.
• Late blight (a water mould): large, dark, greasy-looking patches on leaves and stems, white fuzz on the underside in damp weather, and firm brown rot on fruit and tubers. In cool, wet weather it can destroy a planting within days.

Gentle control: remove lower leaves up to about the first fruit truss, mulch the soil, stake plants for airflow and water at the base. Late blight is different: if you see it, pull and bag the affected plants straight away and warn nearby growers, because spores travel far on the wind.

⚠️ Late-blight plants and tubers must not go on the compost heap or be left lying in the garden; bag them or bin them.

Damping-off in seedlings

How to recognise it: seedlings that were fine yesterday collapse at soil level; the stem looks thin, dark and pinched. Sometimes seeds rot before they even emerge, leaving gaps in the tray.

What favours it: cold, wet compost, crowded sowings, poor light and reused, dirty trays. Several soil organisms are involved.

Prevention:

• Use fresh seed-starting mix, never garden soil.
• Wash and rinse trays and pots before reuse.
• Sow thinly, water from below and let the surface dry slightly between waterings.
• Give plenty of light and some air movement; a small fan on low helps.

Collapsed seedlings cannot be saved. Remove them with the compost around them and resow in clean mix.

Prevention: the habits that matter most

• Airflow: follow the spacing on the seed packet (for tomatoes roughly 18–24 in, estimated), prune crowded growth and keep weeds down.
• Morning watering at the base: leaves dry quickly in daylight; evening sprinkling keeps them wet all night.
• Crop rotation: don't grow the same plant family in the same bed for at least 3 years (tomato, potato, pepper and aubergine count as one family).
• Resistant varieties: catalogue codes or notes such as "mildew tolerant" or "blight resistant" are worth seeking out.
• Healthy plants, sensible feeding: balanced feeding; too much nitrogen gives soft growth that diseases love.
• Clean start: buy healthy-looking transplants and certified seed potatoes.
• Mulch: a layer of straw or compost stops soil from splashing spores onto leaves.

If you decide a fungicide is necessary, choose one approved for home gardens and that crop, and follow the label exactly for rate, timing and harvest interval.

Step-by-step garden hygiene

1. Walk the garden once or twice a week and look under the leaves, especially in humid spells.
2. When you find a diseased leaf, cut it off on a dry day, when spores spread less.
3. Drop removed material straight into a bag, not onto the ground.
4. Wipe or dip pruners in household disinfectant or 70% alcohol between plants when a disease is present.
5. Wash your hands before moving on to healthy plants.
6. At the end of the season, clear away all plant debris from beds that were affected.
7. Seal diseased material in a bag for household waste or burn it where allowed. Hot, well-managed compost can handle mildly affected leaves, but blight, rust and bacterial material should stay out of a typical home heap.
8. Clean stakes, pots and trays before storing them for next year.
9. Note which beds and varieties had problems, so you can rotate and choose better next season.

Quick diagnosis table

Frequently asked questions

Plants often affected by these diseases

ZucchiniTomatoCucumberRoseBasilStrawberryPumpkinPotato