Plant diseases — identifying them from the symptoms

Most problems start the same way: something appears on the plant that was not there yesterday — a coating, a spot, a curling leaf. The name of the disease comes only later. This guide is built the other way round from a typical encyclopedia: it leads from what you see to a specific pathogen and to the decision about what to do with the plant.

Before you reach for a spray, though, it is worth knowing that a good share of reported “diseases” are not diseases at all. Overwatering, a nutrient deficiency, sunscald and the feeding of small pests can look deceptively like a fungal infection — and they are dealt with in a completely different way. We come back to this in the section on telling a disease from a growing mistake.

Quick diagnosis — from symptom to disease

Symptom	Most often on	Likely disease
White, floury coating on the upper leaf surface	roses, cucumbers, squash, apple trees	Powdery mildew
Coating on the leaf underside, yellow angular patches above	cucumbers, onions, grapevines	Downy mildew
Grey, fluffy coating on flowers and fruit	strawberries, peonies, roses	Grey mould
Rusty, dusty clusters of spores on the leaf underside	roses, hollyhocks, pears	Rust
Black spots with a ragged edge, leaves dropping	roses	Rose black spot
Olive, velvety blotches; corky cracking on the fruit	apple trees	Apple scab
Brown spots in concentric rings, starting on lower leaves	tomatoes, potatoes, carrots	Alternaria leaf spot
Dark, sunken lesions; pinkish slime in the centre	beans, peppers, squash, strawberries	Anthracnose
Spreading dark blotches, whitish bloom underneath	potatoes, tomatoes	Late blight
In spring, puckered, thickened, red leaves	peaches, nectarines	Peach leaf curl
Brown fruit rot with beige spore cushions	apple, sour cherry, plum trees	Brown rot
Wilting despite moist soil, brown vascular tissue in the stem	tomatoes, cucumbers, carnations	Fusarium wilt
White, cottony mycelium at the base; hard black lumps	lettuce, beans, cucumbers	White mould
Galls and swellings on the roots	cabbage, cauliflower, radish	Clubroot

Below are the same diseases, but described from the angle of how to tell them apart — because the symptoms in the table can look quite similar to one another.

Coatings — white, grey, rusty

The side of the leaf the coating sits on is the most important piece of diagnostic information.

Powdery mildew produces a white, floury coating on the upper side of leaves, shoots and buds; the leaves yellow, become deformed and drop early. It develops in warm weather and high air humidity, but without the leaves needing to be wet. It is treated with sulphur-based or triazole products, and with a light infection, plant-based preparations and the removal of affected parts are enough.

Downy mildew is an entirely different group of organisms: the coating — greyish-violet or brown — sits on the underside of the leaf, while the upper surface shows yellow patches clearly bounded by the veins, which makes them look angular. It is favoured by moisture and cool nights, and is particularly dangerous for cucumbers, onions and grapevines. Products for powdery mildew will not work here — you need preparations intended for downy mildews.

Grey mould is a grey-brown, fluffy mat of mycelium on flowers, fruit and shoots, preceded by watery, rotting patches. It attacks weakened and damaged tissue above all — hence its fondness for fading petals, bruises on fruit and crowded plantings. It spreads very fast, so affected parts are removed straight away, not “when you get round to it”.

Rust produces rusty-orange, dusty clusters of spores on the leaf underside, matched by yellow patches on the upper surface. Some rusts are host-alternating — they need an intermediate host; pear rust, for instance, spends part of its cycle on juniper. If rust comes back every year despite your treatments, check what is growing nearby.

Spots on the leaves

Rose black spot — black or dark brown spots with characteristically ragged edges, with the leaf blade yellowing around them. The leaves drop en masse, which weakens the bush and reduces flowering. The key measure is not spraying but tidying up: fallen leaves are the source of infection in the following season, so autumn raking breaks the disease cycle.

Apple scab — olive-green, velvety, diffuse blotches on the leaves, and dark corky patches on the fruit that crack and deform it as the fruit grows. Treatments are applied preventively, in spring, during the period of primary infection (professionally according to the Mills table, that is, temperature thresholds and the duration of leaf wetness); once the spots have appeared, spraying will change little that season.

Alternaria leaf spot — brown spots with concentric rings resembling a shooting target, often with a yellow halo, progressing from the oldest, lowest leaves upwards. On fruit and tubers it forms dry, sunken rotten patches.

Anthracnose — dark, sharply defined and sunken lesions; in wet weather a pinkish, slimy mass of spores appears in the centre of them. Characteristic of beans, peppers, cucurbits and strawberries in warm, wet summers.

Potato and tomato late blight — dark brown, rapidly spreading blotches, with a whitish bloom on the leaf underside in damp conditions, plus brown streaks on the stems. In warm, rainy summers it can destroy a crop in a matter of days, which is why prevention counts, not reaction.

Peach leaf curl stands out in that the symptom is not so much a spot as a deformation: in spring the young leaves become thickened, puckered and reddish, with blistered bulges. Here the timing of treatment is absolutely critical — you spray before bud burst, at the stage when the buds are swelling (a further treatment in autumn, after leaf fall, is also worthwhile). Once the leaves have unfolded, spraying is useless and you have to wait until the following spring.

Wilting despite watering

The plant is going limp even though the soil is moist? That is usually a sign that water is not reaching the leaves — the problem lies in the roots or in the vascular tissue.

Fusarium wilt starts in the soil: the fungus enters through the roots into the vascular bundles and blocks the flow of water. The plants wilt first on hot days, often on one side only or from the lower leaves upwards, then yellow and die. The decisive test is simple: cut the stem across — a brown discolouration of the ring of vascular tissue confirms the diagnosis.

White mould attacks at the base and wherever the plant is in contact with the soil: a watery, soft rot appears, covered with dense, white, cottony mycelium, and in the tissue and on it — hard black sclerotia the size of a grain. Those black lumps are a diagnostic feature that cannot be confused with anything else.

It is worth remembering that clubroot also causes wilting in the heat — but in brassicas, and with a completely different picture at the roots (see below).

Rotting fruit

Brown rot, or Monilinia rot, produces spreading brown rotten patches on apples, cherries and plums, with concentrically arranged, greyish-beige cushions of spores. Affected fruit dries out and mummifies, staying on the tree until the following season — and it is precisely these mummies that are the main source of spring infection. The fungus also attacks flowers and shoot tips, causing them to die back. Protective treatments are applied during flowering and before harvest; it is equally important not to let the skin be damaged, because every wound is a gateway for infection (which is why controlling codling moth and scab indirectly limits brown rot).

On strawberries and soft fruit, grey mould dominates; on tomatoes in wet summers it is late blight, in the form of hard, brown rots; and on beans and peppers, anthracnose with sunken, rotting patches.

Symptoms below ground

Some diseases cannot be identified without lifting the plant out of the soil.

Clubroot forms galls and swellings on the roots of cabbage, cauliflower, oilseed rape and radish, distorting the entire root system. The plant grows poorly, yellows and wilts in the heat, because the deformed roots cannot take up water. When the galls start to rot, they release vast quantities of resting spores into the soil.

With fusarium wilt the roots and the base of the stem turn brown, and with white mould black sclerotia are left in the soil around the plant, surviving there for many years.

Fungus, bacterium or a mistake in care?

Most of the “diseases” reported by beginner gardeners are growing mistakes. Before you buy a fungicide, check three things.

Is the symptom progressing and spreading? An infectious disease develops over time: it takes over more leaves, moves onto neighbouring plants, the spots grow larger. A growing mistake usually “stands still” and affects all the leaves to a similar degree.

Can you see a structure, not just a change of colour? A coating, dusty clusters of spores, spore cushions, slime, a distinct margin around the spot — these are signs of a pathogen. A change of colour on its own, with no structure at all, more often means a physiological problem.

What are the roots and the soil doing? Overwatering is the most common cause of symptoms mistaken for a disease: the lower leaves yellow evenly, the plant goes limp despite wet soil, and the roots are brown and soft. If you suspect a watering mistake, start with the guide to watering, not with the sprayer.

Common mix-ups that are not diseases:

• Sunscald — pale, dried-out, papery patches on the leaves most exposed to the sun, usually after the plant has been moved suddenly. It does not spread to other leaves.
• Nitrogen deficiency — even paling and yellowing starting from the oldest, lowest leaves, with no spots and no coating.
• Overfeeding or a salty growing medium — dry, brown leaf edges, with a sharp rather than a blurred margin.
• Spider mites — fine pale speckling from their feeding punctures, the leaves gradually greying and drying out, and where the infestation is heavy, a delicate webbing is visible. They are favoured by warm, dry air, so this is the classic problem of houseplants next to radiators in winter.
• Thrips — silvery streaks and patches on the leaves plus tiny black dots of excrement; that combination distinguishes them from a fungal infection.
• Aphids — the insects themselves are visible to the naked eye, but they excrete sticky honeydew on which a black coating of sooty mould develops. That coating is often taken for a leaf disease, when in fact it does not attack the plant’s tissue and disappears once you get rid of the aphids.

What cannot be cured

The honest answer is: some diseases are not treated at all, and attempts to save the plant only increase the contamination of the soil.

• Fusarium wilt — the pathogen sits inside the vascular bundles, where fungicides essentially do not reach. The plant is removed together with its root ball; under cover, the growing medium is replaced or disinfected.
• Clubroot — incurable on an infested site. Diseased plants are removed, the soil is limed (the pathogen is held back above roughly pH 7.2) and the drainage improved; where infestation is heavy, brassicas are given up on that spot for many years.
• White mould — affected plants are removed along with the surrounding soil, because sclerotia stay viable in it for years.

In these cases the only effective strategy is crop rotation, a change of site and resistant varieties — not spraying.

Do not compost affected plant material in the case of white mould, clubroot, fusarium wilt, late blight and brown rot. A garden compost heap does not reach the temperature that kills sclerotia and resting spores, so by spreading it around the garden you sow the disease onto new beds. Take such material off the plot.

Prevention — five rules that work against most diseases

1. Air movement. Crowding is the most common cause of recurring fungal infections. Keep the recommended spacing when planting, thin out tree crowns and the interiors of shrubs, and remove weeds that trap moisture at ground level.
2. Water at the root, in the morning. Leaves that stay wet all night are the starting condition for most pathogens. Water at the base and do not wet the leaves, flowers or fruit — especially not in the evening.
3. Crop rotation. With soil-borne diseases this is the basic measure: brassicas return to the same spot after 5–7 years at the earliest, and tomatoes and potatoes should neither follow one another nor grow right next to each other.
4. Hygiene of tools and material. Disinfect your secateurs between plants, especially when cutting affected shoots. Buy healthy, certified young plants and seed, and wash pots used for diseased plants before using them again. Do not work among wet foliage — from damp leaves, spores travel on clothing and hands.
5. Clearing up after the season. Rake up and remove fallen rose and apple leaves, collect mummified fruit from the trees and from the ground beneath them, and clear away crop debris. A large share of pathogens overwinters exactly there — not in the air.

On top of that comes the choice of variety: with scab, the mildews, clubroot and fusarium wilt, resistant or barely susceptible varieties make a bigger difference than any spray programme.

Check the details in the disease cards

Each of the diseases described has its own card in Atlas-Flora, with the pathogen, a full description of the symptoms, control and prevention — the complete list is in the disease encyclopedia. Plant cards, in turn, include a threats section, so before planting a new species you can check what it will be exposed to on your own plot.