Guides
Plant problems
Identify the cause before you treat it. Start with the symptom, confirm what is causing it, then choose the simplest useful action.
Pests
These problems are caused by insects, mites, slugs, snails, birds and mammals. Each page shows the damage, explains what to do now, and gives prevention steps that support natural predators where they help.
Aphids
Aphidoidea
Aphids cluster on soft shoots, buds and leaf undersides, leaving sticky honeydew and distorted new growth. Most colonies can be removed by hand, water or pruning without spraying.
Apple maggot fly
Rhagoletis pomonella
A network of brown winding trails through the flesh of a picked apple, made by a fly that switched from wild hawthorn to orchard apple within living memory of European settlement. A pest confined to North America, distinct from codling moth despite sharing a common name in casual use, and one of the clearest examples of a species evolving a new habit in real time.
Asian longhorned beetle
Anoplophora glabripennis
A maple, birch or willow riddled with round exit holes big enough to take a pencil, cut by a large black-and-white beetle with antennae longer than its own body. Native to China, it has broken out repeatedly in North America and Europe since the 1990s, and every outbreak so far has triggered full eradication rather than being managed as an established pest.
Boll weevil
Anthonomus grandis
Boll weevil punctures cotton squares and bolls to feed and lay eggs. It has been eradicated from nearly the entire United States, but remains present in Mexico, Central America and a small corner of south Texas.
Box tree caterpillar
Cydalima perspectalis
Box tree caterpillar leaves webbing, bare brown patches and stripped hedges. Early checks, hand removal, Bacillus thuringiensis and pheromone traps work best before bark damage weakens the plant.
Brown marmorated stink bug
Halyomorpha halys
Fruit dimpled and scarred where a shield-shaped bug fed through the skin, and an unpleasant smell if you crush one indoors. Established across North America, Europe and parts of South America, an active biosecurity target in Australia and New Zealand, and a pest of an extraordinarily wide range of crops.
Brown planthopper
Nilaparvata lugens
A round patch of rice collapsing into brown, dead stems almost overnight, drained dry by thousands of small brown insects clustered at the base of the plants. The most destructive insect pest of rice across Asia and the Pacific, capable of destroying a crop in days once a population builds, and a vector for two serious rice viruses besides.
Cabbage root fly
Delia radicum
Cabbage root fly makes brassica transplants wilt and die, with white maggots feeding in the roots. Fit collars tight to the stem and protect plants before the spring, summer and autumn flights.
Capsid bugs
Miridae
Capsid bugs feed on young leaves and leave small tattered holes with brown edges after the leaf expands. The insect is often gone by the time damage appears, and most plants grow through it.
Carrot fly
Chamaepsila rosae
Carrot fly causes bronzed foliage and rust-brown tunnels through the roots. Low barriers, fine mesh and careful sowing dates protect the crop before each generation lays eggs.
Caterpillars
Lepidoptera
Caterpillars chew ragged holes, leave dark frass and often hide on leaf undersides. Count the prolegs to separate them from sawfly larvae, and use netting early on brassicas.
Coconut rhinoceros beetle
Oryctes rhinoceros
A row of clean, scalloped triangular holes running across an unfurled frond, cut by a large horned beetle while the leaf was still folded inside the crown. Native to Africa and tropical Asia, an increasingly serious invader of Pacific islands including Guam and Hawaii, and a threat to more than just coconut.
Codling moth
Cydia pomonella
Codling moth leaves a tunnel to the apple core packed with wet brown frass. Pheromone traps show when moths are flying, and cardboard bands catch larvae as they leave the fruit.
Colorado potato beetle
Leptinotarsa decemlineata
Colorado potato beetle can strip a potato plant to bare stems within a fortnight. Hand-picking works for gardens where the beetle is established, while suspected sightings in Britain should be reported.
Corn earworm
Helicoverpa zea
A caterpillar working down through the silks to feed inside the tip of the cob, safe from most sprays by the time it is found. An Americas-only pest under one name on maize and another on tomato, with an equally destructive Old World relative filling the same role everywhere else.
Cucumber beetle
Acalymma vittatum
A small yellow beetle on the seedling leaves that matters far more than its own bite, because it can inject a bacterium that kills the whole vine within weeks. Which cucurbits it wilts and which it mostly spares, and why row cover from day one beats any treatment after the fact.
Cutworms
Agrotis and Noctua species
A row of young plants toppled at soil level with the tops left uneaten. The caterpillar responsible is lying curled in the top few centimetres within a hand's width of the stem, and you can find it in a minute.
Cyclamen and broad mite
Phytonemus pallidus and Polyphagotarsonemus latus
New leaves that come out small, thickened and bronzed, and buds that never open. A mite too small to see, routinely misdiagnosed as thrips or a virus, and what separates the three.
Desert locust
Schistocerca gregaria
A grasshopper that spends most of its life alone and harmless, until crowding triggers a change in colour, behaviour and biology that turns it into a swarming, migratory pest able to strip a field bare within hours. The most destructive migratory pest on earth, threatening the livelihoods of tens of millions of people across Africa, the Middle East and South Asia.
Diamondback moth
Plutella xylostella
A small grey-brown moth whose caterpillars pepper brassica leaves with pinholes and windowpanes of skeletonised tissue, then wriggle backward off the leaf on a silk thread when disturbed. The single most economically damaging insect pest of brassica crops worldwide, and the one that has evolved resistance to nearly every insecticide ever used against it.
Earwigs and ants
Earwigs often eat aphids as well as soft petals, and ants protect greenfly for honeydew rather than eating the plant. Control is rarely needed unless flowers, fruit or aphid colonies are being protected.
Eelworm
Globodera, Ditylenchus and Aphelenchoides species
Plant-parasitic nematodes can stunt potatoes, bloat onion necks and thread the leaves of phlox. They are different from the beneficial nematodes sold for pest control, and each crop needs its own prevention plan.
Emerald ash borer
Agrilus planipennis
A metallic green beetle whose larvae tunnel under the bark in winding galleries, killing an ash tree within two to four years of the first infestation. Native to Asia, it has destroyed tens of millions of ash trees across North America since the 1990s and is spreading west from Moscow through Russia and Ukraine, a second major threat to ash trees compounding ash dieback in the parts of Europe it eventually reaches.
Fall armyworm
Spodoptera frugiperda
A grass or grain crop stripped from the growing point outward by a striped caterpillar with a pale upside-down Y on its head. Native to the Americas, established across Africa since 2016, Asia since 2018 and Australia since 2020, and now one of the most consequential crop pests in the world.
Flea beetles
Alticini
Rocket, radish and brassica seedlings peppered with small round holes, and tiny beetles that vanish with a jump. Getting the crop through the seedling stage fast beats every other measure, and the mesh most gardeners own has holes big enough to let them through.
Fungus gnats
Sciaridae
Fungus gnats are small dark flies that rise from damp compost when you water. Their larvae feed mostly on fungi and dead matter, and letting the compost surface dry breaks the cycle.
Galls and gall mites
Eriophyid mites, cynipid wasps and cecidomyiid midges
Pimples on lime leaves, pincushions on wild roses, blisters on pears and fat round buds on blackcurrants. Most galls can be left alone. Blackcurrant big bud mite is the main exception because it can spread reversion virus.
Glassy-winged sharpshooter
Homalodisca vitripennis
A large, plain leafhopper that feeds on woody stems rather than leaves, leaving trails of white sap residue and, far more seriously, carrying the bacterium behind Pierce's disease from vine to vine. Native to the south-eastern United States, it turned a manageable vineyard disease into a major threat once it reached California in the 1990s.
Grape phylloxera
Daktulosphaira vitifoliae
Grape phylloxera is a microscopic aphid-relative that weakens vines by feeding on the roots. It destroyed much of Europe's vineyard area in the nineteenth century, and most wine grapes are now grown on resistant rootstock.
Harlequin bug
Murgantia histrionica
Cloverleaf-shaped white blotches spreading across cabbage and kale leaves, made by a strikingly patterned bug that feeds almost nowhere except the brassica family. A pest of the southern and central United States rather than a global one, with hand-picking and a mustard trap crop doing most of the work against it.
Horse chestnut leaf miner
Cameraria ohridella
Horse chestnut leaf miner browns leaves and makes trees drop them weeks before autumn. The damage looks severe but rarely kills mature trees; bleeding canker on the bark is the more serious problem.
Japanese beetle
Popillia japonica
A shrub or vine with leaves eaten down to a lace of veins, done by a metallic green-and-copper beetle that feeds in mobs and attacks over 300 kinds of plant. Established across most of North America, newly arrived in northern Italy and Switzerland, and a notifiable pest everywhere else in Europe.
Leaf miners
Leaf miners leave pale tunnels and brown blotches while both leaf surfaces stay intact. Sprays rarely reach larvae inside leaves, so control depends on removing affected leaves and clearing leaf fall where it helps.
Leatherjackets and chafer grubs
Leatherjackets leave yellow lawn patches that lift like a carpet, often with birds or badgers tearing at the turf. Separate them from chafer grubs before using nematodes.
Leek moth
Acrolepiopsis assectella
Pale windows mined through the leaves, a caterpillar in the heart of the leek and a net-like cocoon on the surface. A pest that has spread across most of Britain since 2000, and the mesh that stops it.
Macadamia nut borer
Cryptophlebia ombrodelta
A macadamia husk with a tiny entry hole and frass-packed tunnel running to the shell, or the same caterpillar doing identical damage inside a lychee or longan. Native across a huge swathe of Asia and the Pacific, present throughout Australia's macadamia-growing regions, and established in Hawaii but not the US mainland.
Mealybug
Pseudococcidae
Mealybugs look like small blobs of cotton wool in leaf axils, roots and joints. Their waxy covering blocks many sprays, so repeated wiping and isolation matter more than one treatment.
Mediterranean fruit fly
Ceratitis capitata
A small, boldly patterned fly that punctures ripening fruit to lay eggs, leaving a pinprick sting mark that develops into a soft, rotting patch as maggots tunnel through the flesh. One of the most economically damaging fruit pests on earth, attacking more than 300 different fruits and vegetables across a still-expanding range.
Mexican bean beetle
Epilachna varivestis
Mexican bean beetle is a coppery, black-spotted ladybird relative that eats bean leaves instead of aphids. It is confined to the Americas and is worst in the Mid-Atlantic and Appalachian United States.
Moles
Talpa europaea and other Talpidae
Heaps of fine soil across a lawn and a run of raised tunnels through a bed. Moles eat earthworms and grubs rather than roots, so the damage is mechanical, and the animal chewing your bulbs is almost always a vole using the tunnel.
Nut weevil
Curculio nucum
Nut weevil leaves a pinhead-sized hole in a hazelnut and a fat white grub where the kernel should be. Adult control has to happen in early summer, before females lay into soft green nuts.
Oak processionary moth
Thaumetopoea processionea
Caterpillars in a nose-to-tail procession up an oak trunk, and white silk nests on the trunk or a branch fork. Their hairs cause rashes and breathing trouble, so this is a page about reporting it and keeping away rather than dealing with it.
Olive fruit fly
Bactrocera oleae
A pinprick "sting" in the skin of a ripening olive that turns into a brown, collapsing tunnel through the flesh. The single most damaging pest of olives across the Mediterranean and the Middle East, established in California since the late 1990s, and still a biosecurity target in Australia.
Pea and bean weevil
Sitona lineatus
Neat semicircular bites all round the leaf edges of peas and broad beans. The notching alarms people and costs an established plant nothing. The damage that costs you a crop is underground and invisible.
Pink bollworm
Pectinophora gossypiella
A small pink caterpillar that tunnels inside a cotton boll from the flower stage onwards, staining the lint and cutting both yield and fibre quality. One of the most damaging cotton pests worldwide, eradicated from the continental United States after a five-decade campaign but still a major threat across much of Asia, Africa and Latin America.
Plum curculio
Conotrachelus nenuphar
A distinctive curved scar cut into a young apple, peach, plum or cherry by a snouted weevil playing dead the moment it is disturbed. The North American answer to the maggot-in-the-fruit question that plum moth and codling moth answer everywhere else, and a pest confined to the continent it is named for.
Plum moth
Grapholita funebrana
Plum moth leaves a pink caterpillar curled around the stone and fruit that ripens early. Use pheromone traps from May and separate it from plum sawfly, which damages young fruitlets in June.
Pod borer
Helicoverpa armigera
A caterpillar bores into pods, bolls and fruit, then moves on before finishing the first one. Control depends on catching small larvae before they enter the crop and protecting the parasitoids that suppress them.
Queensland fruit fly
Bactrocera tryoni
Fruit that looks sound and turns to a rotting, maggot-riddled mush from the inside within days, caused by a fly native to eastern Australia that attacks nearly two hundred kinds of fruit and vegetable. The pest behind the fruit-fly-free zones and quarantine checkpoints that structure so much of Australian and New Zealand produce movement.
Rabbits and deer
Rabbits cut shoots cleanly at an angle, while deer tear them off ragged. Damage height and the cut surface tell you which animal is feeding and what kind of fence will work.
Raspberry beetle
Byturus tomentosus
Raspberry beetle leaves a small brown grub at the stalk end of the fruit and a dried, hardened feeding patch. Autumn-fruiting varieties usually escape it, while summer varieties need monitoring around flowering.
Red palm weevil
Rhynchophorus ferrugineus
A palm that looks healthy until its whole crown suddenly topples, hollowed from within by a grub that can grow to the length of a finger. Native to South and Southeast Asia, now established across the Middle East, North Africa and the Mediterranean, and one of the reasons ornamental and date palms in those regions are monitored so closely.
Red spider mite
Tetranychus urticae
Spider mites leave fine pale speckling on the upper leaf and webbing at shoot tips. Tap leaves over white paper to confirm them, then raise humidity and wash plants before colonies build.
Root aphids
Pemphigus, Eriosoma and Aphis species
A plant that flags every afternoon, recovers overnight, then stops recovering. Nothing on the leaves explains it, so knock it out of the pot and look for grey aphids and white waxy powder on the roots.
Root-knot nematodes
Meloidogyne species
Root-knot nematodes swell roots into beads and knots that restrict feeding. They have wide host ranges, persist in warm soil and can overcome some resistant tomatoes when soil gets hot.
Rosemary and mint beetles
Chrysolina americana and Chrysolina herbacea
Rosemary and mint beetles are small jewel-bright beetles that strip lavender, rosemary and mint. They feed through cooler months, and hand-picking is the safest control on herbs you eat.
Sawflies
Symphyta
Sawfly larvae strip gooseberries, roses and aquilegias, often leaving only the ribs of the leaf. Count the prolegs to separate them from caterpillars, because some caterpillar treatments will not work.
Scale insects
Coccoidea
Scale insects look like brown or yellowish domes stuck to stems and leaf veins, often with sticky leaves below. Control works best against the mobile crawler stage in summer.
Scarlet lily beetle
Lilioceris lilii
Lily beetles are bright red adults with orange grubs hidden under their own droppings. Hand-pick from March and check lilies and fritillaries before eggs and larvae build up.
Slugs and snails
Gastropoda
Slugs and snails leave smooth-edged holes, missing seedlings and slime trails. Most species do little harm, but grey field slugs, keeled slugs and garden snails need control around young plants and potatoes.
Spotted lanternfly
Lycorma delicatula
Grey-winged, red-flashing insects mobbing a trunk in their hundreds, sooty mould growing on the honeydew beneath, and a vineyard that can die within a few years of the first sighting. Spreading fast across the eastern and midwestern United States, a quarantine pest everywhere else, and a hitchhiker that travels in egg masses stuck to almost anything.
Spotted wing drosophila
Drosophila suzukii
Soft fruit that collapses into a bruised, weeping mush within a day of picking, because this fruit fly lays eggs in ripening fruit while it is still on the plant rather than waiting for it to spoil. Established across North America and Europe since 2008, still a quarantine pest in Australia and New Zealand, and now one of the costliest soft fruit pests worldwide.
Squash vine borer
Melittia cucurbitae
Squash vine borer makes a healthy-looking squash collapse, with sawdust-like frass at the base of the vine. Check stems early, remove larvae where possible and bury damaged sections so they reroot.
Squirrels and mice
Squirrels and mice dig crocuses, take peas before germination and strip bark from young trees. Use barriers, storage and planting depth rather than poison in a garden.
Sweet potato weevil
Cylas formicarius
Sweet potato weevil tunnels through roots and makes them bitter as the plant produces defensive compounds. Deep planting, regular hilling and clean planting material help stop adults reaching the roots.
Thrips
Thysanoptera
Thrips are slender insects that rasp the surface from leaves and flowers, leaving silver streaks and black specks. Their life cycle includes stages in compost, so repeated control is needed.
Tomato hornworm
Manduca quinquemaculata
A plant defoliated overnight by a caterpillar the length of a finger, camouflaged so well it is usually found by its droppings first. Two nearly identical species, why a hornworm carrying white cocoons should be left alone, and why this is a North and South American problem rather than a global one.
Viburnum beetle
Pyrrhalta viburni
Viburnum beetle skeletonises leaves in May and again in August. Check shoot tips for eggs in winter, remove affected growth and choose less susceptible viburnums when replanting.
Vine weevil
Otiorhynchus sulcatus
Adult vine weevils cut neat semicircles from leaf edges, while the grubs eat roots from container plants over winter. Use nematodes while the compost is warm enough and larvae are active.
Whitefly
Aleyrodidae
Whitefly gathers on the undersides of young leaves and leaves sticky honeydew below. Spraying adults does little to the scales, so control depends on repeated washing or parasitoid wasps under cover.
Wireworms
Agriotes, Athous and Limonius species
Thin orange-brown grubs that bore clean round tunnels deep into potatoes, carrots and seedling stems. Worst in the first years after grass is broken up, and the damage is done by a grub that takes four years to grow up.
Wood pigeons
Columba palumbus
Wood pigeons reduce cabbages and sprouts to bare midribs between November and March. Netting works only when it is taut, supported clear of the crop and sealed so birds cannot get trapped.
Woodlice and millipedes
Oniscidea and Diplopoda
Woodlice and millipedes usually eat tissue that is already rotting, so they often get blamed for damage they did not start. They can still graze seedlings under glass, hollow fruit on wet soil and damage potatoes in damp ground.
Woolly aphid
Eriosoma lanigerum
Woolly aphid forms tufts of white wax in bark cracks on apple, cotoneaster and pyracantha, with lumpy swellings beneath. Brush colonies off, protect predators and watch for canker entering the galls.
Diseases
These problems are caused by fungi, water moulds, bacteria and viruses. Several have no cure available to gardeners; those pages say so and explain prevention or resistant planting instead.
Allium white rot
Stromatinia cepivora
Onions and garlic yellow, wilt and pull out with white fluff and black grains at the base. White rot survives in soil for years, so control depends on clean planting stock, hygiene and growing alliums away from infested ground.
Alternaria leaf blight
Alternaria dauci, Alternaria solani and Alternaria brassicae
Dark brown spots with concentric rings and a yellow halo, starting on the oldest leaves. It arrives on the seed as often as on the wind, which is why the same bed goes down to it every year.
Apple and pear scab
Venturia inaequalis
Apple scab causes olive-black blotches on leaves and rough corky patches on fruit. Raking up infected leaves in autumn reduces spores, and resistant cultivars are the best long-term fix.
Apple canker
Neonectria ditissima
Apple canker makes sunken rings of dead bark that can girdle a branch and kill everything beyond it. The fungus enters through wounds and leaf scars, and cutting it out in dry summer weather is the main treatment.
Ash dieback
Hymenoscyphus fraxineus
Ash dieback causes thinning crowns, diamond-shaped stem lesions and bushy growth from the trunk. Many infected trees persist for years, but trees near roads, paths and buildings need professional assessment.
Aster yellows
Candidatus Phytoplasma asteris
Flowers that come out green, deformed and sterile, or turn into tufts of small leaves. A bacterium with no cell wall is rewriting the plant's development, and a leafhopper brought it.
Bacterial canker
Pseudomonas syringae
Bacterial canker makes amber gum weep from sunken bark on cherries and plums, with dead buds and shot holes in the leaves. Prune only in dry summer weather and cut back into healthy wood.
Bacterial soft rot
Pectobacterium and Dickeya species
Tissue collapsing into a wet slimy mush with a foul smell. Bacteria dissolve the glue holding plant cells together, and they can only get in through a wound, which is where the prevention sits.
Barley yellow dwarf
Barley yellow dwarf virus
Circular patches of stunted cereal, yellow in barley and wheat, red in oats. Aphids brought it in the autumn, and the sowing date decides how much of it you get.
Black Sigatoka
Pseudocercospora fijiensis
Dark streaks spreading across a banana leaf and coalescing into dead patches, cutting the leaf area a plant has left to ripen its fruit. The most damaging foliar disease of banana worldwide, controlled mainly by frequent fungicide spraying that few smallholder growers can afford or justify.
Blackleg
Pectobacterium atrosepticum
A potato plant yellowing and toppling with a slimy black stem base. Where blackleg comes from, why certified seed is the whole prevention, and how it turns into soft rot in store.
Blight
Phytophthora infestans
Blight causes brown leaf blotches, stem streaks and rapid crop collapse in tomatoes and potatoes. Watch for Hutton Periods, keep foliage dry and cut potato haulms quickly once infection appears.
Box blight
Calonectria pseudonaviculata
Box blight causes falling brown leaves, black streaks on green stems and spreading bald patches. Reduce clipping, improve airflow and remove infected debris to keep a hedge going.
Bracket fungi and wood decay
Ganoderma, Fomes, Laetiporus and others
A bracket on a tree is the fruiting body of a fungus growing inside the wood. Knocking it off does not remove the decay. Identify the fungus and the tree before deciding whether the tree is unsafe.
Brown rot
Monilinia fructigena
Brown rot makes apples and plums rot on the tree in concentric rings of buff pustules, then shrivel to hard black mummies. Remove mummified fruit in winter to reduce the next infection cycle.
Cassava mosaic disease
Cassava mosaic geminiviruses
Leaves puckered and mottled yellow-green, roots stunted and yields cut by more than half, caused by whitefly-spread viruses found across Africa and South Asia. Cassava originated in the Americas and remains free of this disease there, one of the clearer examples on this site of a crop's homeland not being where its worst problem lives.
Chestnut blight
Cryphonectria parasitica
A sunken, orange-pustuled canker girdling the trunk or a branch, caused by a fungus that reduced the American chestnut from a dominant forest tree to scattered understorey sprouts within decades. Native to East Asia, where local chestnuts tolerate it, and now present across most of Europe and North America.
Citrus greening
Candidatus Liberibacter asiaticus
Citrus greening causes blotchy yellow leaves, small bitter fruit and decline that cannot be cured once a tree is infected. The disease is spread by psyllids, so keeping the vector out is the main defence.
Clubroot
Plasmodiophora brassicae
Clubroot makes cabbages wilt in the sun and recover overnight, with roots swollen into clubs. It persists in soil for years, so liming, drainage, resistant cultivars and strong transplants matter more than treatment.
Coral spot
Nectria cinnabarina
Coral spot forms coral-pink cushions on dead branches and can move into weak or wounded wood. Prune out affected twigs, avoid leaving snags and do not reuse diseased sticks as plant supports.
Damping off
A tray of seedlings that were fine yesterday, toppled over this morning with pinched dark stems. The conditions that cause it, and why the water butt is part of the problem.
Downy mildew
Peronosporaceae
Downy mildew causes angular yellow blotches held between the veins, with a grey or purple bloom underneath. It spreads in wet foliage and still air, unlike powdery mildew on dry-stressed plants.
Ergot and smut
Claviceps purpurea and Ustilago species
Two fungi that replace grain in the ear and could not be more different. One is a hard dark horn carrying the alkaloids behind St Anthony's Fire and behind LSD; the other is a bag of black dust, and one kind of it is eaten as a delicacy.
Fairy rings
Marasmius oreades and others
A ring of lush dark grass, a bare arc inside it, or a circle of toadstools in autumn. A fungus is working outward through the soil at about 30 cm a year, and the dead grass has died of drought rather than infection.
Fireblight
Erwinia amylovora
Fireblight browns blossom overnight, hooks shoot tips and leaves blackened leaves attached to the stem. Cut well below infection in dry weather and check whether the host is in the rose family.
Fusarium head blight
Fusarium graminearum and Fusarium culmorum
Fusarium head blight bleaches spikelets in green ears, sometimes with pink at the base. The fungus dies at harvest, but the toxins it made can remain in the grain.
Fusarium wilt
Fusarium oxysporum
Lower leaves yellow and die while the plant wilts in the heat and recovers overnight. Cut the stem and the water-carrying tissue is stained brown. The fungus is host-specific, so what killed the tomatoes will not touch the beans.
Grey mould
Botrytis cinerea
Fuzzy grey growth on fruit, seedlings and greenhouse plants releases spores when touched. It starts on dead or wounded tissue, then spreads fastest in still, damp air.
Honey fungus
Armillaria
Honey fungus attacks the roots and base of garden trees and shrubs. Confirm it by looking for white fungal sheets under the bark, black rhizomorphs in the soil and honey-coloured toadstools in autumn.
Horse chestnut bleeding canker
Pseudomonas syringae pv. aesculi
Dark sticky liquid weeping from the bark of a horse chestnut and drying into rust-coloured streaks. It looks terminal and often is not, so the first advice is to wait a year rather than to fell.
Laurel wilt
Raffaelea lauricola
Leaves that wilt, turn reddish-brown and dry out but do not drop, on a tree in the laurel family attacked by a fungus carried in by a tiny ambrosia beetle. Spreading through the southeastern United States since the early 2000s, a serious threat to commercial avocado there, and not yet established anywhere else avocado or bay laurel is grown.
Leaf spot
Leaf spot causes dark marks with defined edges, often spreading through wet weather. Hold the leaf to the light to separate fungal spots from bacterial ones, and remove badly affected foliage where needed.
Mosaic virus
Mosaic viruses cause dark and pale green blotching, puckered leaves and misshapen fruit. There is no treatment once a plant is infected, so remove affected plants and control the routes of spread.
Panama disease
Fusarium oxysporum f. sp. cubense
Yellowing leaves collapsing one after another around a splitting pseudostem, caused by a soil fungus with no cure and no way back once it is in the ground. The original strain wiped the Gros Michel banana off supermarket shelves in the twentieth century; a new strain, Tropical Race 4, is doing the same to Cavendish, the variety that replaced it.
Peach leaf curl
Taphrina deformans
Peach leaf curl makes new peach and nectarine leaves emerge thickened, puckered and crimson. Infection happens before the leaves open, so rain protection from winter to bud burst is the main control.
Phytophthora root rot
Phytophthora cinnamomi and other species
A healthy-looking shrub wilts and dies over a few weeks, and the bark at the base is dead. Cut into it and the edge between live and dead wood is a sharp line, which rules out honey fungus. The organism is a water mould, so fungicides aimed at true fungi disappoint.
Plum pox
Potyvirus plumpoxi
Pale rings and blotches on plum, apricot and peach fruit, with the fruit dropping early and tasting of nothing. Aphids spread it in seconds, so spraying them does not help, and in several countries a suspected case has to be reported.
Potato scab
Streptomyces scabiei
Common scab makes corky marks on potato skins that go no deeper than the peel. Keep soil evenly moist while tubers form, and separate it from powdery scab, which favours wetter ground.
Powdery mildew
Powdery mildew makes white patches on leaves and stems, especially when roots are dry and air is humid. Watering, mulching and airflow matter more than spraying, and each mildew has a narrow host range.
Ramorum disease
Phytophthora ramorum
Dark bleeding patches on a trunk, or blackened shoot tips on rhododendron and viburnum. It is notifiable in most countries that have it, so the first action is a report rather than a treatment.
Red thread
Laetisaria fuciformis
Bleached patches in a lawn with pink threads on the grass tips. Red thread looks alarming, kills nothing, and usually means the lawn is short of nitrogen.
Rice blast
Magnaporthe oryzae
Grey-centred, diamond-shaped lesions spreading across the leaves of a rice plant, caused by a fungus that also rots the neck of the panicle so the whole head dies while still green. The single most damaging disease of rice worldwide, destroying enough grain most years to feed tens of millions of people.
Rose black spot
Diplocarpon rosae
Rose black spot causes dark fringed blotches, yellowing leaves and bare stems by late summer. The green rings around spots show where the fungus is still active, and resistant varieties matter more than sprays.
Rust
Pucciniales
Rust diseases make rusty spots under leaves and yellow blotches above. Most rusts are host-specific, so control depends on dry foliage, resistant varieties and removing infected leaves.
Sclerotinia
Sclerotinia sclerotiorum
Sclerotinia makes stems soft and water-soaked, with dense white mould and hard black sclerotia inside. Dead petals often start infection, and the sclerotia can keep soil infectious for years.
Silver leaf
Chondrostereum purpureum
Silver leaf turns leaves on one branch of a plum dull silver and stains dead wood purple. Confirm it with a cut through affected wood, and prune plums in June to reduce infection risk.
Sooty mould
Leaves turning black and sticky under a tree or a houseplant. Sooty mould grows on the honeydew that sap-sucking insects drip, so the fix is whatever is feeding overhead.
Tomato spotted wilt virus
Orthotospovirus tomatomaculae
Tomato spotted wilt virus causes pale rings on fruit and bronze young leaves. Thrips spread it only if they acquired the virus as larvae, so removing infected plants matters more than spraying adults late.
Verticillium wilt
Verticillium dahliae
Verticillium wilt can make a maple lose one branch at a time while the rest looks healthy. A cut through affected wood shows staining, and replanting needs resistant species.
Xylella fastidiosa
Xylella fastidiosa
A bacterium that blocks a plant's own water-conducting tissue, producing a scorched, drought-like browning that no amount of watering fixes. Different strains cause olive quick decline in the Mediterranean, Pierce's disease in American vineyards and citrus variegated chlorosis in Brazil, and there is no cure once a plant is infected.
Disorders
These problems come from growing conditions. Nothing has infected the plant and nothing is eating it, so they usually need a change in care rather than a treatment.
Bitter pit
Bitter pit makes small sunken marks on apple skin and bitter brown flecks in the flesh, worst at the eye end and in store. Steady watering helps the tree move calcium into the fruit.
Blossom drop
Tomatoes, beans and courgettes flowering freely and setting nothing. The temperature limits pollen works within, why courgettes drop their first flowers, and what to do at midday.
Blossom end rot
Blossom end rot makes a black leathery patch on the bottom of a tomato. The plant is failing to move calcium into the fruit, so steady watering usually matters more than adding calcium to the soil.
Bolting
Crops shoot up a flower stem before they are ready to eat. Cold, long days, heat and drought can all trigger it. Once a plant has bolted, sowing time and steady growth matter more than rescue.
Bud drop
Bud drop makes gardenia, camellia and Christmas cactus shed buds before they open. It usually follows movement, uneven watering, temperature swings, dry air or frost followed by early sun.
Dog urine patches
A brown circle the size of a saucer with a collar of lush grass around it. Nitrogen rather than acidity does the damage, so the supplements sold to fix the dog are the wrong answer and a watering can is the right one.
Drought stress
A wilting plant may be dry or drowning, and the treatments are opposite. Check the compost or soil before watering, and soak dry rootballs thoroughly enough for water to reach the roots.
Etiolation
Etiolation makes seedlings thin and weak, and stretches succulents into bare stalks. The pale growth will not shorten again, so recovery means improving the light and pruning back when new growth is strong.
Failure to flower
Failure to flower can come from age, pruning, shade, feeding or damage to developing buds. Check the cause before feeding or pruning, because the right fix depends on the plant.
Fertiliser scorch
Brown crisp margins on the oldest leaves, a white crust on the compost and a plant wilting in soil that is plainly wet. Too much fertiliser stops roots taking up water, and the plant shows drought symptoms while standing in it.
Frost damage
Frost damage blackens soft new growth in spring and browns exposed shrubs in winter wind. Wait before pruning, protect tender growth and harden plants gradually before cold nights.
Fruit splitting
Fruit splits when it swells faster than the skin can stretch, often after rain follows a dry spell. Pick damaged fruit quickly, reduce swings in soil moisture and protect cherries from rain where possible.
Green potatoes
Light turns a tuber green and raises its solanine at the same time. The green is harmless and the alkaloid is not, boiling destroys neither, and a bitter taste is the warning to stop eating.
Guttation
Clear drops along a leaf margin at dawn, gone by mid-morning, leaving a white crust behind. The plant is pushing sap out under root pressure, which is normal, and the crust is what makes people think they have scale insects.
Hard water and salt build-up
Hard water leaves white crusts on compost and pots, browns leaf tips and yellows lime-hating plants. Flush containers, refresh compost and use rainwater for plants that need acid conditions.
Herbicide damage
Herbicide damage twists tomato and bean leaves into fern-like growth and cups the shoot tips. Test manure or compost before spreading it, and keep contaminated material away from sensitive crops.
Leaf scorch
Leaf scorch causes bleached patches and brown crisp margins that appear all at once. It follows sun, wind, frost or water stress rather than infection, so the pattern and timing matter more than sprays.
Moss in lawns
Moss moves into a lawn where the grass has given up, so killing it without changing the conditions buys one season. Which moss you have narrows the cause to shade, compaction, drought or close mowing.
Nutrient deficiencies
Yellow leaves at the bottom of the plant mean something different from yellow leaves at the top. The rule that halves the diagnosis, and why most yellowing is not a shortage at all.
Oedema
Raised watery blisters that dry into rough corky scabs on leaf undersides and stems. Not a fungus and not a pest, whatever the search results say. A watering and light problem with a straightforward fix.
Pot bound
A houseplant drying out within a day, water running straight through, and roots out of the drainage holes. When to pot on, when to root prune instead, and the plants that prefer to stay tight.
Reversion
Reversion sends plain green shoots through a variegated plant, often faster than the patterned growth. Cut reverted shoots out at the source and propagate only from correctly variegated material.
Root rot
Root rot is one of the commonest ways a houseplant dies. Wilting in wet compost means roots are failing, so recovery depends on removing rot, improving drainage and watering less often.
Salt and wind exposure
Salt and wind exposure brown leaf margins, kill buds on one side and scorch plants after coastal gales or road salt. Shelter with permeable barriers and leach salt from soil with fresh water.
Sunscald and frost cracks
Sunken dead bark on the side facing the low winter sun, and long vertical splits that open with a bang on a cold night. Both come from winter sun rather than winter cold, and both are prevented rather than treated.
Tipburn
Young inner leaves with scorched brown edges, often hidden until the head is cut open. Calcium reaches the leaves that transpire, and the leaves buried in the heart of a head do not transpire at all.
Transplant shock
Transplant shock makes a moved plant wilt, drop leaves or stand still for weeks. Protect the roots, water deeply and reduce stress until new roots start growing.
Waterlogging
Waterlogging causes yellow leaves, wilting on wet ground and winter losses. Roots fail when air is pushed out of the soil, so drainage, raised planting and soil structure are the real fixes.
Wind rock
Wind rock makes a shrub or sprout rock until a funnel opens around the stem. Firm plants in autumn, reduce tall growth where needed and stop water collecting around damaged roots.
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