Garden Disease & Pest Database

Late blight (Phytophthora infestans) is the most destructive disease of tomato and potato, capable of wiping out a crop in days during cool, wet weather. It caused the Irish potato famine and remains a serious threat for home and commercial growers alike.

🌡️ 10–25°C · humidity ≥90% · wet leaves / rain

Early blight (Alternaria solani) is a common warm-season disease of tomato and potato that builds through summer, starting on the oldest leaves and working upward, reducing yield and exposing fruit to sunscald.

🌡️ 20–32°C · humidity ≥75% · wet leaves / rain

Powdery mildew is a group of fungi affecting cucurbits, grapes, roses, squash and many ornamentals. Unusually, it thrives in warm, dry conditions with high humidity but does not need leaf wetness, making it common in late summer.

🌡️ 15–28°C · humidity ≥60%

Downy mildew (various Peronospora and Plasmopara species) attacks cucurbits, grapes, basil, lettuce and onions in cool, wet weather. Unlike powdery mildew it needs prolonged leaf wetness and spreads explosively in humid spells.

🌡️ 10–23°C · humidity ≥85% · wet leaves / rain

Gray mold (Botrytis cinerea) is a ubiquitous fungus attacking soft tissue of strawberries, grapes, tomatoes and ornamentals, especially in cool, damp, poorly ventilated conditions and on greenhouse crops.

🌡️ 5–22°C · humidity ≥90% · wet leaves / rain

Apple scab (Venturia inaequalis) is the most important disease of apples and crabapples in temperate climates, infecting leaves and fruit during wet spring weather and disfiguring the crop.

🌡️ 6–26°C · humidity ≥80% · wet leaves / rain

Black spot (Diplocarpon rosae) is the most common and serious disease of roses worldwide, defoliating bushes through warm, wet summers and weakening them before winter.

🌡️ 18–27°C · humidity ≥85% · wet leaves / rain

Anthracnose (Colletotrichum species) is a warm-wet-weather disease of beans, cucurbits, tomatoes, and many trees and shrubs, causing sunken fruit lesions that ruin marketable yield.

🌡️ 20–30°C · humidity ≥85% · wet leaves / rain

Rust fungi are a large group infecting beans, hollyhock, roses, daylilies, cereals and many plants, recognisable by powdery orange pustules and favoured by humid weather with cool nights.

🌡️ 10–24°C · humidity ≥85% · wet leaves / rain

Septoria leaf spot (Septoria lycopersici) is a major tomato disease in humid summers, causing heavy defoliation from the bottom up that exposes fruit and slashes yield.

🌡️ 20–26°C · humidity ≥90% · wet leaves / rain

Fusarium wilt (Fusarium oxysporum) is a soil-borne fungus that plugs the water-conducting tissue of tomatoes, melons, bananas and many crops, thriving in warm soils and persisting for years.

🌡️ 25–34°C · humidity ≥60%

Verticillium wilt (Verticillium dahliae) is a cool-season soil-borne wilt with a very wide host range including tomatoes, potatoes, maples and many trees, clogging vascular tissue and persisting in soil for over a decade.

🌡️ 18–27°C · humidity ≥60%

Clubroot (Plasmodiophora brassicae) is a soil-borne disease of cabbage, broccoli, kale and other brassicas that swells the roots into useless galls and persists in soil for up to 20 years.

🌡️ 12–27°C · humidity ≥70% · wet leaves / rain

White mold (Sclerotinia sclerotiorum) attacks beans, lettuce, carrots and over 400 plant species in cool, moist, dense canopies, producing hard black resting bodies that survive years in soil.

🌡️ 10–24°C · humidity ≥90% · wet leaves / rain

Cercospora leaf spot affects beets, chard, peppers, carrots and many crops in warm humid weather, causing rapid defoliation that reduces root and fruit size.

🌡️ 20–32°C · humidity ≥85% · wet leaves / rain

Peach leaf curl (Taphrina deformans) infects peach and nectarine trees during cool, wet spring weather as buds swell, distorting new growth and weakening trees over years.

🌡️ 7–19°C · humidity ≥85% · wet leaves / rain

Brown rot (Monilinia species) is the most damaging disease of stone fruit — peaches, plums, cherries and apricots — rotting blossoms and fruit in warm, humid weather near harvest.

🌡️ 18–28°C · humidity ≥85% · wet leaves / rain

Cedar-apple rust (Gymnosporangium) needs both junipers and apples to complete its life cycle, spotting apple foliage and fruit after wet spring weather when orange spores release from cedar galls.

🌡️ 8–24°C · humidity ≥85% · wet leaves / rain

Black rot (Guignardia bidwellii) is the most serious fungal disease of grapes in warm humid regions, destroying entire clusters in wet seasons if left unmanaged.

🌡️ 15–27°C · humidity ≥85% · wet leaves / rain

Damping-off (Pythium, Rhizoctonia and Fusarium) kills seedlings before or just after emergence in cold, wet, poorly drained or overcrowded conditions, a common setback for spring seed-starting.

🌡️ 5–20°C · humidity ≥90% · wet leaves / rain

Southern blight (Sclerotium rolfsii) is a hot-weather soil-borne fungus attacking tomatoes, peppers, beans and many vegetables at the stem base, common in warm humid climates.

🌡️ 25–35°C · humidity ≥80% · wet leaves / rain

Leaf mold (Passalora fulva) is a major greenhouse and high-tunnel tomato disease driven by high humidity and poor ventilation, rarely serious outdoors but devastating under cover.

🌡️ 18–27°C · humidity ≥85% · wet leaves / rain

Pear rust (Gymnosporangium sabinae) needs two hosts to complete its life cycle: a juniper, where it overwinters, and a pear, where it does the visible damage. It has spread rapidly across Europe wherever ornamental junipers have been planted near fruit trees.

🌡️ 12–24°C · humidity ≥85% · wet leaves / rain

Common scab (Streptomyces scabies) is a soil-borne actinobacterium that roughens the skin of potatoes without affecting the flesh underneath. It is a cosmetic problem rather than a crop loss, but a severe attack makes the tubers hard to peel and shortens their storage life.

🌡️ 18–30°C · dry air ≤60%

White rot (Sclerotium cepivorum) is the most serious disease of onions, garlic and leeks anywhere they are grown. Its resting bodies survive in soil for twenty years or more without a host, so once a bed is infected it is effectively finished for alliums for a generation.

🌡️ 10–24°C · humidity ≥80%

Shot hole (Wilsonomyces carpophilus, also Stigmina) attacks cherries, plums, peaches and apricots during long wet spells. Its name comes from the way infected leaf tissue dies, dries and falls out, leaving foliage that looks like it has been peppered with birdshot.

🌡️ 8–24°C · humidity ≥85% · wet leaves / rain

Box blight (Calonectria pseudonaviculata) has devastated formal box hedging and topiary across Europe since the 1990s. Warm humid weather and the dense, still air inside a clipped box shape are exactly the conditions it needs, so the tightest, most-prized specimens suffer worst.

🌡️ 18–25°C · humidity ≥90% · wet leaves / rain

Snow mould (Microdochium nivale) is the commonest lawn disease of the cold half of the year. It works at temperatures close to freezing under prolonged wet or snow cover, which is why the damage often only appears as the snow melts in late winter.

🌡️ -2–8°C · humidity ≥90% · wet leaves / rain

Red thread (Laetisaria fuciformis) is the classic disease of an underfed lawn in a mild, damp summer. It is disfiguring rather than destructive: it attacks the leaf blade but not the crown or roots, so a lawn almost always recovers once its nutrition is corrected.

🌡️ 15–25°C · humidity ≥85% · wet leaves / rain

Dollar spot (Clarireedia species) shows up on closely mown turf during warm days followed by heavy dew and cool nights. Like red thread it is strongly associated with low nitrogen and a dry root zone under a wet surface.

🌡️ 15–30°C · humidity ≥85% · wet leaves / rain

Fairy rings are made by soil fungi feeding on buried organic matter — old tree roots, builders’ rubble, buried timber. The ring is the growing edge of a single fungal colony spreading outwards at 15–30 cm a year, and it can persist for decades.

🌡️ 12–28°C

Phytophthora is a water mould rather than a true fungus, and it swims: its spores move through waterlogged soil to find roots. It kills more garden shrubs and trees than any other soil pathogen, and almost every case starts with drainage, not infection.

🌡️ 15–30°C · humidity ≥90% · wet leaves / rain

Leaf spot (Mycosphaerella fragariae) is the commonest leaf disease of strawberries and is almost universal in older beds. It rarely reduces the crop on its own, but a heavy attack weakens the plant going into winter and reduces next season’s flowering.

🌡️ 15–25°C · humidity ≥85% · wet leaves / rain

Cane blight (Leptosphaeria coniothyrium) enters raspberry canes through wounds — cane midge damage, careless tying, hoe cuts — and then kills the cane from that point upwards. It is the usual explanation when a healthy-looking row suddenly dies back the summer after planting.

🌡️ 12–26°C · humidity ≥85% · wet leaves / rain

Sooty mould is a group of harmless fungi that grow on honeydew — the sugary excretion of aphids, scale insects, whitefly and mealybugs. The mould never infects the plant. It is a symptom with an insect cause, and treating the leaf instead of the insect achieves nothing.

🌡️ 15–30°C · humidity ≥75%

Alternaria brassicae and A. brassicicola cause dark leaf spot on cabbage, cauliflower, broccoli, kale and swedes. It is seed-borne as well as splash-spread, which is why an outbreak can appear in a bed with no brassica history.

🌡️ 18–28°C · humidity ≥85% · wet leaves / rain

European canker (Neonectria ditissima) is the most damaging bark disease of apples and pears in a wet climate. It enters through leaf scars, pruning cuts and scab lesions in autumn rain, and then girdles and kills the wood above the infection point.

🌡️ 5–20°C · humidity ≥85% · wet leaves / rain

Corn smut (Ustilago maydis) infects sweetcorn through wounds and through the silks during hot, dry weather following a wet spell. In Mexico the swollen galls are the delicacy huitlacoche; in a European garden they are simply a lost cob.

🌡️ 26–34°C · dry air ≤60%

Chocolate spot (Botrytis fabae) is the main disease of broad beans in a wet spring. Overwintered crops in a still, damp, crowded row are the classic case: the mild form is cosmetic, but the aggressive form can kill a whole stand.

🌡️ 15–22°C · humidity ≥90% · wet leaves / rain

Fire blight (Erwinia amylovora) is a destructive bacterial disease of apple, pear, quince and related plants that spreads rapidly during warm, humid bloom, killing blossoms, shoots and whole limbs.

🌡️ 21–30°C · humidity ≥70% · wet leaves / rain

Bacterial leaf spot (Xanthomonas and Pseudomonas) affects peppers, tomatoes, stone fruit and leafy greens in warm, wet, splashy weather, spreading fast through rain and overhead irrigation.

🌡️ 22–32°C · humidity ≥80% · wet leaves / rain

Bacterial wilt (Erwinia tracheiphila, spread by cucumber beetles, and Ralstonia in solanaceous crops) plugs the vascular system of cucurbits and tomatoes, causing irreversible collapse in warm weather.

🌡️ 25–37°C · humidity ≥70%

Bacterial canker (Clavibacter michiganensis in tomato; Pseudomonas in stone fruit) is a serious seed- and water-borne disease that causes wilting, cankers and fruit spotting in cool, wet conditions.

🌡️ 16–28°C · humidity ≥80% · wet leaves / rain

Crown gall (Agrobacterium tumefaciens) is a soil-borne bacterium that induces tumour-like galls on roots and stems of roses, fruit trees, grapes and many woody plants, entering through wounds.

🌡️ 20–30°C · humidity ≥60%

Bacterial speck (Pseudomonas syringae pv. tomato) is a cool, wet-weather tomato disease that blemishes fruit and spots foliage, most damaging early in the season and in spring transplants.

🌡️ 13–25°C · humidity ≥80% · wet leaves / rain

Soft rot (Pectobacterium and Dickeya species) dissolves the pectin holding plant cells together, turning firm tissue into slime. It attacks potatoes, carrots, onions, brassicas and stored bulbs, and needs a wound plus water plus warmth — remove any one and it cannot start.

🌡️ 20–32°C · humidity ≥90% · wet leaves / rain

Halo blight (Pseudomonas savastanoi pv. phaseolicola) is a seed-borne bacterial disease of French and runner beans. It favours cool wet weather — unusually for a bacterial disease — and spreads explosively through a row when the foliage is worked while wet.

🌡️ 16–23°C · humidity ≥90% · wet leaves / rain

Mosaic viruses (Tobacco mosaic, Cucumber mosaic and relatives) infect tomatoes, peppers, cucurbits and many ornamentals, spread by handling, aphids and contaminated tools, with no cure once infected.

🌡️ Spread by handling, insects or soil — not weather-triggered

Tomato spotted wilt virus is spread by thrips and infects tomatoes, peppers, lettuce and hundreds of ornamentals worldwide, causing severe yield and quality loss with no cure.

🌡️ Spread by handling, insects or soil — not weather-triggered

Leaf curl viruses (such as Tomato yellow leaf curl) are whitefly-transmitted viruses that devastate tomatoes and peppers in warm climates, stunting plants and slashing fruit set.

🌡️ Spread by handling, insects or soil — not weather-triggered

Plum pox virus is the most serious disease of stone fruit in Europe and is a notifiable quarantine organism in many countries. Spread by aphids and by grafting infected material, it has no cure — an infected tree stays infected and becomes a source for every plum, peach and apricot nearby.

🌡️ Spread by handling, insects or soil — not weather-triggered

Cucumber mosaic virus has the widest host range of any plant virus — over 1,200 species, from courgettes and tomatoes to spinach, lilies and delphiniums. Aphids move it from plant to plant in seconds, and many common weeds carry it silently through the winter.

🌡️ Spread by handling, insects or soil — not weather-triggered

Potato virus Y is the main reason gardeners are told never to replant their own saved tubers year after year. It builds up invisibly in seed stock, cutting yields by up to 80% over a few generations, and aphids carry it between plants without ever needing to colonise them.

🌡️ Spread by handling, insects or soil — not weather-triggered

Aphids are soft-bodied sap-sucking insects that multiply explosively in warm spring and summer weather on almost every crop, weakening plants and spreading viruses as they feed.

🌡️ 16–28°C

Spider mites are tiny sap-sucking arachnids that explode in numbers during hot, dry, dusty weather, especially on beans, tomatoes, cucumbers and houseplants, and are favoured by drought stress.

🌡️ 27–40°C · dry air ≤55%

Whiteflies are tiny sap-sucking insects that swarm up in clouds when plants are disturbed, thriving in warm weather and greenhouses on tomatoes, brassicas and ornamentals, and spreading viruses.

🌡️ 21–33°C

Scale insects are immobile sap-suckers that attach to stems and leaves of fruit trees, citrus, houseplants and shrubs under protective waxy shells, slowly draining vigour.

🌡️ 18–34°C

Caterpillars — the larvae of moths and butterflies such as cabbage worms, hornworms and loopers — chew foliage and fruit of vegetables and ornamentals through the warm season.

🌡️ 15–33°C

Slugs and snails are night-feeding molluscs that thrive in cool, wet, shady conditions, shredding seedlings, leafy greens and soft growth, and are most destructive after rain.

🌡️ 5–22°C · humidity ≥80% · wet leaves / rain

Thrips are slender rasping insects that scar flowers and foliage of vegetables, fruit and ornamentals in warm dry weather and greenhouses, and transmit damaging viruses such as spotted wilt.

🌡️ 20–35°C · dry air ≤60%

Japanese beetles are metallic green-and-copper beetles that swarm in midsummer to skeletonise the leaves of roses, grapes, fruit trees and hundreds of plants, while their grubs damage lawn roots.

🌡️ 21–35°C

The Colorado potato beetle is the most important insect pest of potatoes and also attacks tomatoes, peppers and eggplant, defoliating plants rapidly and notorious for developing insecticide resistance.

🌡️ 18–33°C

The codling moth is the key pest of apples and pears worldwide; its larvae tunnel into the core of developing fruit, causing the classic 'wormy apple' and major harvest loss.

🌡️ 15–32°C

Mealybugs are soft sap-sucking insects coated in white waxy fluff that infest houseplants, citrus, grapes and greenhouse crops, thriving in warm, sheltered conditions year-round indoors.

🌡️ 20–32°C

Squash bugs are shield-shaped sap-sucking insects that attack squash, pumpkins and other cucurbits through summer, causing wilting that can be mistaken for bacterial wilt and killing young plants.

🌡️ 21–35°C

Carrot fly (Psila rosae) finds its host entirely by smell, homing in on the scent released when carrot foliage is crushed or thinned. Its larvae then tunnel through the roots of carrots, parsnips, celeriac and parsley, ruining them for storage.

🌡️ 12–26°C

Cabbage root fly (Delia radicum) lays eggs at the base of brassica stems, and the larvae eat the fine roots away. Transplants are far more vulnerable than direct-sown plants because their root systems are small and already disturbed.

🌡️ 10–25°C

Flea beetles (Phyllotreta species) are tiny shiny beetles that jump like fleas when disturbed. They shot-hole the leaves of radish, rocket, turnip, cabbage seedlings and other brassicas, and they are at their worst in hot, dry weather on young plants.

🌡️ 18–32°C · dry air ≤55%

Vine weevil (Otiorhynchus sulcatus) is the pest that kills container plants overnight in autumn. The adults notch leaves harmlessly; it is the fat white grubs eating the roots below the compost that do the damage, and by the time the plant wilts the roots are already gone.

🌡️ 5–25°C

Box tree moth (Cydalima perspectalis) arrived in Europe from East Asia in 2007 and has spread across the continent since. Its caterpillars can strip a mature box hedge to bare twigs in under two weeks, and there are two or three generations a year.

🌡️ 12–32°C

The horse chestnut leaf miner (Cameraria ohridella) turns whole avenues of chestnuts brown by July across central Europe. It looks catastrophic and worries people every summer, but it does not kill established trees — the leaves have already done most of their year’s work by the time the damage shows.

🌡️ 10–30°C

Unlike ordinary fruit flies, spotted wing drosophila (Drosophila suzukii) has a serrated ovipositor that cuts into sound, ripening fruit rather than waiting for it to spoil. Since arriving in Europe in 2008 it has become the main pest of cherries, raspberries, blueberries and late strawberries.

🌡️ 15–30°C · humidity ≥70%

Raspberry beetle (Byturus tomentosus) is the reason for the small white grub found in a picked raspberry. Adults feed on the flowers in May and June and lay eggs in them; the larvae then develop in the ripening fruit and feed at the stalk end.

🌡️ 15–28°C

Plum fruit moth (Grapholita funebrana) is the plum equivalent of codling moth. The caterpillar bores to the stone and feeds around it, and an infested plum ripens and drops early — which is why the crop seems to fall in June and taste fine, until you cut one open.

🌡️ 15–30°C

Pear psylla (Cacopsylla pyri) is a sap-sucking insect specific to pears. The honeydew it produces is the real problem: it coats the leaves and fruit, sooty mould grows on it, and a heavily infested tree looks scorched and sticky by midsummer.

🌡️ 10–30°C

The scarlet lily beetle (Lilioceris lilii) is a brilliant red beetle that has spread through Europe and North America over the last few decades. Both adults and larvae eat lily and fritillary foliage, and an unwatched clump can be stripped to bare stems in a fortnight.

🌡️ 10–30°C

Asparagus beetle (Crioceris asparagi) is a black-and-cream beetle with red margins that feeds only on asparagus. Damage to the summer ferns matters more than damage to the spears, because the ferns are what build the crown for next year’s crop.

🌡️ 12–30°C

Gooseberry sawfly (Nematus ribesii) can defoliate a gooseberry or currant bush completely in under a week. There are three generations a year, and the first is usually well advanced inside the bush before anyone notices anything from outside.

🌡️ 12–28°C

Wireworms are the larvae of click beetles and spend three to five years in the soil before pupating. They are the classic problem of a bed made from old grassland — the population arrives with the turf and takes several years of cultivation to decline.

🌡️ 8–25°C

Cutworms are the soil-dwelling caterpillars of several noctuid moths. They feed at night, severing young plants at ground level, and hide just below the surface by day — which is why a gardener finds toppled seedlings each morning and no culprit anywhere.

🌡️ 12–30°C · dry air ≤60%

Root-knot nematodes (Meloidogyne species) are microscopic roundworms that invade roots and force the plant to build galls around them. They are worst in light, sandy soil under glass and in polytunnels, where warmth lets several generations complete in a single season.

🌡️ 18–30°C

Allium leaf miner (Phytomyza gymnostoma) spread across Europe from the 1990s and is now a major pest of leeks, onions, garlic and chives. There are two flight periods — roughly March to April and October to November — and almost all the damage is done in those two windows.

🌡️ 8–25°C

Leaf miners are the larvae of various flies, moths and sawflies that live and feed between the two surfaces of a leaf. On beet, chard, spinach and many ornamentals the damage looks alarming but is largely cosmetic — the plant keeps growing around the mines.

🌡️ 12–30°C

Earwigs are mostly beneficial: in fruit trees they are among the best predators of aphids, psylla and codling moth eggs. They only become a pest on soft flowers — dahlias, chrysanthemums, clematis — where night feeding shreds the petals.

🌡️ 10–30°C · humidity ≥70%

Leatherjackets are the larvae of crane flies (daddy-long-legs), and they eat grass roots through autumn, winter and spring. Damage is worst after a wet late summer, because the adults lay far more eggs into consistently damp turf.

🌡️ 5–22°C · humidity ≥80% · wet leaves / rain

Nitrogen deficiency is one of the most common nutrient problems, since nitrogen is mobile and leaches readily from soil, leaving plants pale and slow-growing, especially in wet springs and sandy soils.

🌡️ Spread by handling, insects or soil — not weather-triggered

Iron chlorosis is a common deficiency in high-pH (alkaline) and waterlogged soils, where iron becomes unavailable even when present, affecting blueberries, rhododendrons, citrus and many ornamentals.

🌡️ Spread by handling, insects or soil — not weather-triggered

Magnesium deficiency is widespread on sandy, acidic and heavily potassium-fertilised soils, common in tomatoes, potatoes and roses, since magnesium is the central atom of chlorophyll.

🌡️ Spread by handling, insects or soil — not weather-triggered

Potassium deficiency reduces fruit quality and disease resistance, common on sandy and light soils and in heavy-fruiting crops like tomatoes, potatoes and fruit trees.

🌡️ Spread by handling, insects or soil — not weather-triggered

Phosphorus deficiency is usually a temperature problem rather than a supply problem. Roots cannot take phosphorus up efficiently below about 12°C, so plants set out too early in cold soil show classic symptoms even in a well-fed bed, then grow out of it as the soil warms.

🌡️ ≤12°C

Calcium is carried through a plant by the transpiration stream, so it reaches the parts that lose the most water — the leaves — and struggles to reach fruit, growing tips and hearts. Almost every case in a garden is a water-supply failure rather than a shortage of calcium in the soil.

🌡️ dry air ≤45%

Manganese becomes unavailable in alkaline, over-limed or waterlogged soil, and in peaty soils that are naturally low in it. It is commonly confused with magnesium and iron deficiency, and the pattern of yellowing is what separates them.

🌡️ Spread by handling, insects or soil — not weather-triggered

Boron is needed in tiny amounts but is essential for cell wall formation and pollination. It leaches readily from light soils and becomes unavailable in dry weather and in over-limed ground, so beetroot, celeriac, swede and cauliflower are the usual casualties.

🌡️ dry air ≤45%

Blossom end rot is a physiological disorder of tomatoes, peppers, squash and melons caused by a calcium shortage in the fruit, usually triggered by uneven watering rather than low soil calcium.

🌡️ Spread by handling, insects or soil — not weather-triggered

Sunscald is a physiological injury to fruit and bark from intense sun and heat, common on tomatoes, peppers and apples when foliage is thin from pruning, disease or defoliation.

🌡️ 32–45°C · dry air ≤50%

Fruit cracking is a physiological disorder of tomatoes, cherries, plums and melons caused by rapid water uptake after dry spells, especially when heavy rain follows drought near ripening.

🌡️ wet leaves / rain

Frost damage happens when ice forms inside plant tissue and ruptures the cell walls. A late spring frost on soft new growth and open blossom does far more harm than a much colder night in midwinter, because dormant tissue is genuinely frost-hardy and new growth is not.

🌡️ ≤2°C

Bolting is a plant deciding it is running out of time and switching from leaf or root growth to setting seed. Heat, long days, drought, root disturbance and a cold spell in the seedling stage can all trigger it, and once it starts it cannot be reversed.

🌡️ ≥25°C · dry air ≤50%

Blossom drop is a plant aborting flowers it cannot support. In tomatoes, peppers and beans the trigger is almost always temperature: pollen becomes sterile above about 32°C by day or below about 13°C at night, and the unpollinated flower is shed within days.

🌡️ ≥32°C · dry air ≤40%

Roots need oxygen as much as leaves do. When soil pores fill with water the oxygen is gone within a day or two and roots begin to suffocate and die — which is why a waterlogged plant shows exactly the same wilting as a drought-stressed one.

🌡️ humidity ≥90% · wet leaves / rain

Tipburn is localised calcium starvation at the fastest-growing leaf margins of lettuce, cabbage, chinese leaves and strawberries. The soil almost always has plenty of calcium; the plant simply cannot move it fast enough to a leaf edge that is expanding in hot, dry, still air.

🌡️ ≥24°C · dry air ≤50%

Hormone weedkiller damage is regularly mistaken for a virus. It reaches a garden three ways: spray drift from a neighbouring lawn, contaminated manure or grass clippings from treated pasture, and the residue left in a watering can used for weedkiller. Tomatoes are the classic indicator plant.

🌡️ Spread by handling, insects or soil — not weather-triggered

Oedema happens when roots take up water faster than the leaves can transpire it away. Cells on the leaf underside swell and burst, leaving corky bumps. It is a watering and humidity problem, common on greenhouse plants in dull cool weather and on succulents and pelargoniums in winter.

🌡️ ≤18°C · humidity ≥85%