Pests & Diseases
Identify, then treat
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Start with these four
Four problems that between them cover the ways a plant tells you something is wrong: chewed, sucked, rotted and starved.
Plates are illustrations, not photographs of a specific plant, and are not evidence of a variety, place or season.
All pests & diseases
The full section as a record. Every entry answers the same questions in the same order, so two can be read line for line.
How to work out what is actually wrong with a plant
The hardest part of plant diagnosis is not identifying an organism. It is resisting the urge to identify one at all, because most of what looks like a pest or a disease is neither, and the treatments for the real causes are free.
What follows is the order that eliminates the most possibilities fastest. It is deliberately not a list of pests, because a list of pests is only useful once you already know you have one.
First: is the damage getting worse?
Mark the edge of the damage, with a photograph or by tying a loose tie on the affected branch, and look again in three days. This single check separates the two categories that matter.
Damage that is not spreading is history. Hail, a late frost, a scorching afternoon, a chemical drift, a dog: all of them leave marks that stay exactly as they are while the plant grows past them. Nothing needs treating, and treating it wastes the season.
Damage that is spreading is live, and only then is it worth asking what is causing it.
This is the check that most often prevents an unnecessary spray, and almost nobody does it, because standing still for three days feels like doing nothing.
Second: rule out the causes that are not organisms
A large share of what gets reported as disease is a watering, light or root problem wearing a convincing costume. These are worth eliminating before any organism is considered, because they are common, they are free to fix, and no spray touches them.
- Water: dig two inches down and feel it. Both too wet and too dry produce wilting, and they look identical above ground
- Roots: tip a container plant out. Brown mushy roots answer the question immediately
- Light: count actual hours of direct sun on that plant, not the brightness of the garden
- Feed: pale new growth points at nutrients, pale old growth usually does not
- Damage: check the base of the trunk for mower, strimmer or animal damage before blaming the leaves
A plant that is wilting in soil that is soaking wet is drowning, not thirsty, and the reflex to water it is what finishes it. That one confusion accounts for more dead houseplants than every insect combined.
Third: read the pattern, not the symptom
Symptoms are almost useless on their own. Yellow leaves mean nothing until you say which leaves, in what pattern, and whether the leaf is firm or soft. The pattern is the diagnosis.
- Oldest leaves yellow first, plant otherwise fine: normal aging, or nitrogen
- Yellow between green veins on new leaves: a nutrient the plant cannot take up, usually a pH problem rather than a shortage
- Uniform yellowing with a soft stem base: overwatering and root rot
- Brown dry edges on otherwise green leaves: water supply, dry air or salt, never a fungus
- Holes with a slime trail: slugs. Holes with droppings: caterpillars. Small round shot-holes: flea beetles
- Stippled dusty leaves with fine webbing: spider mites, which follow dry air
- Sudden complete wilt of one branch or one side: a vascular wilt, and usually not curable
Fourth: decide whether it is worth acting on
Not every problem deserves a response, and treating everything is how a garden ends up with resistant pests and no predators left to eat them. Problems fall into three groups and the difference decides everything.
Cosmetic problems arrive late, damage leaves the plant was about to drop anyway, and end with the season. Powdery mildew on a zucchini in September is the classic example. The correct response is nothing.
Slow problems cost the plant vigor over months or years, and are worth addressing when convenient rather than urgently. Most scale insects, most leaf spots and most aphid colonies sit here.
Fast problems kill within days and are the only ones that justify dropping everything: bacterial wilt in cucurbits, fire blight in an apple, a hornworm on a young tomato, an aphid outbreak on seedlings.
The honest test is to ask what happens if you do nothing for a fortnight. If the answer is a slightly uglier plant, the answer is do nothing for a fortnight.
Fifth: start with the least drastic thing that works
Where action is warranted, the order runs from physical to chemical, and the first two steps solve most cases.
- Remove it by hand: hornworms, scale, mealybugs and affected leaves all come off with fingers, a cloth or a jet of water
- Change the conditions: airflow, spacing, watering at the base in the morning, and mulch prevent more disease than any product
- Encourage what eats it: ladybirds, lacewings, parasitic wasps, birds and ground beetles all work for free, and broad-spectrum sprays remove them first
- Use a targeted product: something that names the specific pest, applied at the stage the label specifies
- Remove the plant: only for the few problems that spread readily and cannot be cured
Broad-spectrum insecticides are last for a practical reason rather than a moral one. They kill the predators faster than the pests, because predators are fewer and slower to return, so the second outbreak is usually worse than the first.
Organic pest control: what each product actually does, and what it does not
Organic does not mean harmless and it does not mean broad. Almost every product on this list works on a narrow group of pests, at a particular life stage, under particular conditions, and most disappointments come from using one against something it was never going to touch. The column that matters is the last one.
| Control | What it works on | What it will not do |
|---|---|---|
| Insecticidal soap | Soft-bodied insects it physically wets: aphids, spider mites, whitefly nymphs, young scale | Nothing it does not directly hit, and nothing after it dries. It has no residual action, so coverage under the leaf is the whole job |
| Horticultural oil | Smothers eggs, scale and mite stages, including overwintering eggs when applied as a dormant spray | Work on anything it misses, and it damages foliage if applied in heat or on a plant already short of water |
| Neem | Deters feeding and disrupts the growth of young insects over days | Knock anything down quickly. It is slow by design, and it harms predators that feed on treated tissue |
| Bt, Bacillus thuringiensis | Caterpillars, and only caterpillars that eat the treated leaf | Touch beetles, aphids, mites or anything that does not chew foliage. Sunlight breaks it down within days, so timing is everything |
| Spinosad | Caterpillars, thrips, leafminers and some beetles | Spare bees while it is wet. Apply at dusk so it dries before they fly |
| Diatomaceous earth | Soft-bodied crawling insects, by abrasion, while it stays dry | Work at all once it is damp, and it kills ground beetles and other beneficials just as readily |
| Iron phosphate slug bait | Slugs and snails, and it is the bait to use where pets and wildlife are around | Act instantly. Slugs stop feeding first and die out of sight days later |
| Row cover and netting | Excludes almost everything, which makes it the most effective control here | Help if it goes on after the pest arrives, and it has to come off fruiting crops for pollination |
Two rules cover the whole table. Read what the label names, because a product that names one pest does far less damage to the predators in your garden than one that names everything. And spray at dusk rather than in the middle of the day: it is cooler, so there is less leaf damage, and the pollinators are not flying.
Companion planting, with the parts that are actually supported
Companion planting is where natural pest control advice gets furthest ahead of the evidence. Some of it works, some of it is folklore repeated until it sounded like fact, and it is worth separating the two.
- What works: flowers that feed predators and parasitic wasps. Small flat flowers, dill, fennel, yarrow, alyssum and cilantro left to bloom, bring in the insects that eat aphids and caterpillars for you all season.
- What works: trap crops. A row of something a pest prefers, planted away from the crop you care about, concentrates the problem where you can deal with it.
- What works: physical interruption. Mixed planting makes a pest that finds its host by sight and smell work harder than it does in a solid block of one crop.
- What is overstated: marigolds as a general repellent. Certain French marigolds suppress some soil nematodes when grown as a cover crop and turned in, which is a long way from a border of them protecting tomatoes from insects.
- What is folklore: most of the plant-repels-plant pairings passed around online. If a claim has no mechanism attached to it, treat it as decoration rather than as pest control.
What prevention actually consists of
Almost all fungal disease needs the same three conditions: still air, wet leaves and crowded plants. Every one of those is decided by the gardener rather than by luck.
Space plants to their mature width rather than their nursery width. Water the soil rather than the foliage, and do it in the morning so anything splashed dries before nightfall. Mulch, which stops soil and spores splashing onto lower leaves in heavy rain. Clear fallen leaves from plants that carry disease over on them, roses and peaches especially. Rotate annual crops so soil-borne problems do not accumulate.
None of that is a treatment and all of it works better than one. It is also why the entries in this section keep returning to spacing and watering rather than to products.
Common garden pests, once the checks above point at an organism
The order matters more than the list. Most people start here, name a pest from a photograph, and treat for something that was never the problem. Work through the five checks first. Then this is the short list of common garden pests that actually turn up in home gardens, sorted by the damage they leave rather than by the name of the insect, because the damage is what you have in front of you.
| Pest | What you see | What it goes for | First thing that works |
|---|---|---|---|
| Aphids | Dense clusters on new shoots and leaf undersides, curled and distorted growth, sticky honeydew and the black sooty mold that grows on it | Soft new growth on almost anything, indoors and out | A jet of water, repeated over several days. Ladybugs, lacewing larvae and parasitic wasps finish the rest if you leave them alone |
| Slugs and snails | Ragged holes with smooth edges, whole seedlings gone overnight, silvery slime trails in the morning | Seedlings, lettuce, hosta, strawberries, basil | Hand-picking after dark, barriers around vulnerable beds, and watering in the morning so the surface is dry at night |
| Caterpillars: cabbage worms, hornworms, armyworms | Large ragged holes, and dark droppings on the leaves below the damage | Brassicas, tomatoes and most leafy crops | Hand-picking, which is faster than it sounds once you learn to look under the leaf. Bt for a heavy infestation on brassicas |
| Spider mites | Fine pale stippling that makes the leaf look dusty, and fine webbing in the leaf joints | Anything in hot dry air, and houseplants in winter | Raise the humidity and wash the leaf undersides. Avoid broad-spectrum sprays, which kill the predatory mites first |
| Whiteflies | Tiny white insects that fly up in a cloud when the plant is disturbed, plus honeydew below | Tomatoes, brassicas and many houseplants | Yellow sticky traps and washing. Check the undersides for the flat scale-like nymphs, which are the stage doing the damage |
| Thrips | Silvery streaks and tiny black specks, and buds that open distorted | Onions, roses, many ornamentals, indoor plants | Remove the affected buds, use blue sticky traps, and leave the predatory mites alone |
| Scale insects | Immobile brown or white bumps along stems and leaf veins, with sticky leaves beneath them | Woody plants, citrus, houseplants | Scrape them off with a fingernail or a cloth. Horticultural oil timed to the crawler stage for a heavy infestation |
| Mealybugs | White cottony masses in the leaf axils, and sometimes on the roots | Houseplants, succulents, citrus | A cotton swab dipped in alcohol, repeated weekly, with the plant isolated until it is clear |
| Japanese beetles | Leaves skeletonized down to the veins, and beetles feeding in groups in full daylight | Roses, grapes, beans, linden | Hand-pick into soapy water in the morning while they are slow. Skip the pheromone traps, which draw more beetles in than they catch |
| Flea beetles | Small round shot-holes peppered across young leaves | Eggplant, brassicas and radish seedlings | Row cover until the plants are established. On a mature plant the damage is cosmetic |
| Squash bugs | Wilting vines, gray-brown shield-shaped bugs, and bronze egg clusters on leaf undersides | Squash, pumpkin and zucchini | Crush the egg clusters weekly, which is the only control that reliably works, and hand-pick the adults early |
| Fungus gnats | Small dark flies drifting over the potting soil, and seedlings that fail for no visible reason | Houseplants and seed trays kept too wet | Let the surface dry between waterings, and use yellow sticky traps for the adults |
| Cutworms | Seedlings cut clean through at soil level overnight, with the tops left lying beside them | Any freshly transplanted seedling | A collar pushed into the soil around each stem at planting |
| Vine weevil | Notched, scalloped leaf edges first, then sudden collapse as the grubs eat the roots | Container plants, heuchera, primula, strawberries | Check the rootball for white C-shaped grubs, and water in beneficial nematodes in late summer |
| Deer, rabbits and voles | Stems cut cleanly at a set height, gnawed bark at the base, shallow surface tunnels | Everything, seasonally | Physical exclusion. Repellents work until it rains, and trunk guards go on before the first winter |
Two things about that list are worth saying plainly. The first is that almost every pest on it is controlled by removing it and changing the conditions rather than by a product. The second is that a garden with no pests at all has no predators either, and it is the second outbreak in such a garden that does the damage.
The insects working for you, and how not to kill them
Most garden pest control is already happening without you. A healthy garden carries a standing population of predators that eat aphids, caterpillars, mites and slug eggs continuously, and the single most damaging thing a gardener can do is remove them with a broad-spectrum spray. Predators are fewer in number and slower to recover than the pests they eat, so the garden comes back out of balance and the second outbreak is worse than the first.
| Beneficial insect | What it eats | How to keep it |
|---|---|---|
| Ladybugs and their larvae | Aphids, in large numbers, and the larvae eat more than the adults do | Learn to recognize the larvae, which look nothing like the adult and get killed by gardeners who mistake them for pests |
| Lacewing larvae | Aphids, thrips, mites, small caterpillars and insect eggs | Leave some aphids in place. Predators need prey to stay in the garden at all |
| Parasitic wasps | Aphids, caterpillars and whiteflies, from the inside | Grow small flat flowers: dill, fennel, yarrow, alyssum, and cilantro left to bloom |
| Hoverflies | Aphids, as larvae, while the adults pollinate | The same flowers, and no insecticide anywhere during bloom |
| Ground beetles | Slugs, slug eggs, cutworms and root maggots, at night | Mulch and permanent plantings, which give them cover during the day |
| Predatory mites | Spider mites | Avoid broad-spectrum sprays entirely, because those controls kill mites of every kind |
| Spiders and birds | Everything, continuously | Water, cover, and a garden that is not sprayed |
Organic controls work the same way as any other control in one respect: they still have to be targeted. A product that names one pest and one life stage does far less damage to that standing population than one that names everything.
Beetles, larvae and the life stage that decides the control
A large share of garden insect damage is done by larvae rather than by the adult insect, and controls that work on one stage do nothing to the other. That is the most common reason a treatment fails on beetles: the adults get sprayed while the larvae, which are doing the real damage somewhere else, are untouched.
| Beetle | Adult damage | Larval damage | Where control actually works |
|---|---|---|---|
| Japanese beetle | Leaves skeletonized in daylight, in groups | Grubs feeding on grass roots in the lawn | Hand-pick the adults into soapy water. Treat grubs only where lawn damage is proven, never as insurance |
| Cucumber beetle | Chewed leaves and flowers, and it carries bacterial wilt into the plant | Larvae feed on the roots | Row cover until the plants flower, then scout daily, because the disease matters more than the chewing |
| Squash vine borer | The adult is a moth that lays eggs at the base of the stem | The larva tunnels inside the vine and the plant collapses in a day | Check the stem base for sawdust-like frass from midsummer, and bury or wrap the lower stem before it starts |
| Colorado potato beetle | Adults strip potato and eggplant foliage | The larvae do the same on the same plants, faster | Crush the orange egg clusters on the leaf undersides. It is the only step that keeps ahead of them |
| Flea beetle | Shot-holes across seedling leaves | Larvae feed on roots and rarely matter | Row cover over the seedlings until the plants are big enough not to care |
| Ground beetle | None. It is a predator, not a pest | The larvae eat slugs and cutworms | Leave it alone. It is the beetle most often killed by mistake |
The pattern to take from that table is that timing beats product choice. Most beetle and larvae problems in a home garden are solved by a physical barrier put on before the insect arrives, or by removing eggs in the week they are laid, and neither of those is something you can do after the damage appears.
Common plant diseases, and which of them are worth fighting
Fungal problems make up the large majority of garden plant disease, and nearly all of them need the same three conditions: still air, wet leaves and crowded plants. That is why the prevention section above is more useful than any product, and why several of these need no treatment at all.
| Disease | What you see | What it needs to take hold | Response |
|---|---|---|---|
| Powdery mildew | A white dusty coating on the upper leaf surface, spreading over weeks | Warm days, cool nights, crowded plants, dry roots | Late in the season it is cosmetic and needs nothing. Earlier, thin and space the plants and water at the base |
| Downy mildew | Yellow patches on top, with gray or purple fuzz on the underside directly beneath them | Cool wet weather and leaves that stay wet for hours | Remove affected leaves, improve airflow, and choose resistant varieties next season |
| Black spot | Round black spots with fringed edges on rose leaves, followed by yellowing and leaf drop | Six or more hours of leaf wetness | Clear and bin the fallen leaves, water at the base only, and grow resistant cultivars |
| Septoria leaf spot | Small dark spots with pale centers, starting on the lowest tomato leaves and moving up | Splashing rain and warm humid nights | Mulch to stop soil splashing up, remove the lowest leaves, and rotate the bed |
| Rust | Orange or brown powdery pustules on the leaf undersides | Humidity and still air | Remove the affected leaves. It is disfiguring and rarely fatal |
| Botrytis, or gray mold | Fuzzy gray mold on fading flowers, soft fruit and damaged tissue | Cool, damp, still air | Remove spent flowers and damaged tissue, and open the plant up to moving air |
| Fusarium and verticillium wilt | One branch or one side wilts suddenly, with brown staining inside a cut stem | Soil-borne, and it persists in the ground for years | Not curable. Remove the plant and keep that family out of that ground for three to four years |
| Bacterial wilt | A whole cucumber or melon vine collapses within days, and cut sap strings when the stem ends are pressed together and drawn apart | Carried into the plant by cucumber beetles | Not curable. Remove the plant and control the beetles on what is left |
| Fire blight | Shoots blacken and curl over like a shepherd’s crook on apple and pear | Warm wet weather during bloom | Cut well below the visible damage in dry weather, sterilizing the blades between every cut |
| Root rot | Yellowing and wilting in soil that is wet, and brown mushy roots when the plant is lifted | Waterlogged soil with nowhere to drain | Stop watering, repot into a drier medium, and fix the drainage rather than the plant |
| Damping off | Seedlings collapse at soil level in the tray, in a spreading patch | Cold wet compost and still air under a cover | Sow thinly, water from below, and ventilate. It cannot be reversed once a seedling has fallen |
| Mosaic viruses | Irregular yellow, light green and white mottling across a leaf, with distortion | Spread by aphids and by handling infected plants | Not curable. Remove the plant, and wash hands and tools before touching anything else |
The pattern in the last column is the useful part. Most fungal leaf diseases are managed rather than cured, and they cost the plant far less than gardeners assume. The ones that justify removing a plant are few, and they are the vascular and viral problems, where nothing reaches the pathogen and the plant becomes a reservoir for the rest of the garden.
Telling a pest problem from a disease problem
The separation is easier than it looks, and it decides everything that follows.
- Look for the organism first. Turn the leaf over. Aphids, mites, scale, mealybugs and caterpillars are all visible with reading glasses, and the sticky honeydew they leave is visible without them.
- Chewed tissue means a pest. Something has to have a mouth to take a piece out of a leaf, and disease removes color rather than material.
- Spots with defined edges, halos or fuzz mean a fungus or a bacterium. Spots that are simply missing tissue mean something ate it.
- Sudden wilt with no visible cause points inside the plant, at the roots or at a vascular disease. Both are checked by lifting the plant or cutting a stem, and neither is a spray problem.
- Damage that stops at a straight line, or at a set height, is physical or animal: mowers, frost, chemical drift, deer.
Which pests and diseases go for which plants
Most garden pests are not general feeders. They arrive on a short list of plants, at a predictable point in the season, and knowing which pests your crops attract turns scouting from a vague worry into a two-minute job. Check the plants on this list first, and check the leaf undersides while you are there.
| Plant | Pests it attracts | Diseases it gets | What to watch for |
|---|---|---|---|
| Tomato | Hornworm, aphids, whitefly, flea beetle | Septoria leaf spot, early blight | Dark droppings under a stripped stem mean a hornworm above it. Spots start on the lowest leaves |
| Squash, zucchini and pumpkin | Squash bugs, squash vine borer, cucumber beetle | Powdery mildew, bacterial wilt | Bronze egg clusters on leaf undersides, and sawdust-like frass at the stem base from midsummer |
| Cucumber | Cucumber beetle, aphids, spider mites | Bacterial wilt, downy mildew | A vine that collapses in a day is bacterial wilt carried by the beetles, not drought |
| Cabbage, broccoli and kale | Cabbage worm, cabbage looper, aphids, flea beetle, cabbage root maggot | Black rot, clubroot | White butterflies over the bed in June mean eggs on the leaf undersides within the week |
| Beans | Mexican bean beetle, aphids, slugs | Rust, anthracnose | Yellow larvae feeding from the underside, which is where any control has to be aimed |
| Lettuce and salad greens | Slugs, snails, aphids | Downy mildew | Slime trails at first light, and aphids down in the heart rather than on the outer leaves |
| Potato and eggplant | Colorado potato beetle, flea beetle | Early blight, verticillium wilt | Orange egg clusters on the leaf undersides, which is the stage to crush |
| Onion and garlic | Onion maggot, thrips | Downy mildew, white rot | Silvery streaking in the leaf folds, where thrips shelter |
| Corn | Corn earworm, cutworm, squirrels | Rust, smut | The silks are where the earworm enters, so that is where to look |
| Roses | Aphids, Japanese beetle, thrips | Black spot, powdery mildew | New shoot tips for aphids in spring, and fallen leaves for black spot all season |
| Apple and pear | Codling moth, aphids, borers | Scab, fire blight, brown rot | The trunk base for borer holes, and the fruit for a single entry hole with frass around it |
Read that table alongside the crop rotation advice in the vegetable section. Several of the worst problems on it, the cabbage root maggot, the Colorado potato beetle and the soil-borne wilts, build up where the same plant family goes back into the same ground year after year, and rotation is the control that costs nothing.
What to do in the garden, for your zone
One email at the start of each month: what to plant, prune and order in the next four weeks, written for the zone you pick rather than for a national average.
Twelve emails a year, no more. An unsubscribe link in every one, and the zone is used for nothing except deciding which version you get.
When pests & diseases go wrong
Three failures account for most of what readers ask about in this section.
Questions about pests & diseases
Exclusion first: row cover and netting stop more damage than any spray. After that, match the product to the pest and the life stage. Insecticidal soap and horticultural oil work only where they land, Bt works only on caterpillars that eat the leaf, and diatomaceous earth stops working the moment it gets damp.
Parts of it. Flowers that feed predators and parasitic wasps genuinely reduce aphid and caterpillar pressure, and trap crops work. The plant-repels-pest pairings passed around online mostly have no mechanism behind them, and marigolds suppress some soil nematodes as a turned-in cover crop rather than repelling insects from a border.
Aphids, slugs and snails, and caterpillars account for most of what gardeners actually meet, followed by spider mites indoors and Japanese beetles, squash bugs and flea beetles outdoors. All of them are identified faster by the damage they leave than by trying to match an insect to a photograph.
Chewed or missing tissue means something with a mouth. Discolored tissue that is still there means a disease, a nutrient problem or a watering problem. Turn the leaf over before deciding, because most sap-feeding pests live on the underside and leave sticky honeydew as evidence.
Ragged holes with a slime trail are slugs or snails. Ragged holes with dark droppings below are caterpillars. Small round shot-holes across young leaves are flea beetles. Leaves eaten down to the veins in daylight are Japanese beetles or sawfly larvae.
Almost always powdery mildew. On a squash or a zinnia in late summer it is cosmetic and the correct response is nothing. Earlier in the season, or on a plant you need productive, thin the growth for airflow and water the soil rather than the foliage.
Usually not. Hand removal and a change of conditions solve most garden pest problems, and broad-spectrum insecticides make the next outbreak worse because they kill the predators faster than the pests. Where a product is genuinely needed, one that names the specific pest beats one that names everything.
The vascular wilts, bacterial wilt in cucumbers and melons, and the mosaic viruses. In each case nothing reaches the pathogen inside the plant, and leaving the plant in the ground keeps a source of infection next to everything else.
Some are and most are not. A jet of water, soapy water for hand-picked beetles, and a rubbing-alcohol swab for mealybugs all work. Baking soda, vinegar, dish soap sprays and cinnamon are either weakly effective or actively damaging to leaves, and none of them fixes the conditions that caused the problem.
Yellowing has at least half a dozen common causes, and the pattern tells you which. Oldest leaves first usually points to nitrogen or to normal aging; yellow between green veins points to a nutrient the plant cannot take up, often because of pH; uniform yellowing with a soft stem base points to overwatering and root rot. Start from the pattern rather than the color.
No, and treating everything is how gardens end up with resistant pests and no predators. Plenty of leaf damage is cosmetic, arrives late in the season, and costs the plant nothing. Each entry states whether the problem is worth acting on at all.
Rarely, and only for the small number of problems that spread readily and cannot be cured, where the entry says so explicitly. For most things, removing affected leaves, improving airflow and correcting the watering does more than removal.
Where a treatment carries a restriction, the entry states it. Products change and labels differ by state, so the label on the container you own is the authority, and it takes precedence over anything written here.