Apple

The classic 'worm in the fruit': larvae of a small gray moth tunnel in from the skin and feed at the core or kernel, packing the entry hole with sawdust-like frass and ruining the crop for keeping. The same moth works apples, pears and walnuts.

Watch for: Small entry holes ringed with brown frass, early fruit or nut drop, and a tunnel running to the seed cavity or kernel when you cut or crack a damaged one open.

Prevent: Hang pheromone traps to time any spray; gather and destroy dropped fruit and nuts promptly to break the cycle; bag individual fruits or apply kaolin clay for spray-free control.

Treat: Spinosad timed to egg hatch (the traps tell you when), or codling moth granulosis virus (CpGV) as a targeted biological. Bt does not work on codling moth - the newly hatched larva bores straight into the fruit within hours and never eats enough surface tissue to take in a dose. Use only a product labeled for the crop you are treating, follow the label exactly, and observe the pre-harvest interval before you pick. Sanitation and fruit-bagging matter more than any single spray.

Usually two to three generations a season - the first, at petal fall, is the one to stop. Bt does not control codling moth on apple: the newly hatched larva bores straight into the fruit and never eats enough surface tissue to take in a dose. Time spinosad to egg hatch, or use codling moth granulosis virus (CpGV), and lean hardest on sanitation and fruit-bagging.

The most common apple disease: a fungus that produces olive-green to brown velvety spots on leaves and fruit, causing corky scabs, leaf drop, and cracked, cosmetically ruined apples in wet springs.

Watch for: Olive-brown fuzzy blotches on new leaves, matching corky spots and cracks on the fruit, and early yellowing and leaf fall.

Prevent: Rake and destroy fallen leaves in autumn (the spores overwinter there), prune for airflow, and above all plant scab-resistant varieties like Liberty or Enterprise where springs are wet.

Treat: On susceptible trees, protective fungicide sprays from bud break through the wet spring period; there's no cure once fruit is scabbed, so prevention is the whole game.

Worst in cool, wet springs and in humid eastern climates; drier regions see far less.

A fast, destructive bacterial disease (Erwinia amylovora) of the rose family. It enters through open blossoms and soft shoot tips in warm, humid weather, then runs back down the shoot, blackening leaves and wood as though scorched and bending the tip into a telltale 'shepherd's crook'. From there it moves into limbs as dark sunken cankers, and it can kill a young tree in a single season. Apple, pear, quince, loquat, hawthorn, serviceberry and toyon are all hosts, and susceptibility varies enormously between species and between varieties.

Watch for: Sudden wilting and blackening of blossoms and shoot tips that look burned rather than diseased, shoots bent over into a hook, dark sunken cankers on limbs, and amber bacterial ooze on the bark in warm humid weather.

Prevent: Where a resistant option exists for your crop, variety choice matters more than any spray - check before you buy. Avoid heavy nitrogen and hard pruning, both of which force the soft succulent growth the bacteria enter through, and never prune or work the plant in wet weather. Keep susceptible hosts spaced apart rather than planted hard against each other, and do not prune two of them in the same wet spell.

Treat: Cut strikes out at least 8 to 12 inches below the visible damage, sterilizing the saw or pruners between every single cut, and remove and destroy the prunings rather than leaving them lying. Cut cankers out in dry dormant weather. Copper or biological sprays at bloom help in high-pressure areas - use only a product labeled for the crop you are treating, follow the label exactly, and observe the pre-harvest interval before you pick.

Explodes in warm, humid springs, and the shepherd's-crook shoot tip is the unmistakable sign. Apple is susceptible but less so than pear or quince, and resistant varieties are widely available - worth choosing where blight is common in your area.

Japanese beetles are metallic green-and-copper beetles about half an inch long that emerge in early summer and feed for six to eight weeks. They feed in aggregating masses - one beetle's feeding releases a scent that draws in more - so a plant goes from untouched to covered in a couple of days. They chew the soft tissue between the leaf veins and leave the veins standing, so the leaf ends up as a brown lace skeleton, and they shred open flowers and sometimes ripening fruit as well. They work the sunniest upper parts of a plant on warm afternoons. The white C-shaped grubs spend the rest of the year in turf and pasture soil eating grass roots, and the adults then fly in from wherever they hatched, which can be some distance away.

Watch for: Clusters of shiny metallic beetles massed together on the upper, sunniest foliage and down inside open blooms, heaviest on warm sunny afternoons from around late June. Leaves reduced to lace, with the veins left intact and the tissue between them gone - unmistakable, and quite different from the ragged chewing of a caterpillar. Ragged, chewed petals and blooms that never open properly. Beetles that let go and drop straight to the ground when you disturb the plant. Feeding damage on ripening fruit, on the plants they also take fruit from. Whole branches going brown and lacy at the top of a plant while the shaded lower growth stays untouched.

Prevent: Hand-picking is genuinely the main defense in a home garden. Go out in the cool of the morning while the beetles are sluggish, hold a tub of soapy water underneath, and knock them in - they drop when disturbed, so most fall straight into it. Do it daily through the flight: because the beetles call each other in, taking them off early stops a plant becoming a beacon. Do not put pheromone or lure traps anywhere near the garden - they demonstrably pull in far more beetles than they catch. And do not count on lawn grub control: milky spore has performed poorly and inconsistently in field trials across most of the US, and treating your own turf does not reduce the adults feeding on your plants, because they fly in from surrounding properties. Where it is practical, row cover or netting through the peak flight excludes them outright. Otherwise riding out the six-to-eight-week wave is a legitimate strategy, since most established plants absorb the damage.

Treat: Persistent morning hand-picking into soapy water is the most effective home control there is, and on a vigorous established plant the damage is usually cosmetic and needs nothing else. Neem deters feeding rather than killing, and has to be reapplied. For heavy pressure, spinosad or a targeted insecticide labeled for the plant will work - but time any spray away from bloom and apply it at dusk to protect bees, and observe the pre-harvest interval on anything you eat. On flowering plants, some gardeners simply shear the blooms off for a couple of weeks and let the plant rebloom once the wave has passed, which removes the lure as well as the damage. Young plants and young trees are the ones worth defending: a mature tree or an established vine tolerates several weeks of leaf loss without lasting harm.

A few weeks of feeding rarely hurts an established tree; young trees may need protection.

Deer are the largest garden problem across much of the country, and where pressure is high they set the limit on what you can grow at all. They browse at night and around dawn and dusk, taking tender shoots, leaves, buds, flowers, and fruit within about five feet of the ground, and they come back night after night to anything they have found and liked. Because they have no upper incisors they tear rather than cut, leaving ragged stem ends - that is how you tell deer damage from a rabbit's clean angled cut. In fall, bucks rub their antlers on young trunks and can strip the bark right around a stem, which kills a young tree outright. Palatability varies enormously between plants and between seasons: a herd short of food in late winter eats things it ignores in June, and no plant is truly deer-proof.

Watch for: Growth cleanly missing overnight, with the plant browsed back to a level line about as high as a deer can comfortably reach. Ragged, torn stem ends rather than the clean angled cut a rabbit leaves - the single most reliable identifier. Buds and flowers gone while the foliage below is untouched, since deer often take the best part first. Hoof prints and droppings in soft ground near the damage. Repeat visits, with fresh damage each morning once a planting has been found. On young trees, shredded and stripped bark and broken low branches in autumn - that is antler rubbing rather than feeding.

Prevent: Fencing is the only thing that reliably works. Eight feet is the standard for excluding deer, or two parallel four-foot fences a few feet apart, which they will not jump because they cannot judge the landing. Short of that, cage individual plants: a sturdy wire cage or trunk guard around every young tree, left on until the trunk is thick and the canopy is above browse height, prevents both browsing and antler rubbing and is the single highest-value thing you can do for a young tree. Repellent sprays do help at moderate pressure, but they wash off and deer habituate, so reapply after rain and rotate between different types through the season. Motion-activated sprinklers, scent deterrents, and noise devices work for a while and then stop. Siting matters too: plants close to the house, in daily traffic, and screened behind other planting get browsed less than the same plant out at the boundary.

Treat: There is no chemical fix, only exclusion and recovery. Get physical protection up immediately after the first damage, because deer return to a known meal, and reinforce or rotate whatever you were relying on before. Most plants regrow from browsing if the crown and roots are intact - feed and water a grazed plant and protect the regrowth while it comes back. What does not recover in the same season is anything whose buds or flowers have been taken, and a young tree whose bark has been rubbed right around the trunk is usually lost. Where deer pressure is genuinely heavy, the honest answer is to fence properly or to grow the plants deer leave alone, rather than to keep replacing losses.

Young trees are far more vulnerable than mature ones - the first few years are the risk window, for antler rubbing as much as for browsing.

Aphids are small soft-bodied sap-suckers - green, black, gray, pink, or waxy white, and mostly wingless - that gather in dense colonies on the tenderest new growth: shoot tips, buds, and the undersides of young leaves. They breed extremely fast in mild weather, so a few can become hundreds inside a week. Their feeding curls and distorts soft growth, and the sticky honeydew they excrete coats everything below and turns black with sooty mold. Ants often farm and defend the colonies for that honeydew. A few aphids are normal on almost any plant and a healthy plant shrugs them off; only dense, persistent colonies do real damage, and some aphids also spread plant viruses as they feed.

Watch for: Dense clusters of tiny insects packed onto shoot tips, flower buds, and the undersides of young leaves. Curled, cupped, or distorted new growth that never opens properly. A sticky shine on the leaves - and on whatever sits under the plant - that later goes black with sooty mold. Tiny white cast skins left behind where a colony has been feeding. Ants running steadily up and down the stems, which is often the first thing you notice. Mottled, streaked, or misshapen leaves that do not grow out can mean a virus the aphids brought with them rather than the feeding itself.

Prevent: Go easy on nitrogen: the soft, lush growth that heavy feeding produces is exactly what aphids breed on. Check new growth once a week through spring and early summer so you catch colonies while they are still small, and rub the first few out with your fingers. Leave flowering plants nearby and skip broad-spectrum sprays - ladybugs, lacewings, hoverflies, and parasitic wasps do most of this work for free, and they only stay if there is something for them to eat. Keep ants off the plant where you can, since they protect aphids from those predators. Water steadily so plants are not stressed, and check any new plant over before you bring it home or move it indoors, since aphids usually arrive on something else.

Treat: Knock the colonies off with a hard jet of water and repeat every few days - on most plants that alone ends it. Pinch out or prune off the worst-infested tips rather than treating the whole plant. If colonies keep rebuilding, spray insecticidal soap or horticultural oil directly onto the insects, covering leaf undersides, and repeat weekly until they are gone; treat early or late in the day, and never spray a wilted or heat-stressed plant. Avoid broad-spectrum insecticides - they kill the ladybugs, lacewings, and parasitic wasps that were about to solve the problem for you, and aphid numbers usually come back worse afterward. Anything you spray must be labeled for the plant you are treating; follow that label and observe its pre-harvest interval. On anything you eat, wash leaves, heads, and buds well before use, since aphids hide down in crowns, folds, and tight growth.

Woolly apple aphid is the distinctive one here: white cottony masses on bark, pruning wounds and roots rather than open colonies on leaves. Mostly a young-tree and nursery-stock concern; mature trees shrug it off.