Potato

The signature potato pest: a rounded yellow-and-black-striped beetle whose humpbacked brick-red larvae can strip a plant's foliage in days, cutting yield hard.

Watch for: Yellow-orange egg clusters on leaf undersides, humpbacked reddish larvae feeding in groups, and skeletonized leaves from the top down.

Prevent: Rotate potatoes far from last year's bed (adults overwinter in soil and walk to the nearest nightshade); use straw mulch and row cover on young plants; interplant beans.

Treat: Hand-pick adults and crush egg masses daily - the single most effective home control; spinosad or Bt (tenebrionis strain) for heavy pressure. They develop resistance fast, so rotate any spray.

If you grow potatoes and eggplant, expect this beetle; scouting from emergence and squashing eggs before larvae hatch prevents most damage.

The Irish-famine pathogen (Phytophthora): greasy gray-green lesions that can collapse a planting in under a week of cool, wet weather.

Watch for: Large greasy-looking dark patches on leaves and stems, white fuzz under leaves in humidity, and firm brown-to-purplish rot on tubers.

Prevent: Plant certified clean seed and resistant varieties; give maximum airflow; keep foliage dry; destroy volunteer potatoes and cull piles that harbor it over winter.

Treat: There is no cure - remove and bag whole infected plants immediately to protect neighbors, and cut down foliage before digging so spores don't reach the tubers. It spreads on wind for miles.

The disease that caused the Irish potato famine; it infects both potato and tomato, so keep them apart and never compost blighted plants.

Flea beetles are tiny dark beetles - often shiny black or bronze, rarely more than a couple of millimeters long - with enlarged back legs that let them spring away like fleas the moment you lean in. The adults chew countless small round pits and shot-holes through leaves, giving the foliage a peppered, buckshot-riddled look. They overwinter as adults in leaf litter, hedgerows, and crop debris, emerge hungry in spring, and are worst in warm dry spells. The damage is mostly a seedling and transplant problem: a small plant with a few leaves can be checked, stunted, or killed outright, while the same plant shrugs them off once it is large and growing strongly. Different species specialize on different plant families, so the beetles hammering your brassicas are not the ones on your nightshades. Some species also have larvae that feed on roots and tubers underground.

Watch for: Dozens of pinhead-sized round holes peppered across the leaves, thickest on the youngest, most tender growth. Tiny beetles that spring away as you brush the plants or lean in for a closer look - that jump is the identification. Seedlings that go lacy and stop growing, and transplants that sit and sulk instead of taking off. Damage heaviest in warm, dry weather and along the edge of a bed nearest weedy ground or last year's crop debris. On root crops, shallow narrow grooves or channels scarring the skin at harvest, from larvae feeding underground.

Prevent: Floating row cover or fine insect mesh, laid over the bed from the day of sowing or transplanting and sealed at the edges so nothing can walk in underneath, is the single best defense - and on the crops they favor it is close to essential. Leave it on until plants are large and growing strongly, or until flowering on anything that needs pollinating. Push growth: a well-watered, unchecked plant outgrows the damage, so the whole game is getting seedlings through their vulnerable weeks fast. Keep beds weed-free, since related weeds host the beetles alongside your crop, and clear crop debris at the end of the season where the adults overwinter. Rotate away from a bed that grew a related crop last year. Where you can, shift sowing dates - cooler-season and fall plantings often miss the peak of adult feeding entirely.

Treat: Row cover and healthy fast growth carry most crops through, and on an established plant the damage is usually cosmetic and needs nothing at all. Where a young planting is genuinely being stripped, kaolin clay coats the leaves and deters feeding, and spinosad will knock the beetles back. Spinosad is acutely toxic to bees while the spray is still wet - apply it at dusk when bees are not flying, never onto plants in flower, and follow the product label. Anything you spray must be labeled for the plant you are treating, and you must observe its pre-harvest interval on a crop you are picking. Yellow sticky traps are useful for telling you when the beetles have arrived. Where the damage falls on the part you eat, harvesting young often beats them outright; where you are growing for a root or a fruit, leaf damage can simply be tolerated. Larvae feeding underground cannot be reached by a foliar spray, so rotation and clean beds are what work against those.

Mostly cosmetic on established potatoes, but they can check emerging plants and spread bacterial disease; protect new growth in spring.

Fungal disease (Alternaria) causing brown spots with concentric target rings, starting on the lowest leaves and climbing - the same nightshade disease that hits tomatoes and potatoes.

Watch for: Bulls-eye-patterned brown spots on lower leaves, yellowing around the spots, and leaf drop moving upward; dark sunken spots on tubers.

Prevent: Mulch to stop soil splash, water at the base, rotate out of nightshade beds for 2-3 years, and keep plants well-fed (stressed plants suffer worse).

Treat: Remove and trash (never compost) affected leaves at first sight; copper or biological fungicides slow the spread through the canopy.

Rarely fatal on its own but weakens the plant and shortens the bulking window; more of a slow drain than late blight's sudden collapse.

A soil fungus (Verticillium dahliae and relatives) that enters through the roots and grows upward inside the water-conducting tissue, plugging it from within. The plant is not being eaten or rotted, it is being cut off from its own water supply, which is why the signature symptom is one branch or one side wilting and browning while the rest looks perfectly healthy. Cut into affected wood or stem and the tissue is streaked olive-green, grey or brown. The host range is enormous - maples, redbud, olive, tomato, potato, pepper, eggplant, strawberry and many more - and the fungus survives in soil for years as tough resting structures, so infected ground stays infected. Grasses and conifers do not get it.

Watch for: One branch, one limb or one side of a plant wilting, yellowing and browning in summer while the rest carries on normally - the lopsided pattern is the tell - and leaves that wilt and then hang on rather than dropping cleanly. Cut into affected wood and look for olive-green, grey or brown streaking along the grain; in a cut vegetable stem it shows as a ring of discolored dots. On vegetables, lower leaves yellow in a wedge from the margin inward, and the plant wilts at midday but recovers overnight in the early stages.

Prevent: There is no way to sterilize garden soil of it, so prevention is plant choice and plant vigor. Do not replant a susceptible species where one has already died of it. Grasses, conifers, ginkgo, hawthorn, hornbeam, honeylocust, oak and beech do not get it, and many vegetable varieties carry bred-in resistance - the V in a VFN code on the seed packet. Rotate susceptible vegetables on a long cycle and keep them off ground where the disease has shown. Buy clean stock, avoid wounding roots with spades and machinery, and keep woody plants steadily watered, mulched and only lightly fed, because a vigorous plant walls the fungus off far better than a stressed one.

Treat: There is no cure and nothing to spray. On a tree or shrub the aim is to keep it alive long enough to compartmentalize the fungus, which many plants manage for years: prune out dead and wilting branches well below the discolored wood, sanitize the pruners between cuts, water deeply through dry spells, mulch, and feed lightly rather than pushing it with a heavy feed. On vegetables, pull and bin affected plants rather than composting them, and move that crop to clean ground or to containers of fresh mix. Do not replant the same species where one has died.

Part of the 'early die' complex in potatoes, and it shows as brown streaking in the tuber's vascular ring as well as the stem. A bed that keeps killing potatoes weeks early is a sign to rotate hard away from all nightshades.

A soil bacterium (Streptomyces) that produces rough, corky, raised or pitted lesions on tuber skins; cosmetic but unsightly, worst in dry, alkaline soils.

Watch for: Brown, corky, scabby patches or star-shaped pits on the potato skin at harvest; the flesh underneath is fine.

Prevent: Keep soil evenly moist during tuber formation (the key control), don't lime the potato bed (keep pH 5.0-5.5), avoid fresh manure, and grow scab-resistant varieties.

Treat: No treatment once tubers are affected - they're still edible; peel and eat normally, and fix the soil moisture and pH for next year.

Cosmetic only: scabby potatoes are perfectly good to eat. The best prevention is consistent watering from flowering onward.

The tough, wiry, orange-brown larvae of click beetles, which live in soil for years and bore narrow holes and tunnels through storage roots.

Watch for: Thin, clean holes and shallow tunnels in roots at harvest, and stiff orange-brown 'worms' in the soil - worst in beds recently converted from lawn or sod.

Prevent: Avoid planting potatoes into ground that was grass/sod the year before; rotate; cultivate the soil to expose larvae to birds; bait with buried potato chunks and remove them with their feeders.

Treat: No rescue for holed tubers - trim and eat them soon (they don't store); trap with buried potato or carrot pieces before planting to gauge and reduce numbers.

Damage is cosmetic tunneling rather than plant death, but heavy populations riddle a crop; new gardens broken from lawn are the classic hotspot.

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.

The real danger on potatoes isn't the feeding but the viruses aphids spread (leafroll, PVY) - which is another reason to plant clean certified seed and control aphids early.