Onions, Bulb

Rapid foliar blight reduces plant vigor and yield; severe infections can kill leaves while stems and fruit often remain initially intact.

Favored By

Cool to moderate temperatures, prolonged leaf wetness, high humidity, cloudy weather, dense foliage, and windborne spores.

What It Looks Like

Angular yellow or pale-green spots on upper leaf surfaces with gray, purple, or dark downy growth beneath, most visible on humid mornings.

Prevention

Plant resistant cultivars where available; improve airflow; use drip irrigation; avoid overhead watering late in the day; remove volunteer hosts.

Management

Remove infected leaves or plants when practical; monitor regional forecasts for cucurbit downy mildew; use a crop-labeled preventive fungicide when local risk is high.

Seeds rot before emergence or young seedlings collapse, producing thin stands and sudden losses in flats or garden rows.

Favored By

Cold or waterlogged soil, contaminated mix or containers, poor drainage, deep sowing, dense seedlings, low light, and overwatering.

What It Looks Like

Pre-emergence seeds become soft and brown; emerged seedlings develop a water-soaked or pinched stem at the soil line and topple.

Prevention

Use sterile seed-starting mix and clean containers; sow at the proper depth; provide warmth, drainage, light, and airflow; avoid crowding and overwatering.

Management

Discard affected seedlings and nearby mix; reduce watering; improve light and airflow. Fungicides have limited value after collapse and must be specifically labeled.

The pathogen girdles stems at the soil line and rots crowns and roots, causing sudden wilt and plant collapse across a wide host range.

Favored By

Hot, humid weather, moist soil, dense mulch or debris against stems, poor airflow, and repeated susceptible crops.

What It Looks Like

White fanlike fungal growth at the stem base and nearby soil, followed by numerous mustard-seed-like tan to brown sclerotia.

Prevention

Rotate with nonhosts such as corn or other grasses; improve drainage; keep mulch away from crowns; remove debris; avoid moving contaminated soil.

Management

Remove plants plus surrounding soil before sclerotia spread; deep burial may help in tilled areas; soil solarization can reduce survival in hot regions.

Stem and crown infections cause watery rot, wilting, lodging, and plant death in beans, lettuce, tomatoes, cucurbits, and many other crops.

Favored By

Cool moist weather, dense canopies, prolonged leaf wetness, poor airflow, flowering plants, and soils containing long-lived sclerotia.

What It Looks Like

Water-soaked lesions become bleached and covered with cottony white growth; hard irregular black sclerotia form inside or on stems.

Prevention

Rotate to grasses or other nonhosts; space and stake plants; use drip irrigation; control broadleaf weeds; remove infected plants before sclerotia mature.

Management

Pull and discard infected plants and nearby debris; reduce canopy moisture; avoid cultivating contaminated soil into new beds; fungicides are preventive, not curative.

For Onions, Bulb: Reviewed Research Library baseline; local occurrence and treatment timing vary.

Numerous lesions coalesce and kill foliage, reducing beet, chard, carrot, okra, and other crop growth and quality.

Favored By

Warm humid weather, long leaf wetness, overhead irrigation, infected residue, dense plantings, and susceptible weeds.

What It Looks Like

Small circular tan or gray spots with reddish-purple or dark borders; tiny dark fruiting bodies may be visible in pale centers.

Prevention

Rotate 2–3 years; remove residue and host weeds; space plants; irrigate at soil level early in the day; use resistant cultivars.

Management

Remove heavily infected leaves; do not overfertilize with nitrogen; apply a labeled fungicide preventively if disease has a history of severe damage.

For Onions, Bulb: Reviewed Research Library baseline; local occurrence and treatment timing vary.

Wounded vegetables and storage organs rapidly become watery, foul-smelling, and unmarketable in the garden or after harvest.

Favored By

Warm wet conditions, wounds, insect injury, contaminated tools or wash water, poor ventilation, and warm storage.

What It Looks Like

Tissue becomes soft, water-soaked, cream to tan, and collapses into a wet mass; outer skin may remain briefly intact and odor is often strong.

Prevention

Prevent wounds; control insects; harvest in dry weather; sanitize tools and containers; cool produce quickly; provide drainage and ventilation.

Management

Remove rotting produce and contaminated debris; disinfect containers; do not store damaged vegetables with sound produce. No spray cures established soft rot.

Sap-feeding colonies distort tender growth, cause yellowing and sticky honeydew, and may transmit plant viruses.

Favored By

Soft new growth, excess nitrogen, sheltered plants, and warm moderate weather.

What It Looks Like

Tiny (about 1/16–1/8 in), soft, pear-shaped insects; green, black, yellow, gray, or pink; two tailpipe-like cornicles.

Prevention

Avoid excess nitrogen; inspect leaf undersides; conserve lady beetles, lacewings, hoverflies, and parasitoid wasps; control ants that protect aphids.

Management

Knock off with water; prune badly infested tips; use insecticidal soap or horticultural oil with thorough contact coverage. Repeat only as label directs.

Rasping-sucking feeding scars leaves, buds, flowers, and fruit; some species transmit viruses.

Favored By

Hot, dry weather; flowers and tender growth; weeds and plant debris; stressed plants.

What It Looks Like

Very slender, usually 1/25–1/8 in, yellow, tan, brown, or black; adults have narrow fringed wings.

Prevention

Start with clean plants; remove weeds and spent blossoms; keep plants evenly watered; conserve minute pirate bugs and other predators.

Management

Dislodge with water; remove heavily damaged tissue; use insecticidal soap, horticultural oil, or spinosad where labeled, timed to susceptible stages.

Adults chew numerous small shot holes in leaves; seedlings can be stunted or killed. Some larvae feed on roots.

Favored By

Warm sunny weather, young seedlings, nearby weeds or crop residue, and repeated planting of susceptible crops.

What It Looks Like

Tiny (about 1/16–1/8 in), shiny dark or striped beetles with enlarged hind legs that jump when disturbed.

Prevention

Use row cover immediately after planting; remove weeds and residues; rotate crops; plant vigorous transplants or delay planting where practical.

Management

Remove cover at flowering when pollination is needed; use labeled kaolin clay or insecticides only when feeding threatens establishment.

For Onions, Bulb: Reviewed Research Library baseline; local occurrence and treatment timing vary.

Night-feeding caterpillars sever seedlings at soil level; climbing species also feed on foliage and fruit.

Favored By

Weedy ground, plant residue, green manure, and recently converted sod; small transplants are most vulnerable.

What It Looks Like

Smooth, stout, gray, brown, green, or black caterpillars, often striped or spotted; curl into a C when disturbed.

Prevention

Remove weeds and residue before planting; till before planting and in fall; use compost rather than fresh green manure; place collars around transplants.

Management

Search soil near damaged plants at dusk and handpick; treat small larvae with a labeled biological or insecticide only when damage is active.

Larvae tunnel into onion-family bulbs and seedlings, causing wilting, rot, and plant death.

Favored By

Cool wet soils, undecomposed organic matter, cull onions, and repeated allium crops.

What It Looks Like

Adult is a small gray fly; larvae are white, legless, tapered maggots up to about 1/3 in.

Prevention

Rotate alliums; remove culls and volunteers; avoid fresh manure before planting; encourage rapid emergence; use row cover before adult flight.

Management

Remove and destroy infested plants and bulbs; there is little rescue treatment once larvae enter bulbs; use labeled preventive treatments only where locally recommended.

They rasp irregular holes in leaves, stems, seedlings, and low fruit and leave silvery slime trails.

Favored By

Moist shade, dense mulch, weeds, boards, stones, and frequent evening irrigation.

What It Looks Like

Soft-bodied mollusks; snails carry shells while slugs do not; active mostly at night or during wet weather.

Prevention

Water in morning; improve airflow and drainage; remove hiding places and weeds; keep mulch away from seedling stems; use barriers where practical.

Management

Handpick at night; use board traps; apply iron phosphate bait according to label and protect it from children, pets, and wildlife.

For Onions, Bulb: Reviewed Research Library baseline; local occurrence and treatment timing vary.

Microscopic roundworms induce root galls, restricting water and nutrient uptake and causing stunting, yellowing, and midday wilt.

Favored By

Warm sandy soil, susceptible crops, contaminated transplants or soil, and repeated host crops.

What It Looks Like

Microscopic; diagnosis relies on swollen knots or galls on roots, often with a weak uneven crop canopy.

Prevention

Use clean transplants; grow resistant cultivars; rotate with poor hosts; solarize soil in hot climates; add organic matter and clean tools.

Management

Remove heavily infested roots; use resistant crops and rotation over multiple seasons; soil-applied products are limited and must be labeled for home gardens.

Larvae chew seeds, roots, and underground stems or tubers, causing poor stands and holes in potatoes and root crops.

Favored By

Recently converted sod or weedy ground, cool moist soil, and high organic matter.

What It Looks Like

Slender, hard-bodied, shiny yellow to brown larvae with three pairs of small legs; adults are click beetles.

Prevention

Avoid planting susceptible crops immediately after sod; till to expose larvae; control grassy weeds; use bait stations before planting to assess risk.

Management

Trap with buried potato or carrot pieces and remove larvae; rotate to less susceptible crops; rescue treatments after planting are generally ineffective.

Browse foliage, buds, flowers, and fruit; damage often occurs high above the ground with ragged torn stems.

Favored By

High deer density, drought or winter food shortage, gardens near woods, and unfenced plantings.

What It Looks Like

Large hoofed mammal; cloven tracks and pellet droppings; browsing can reach roughly 6 ft.

Prevention

Install an 8-ft deer fence or properly designed electric fence; use cages and netting; choose less-preferred plants while recognizing none are deer-proof.

Management

Exclude first; rotate repellents labeled for edible crops and reapply after rain; protect young trees with guards. Follow local wildlife laws.

Clip seedlings and tender stems, browse leafy greens and beans, and gnaw bark in winter.

Favored By

Dense cover, brush piles, fence gaps, drought, and abundant tender vegetation.

What It Looks Like

Small mammal with long ears and short white tail; leaves round pellets and clean, angled cuts close to the ground.

Prevention

Use 1-in poultry mesh at least 2 ft high with bottom buried 3–6 in; remove nearby cover; use trunk guards.

Management

Exclude and protect individual beds; use repellents labeled for edible crops; trapping or lethal control is regulated and should follow local law.

Gnaw roots, crowns, bulbs, and bark and create surface runways, sometimes killing young trees and perennials.

Favored By

Dense grass, heavy mulch against trunks, snow cover, weeds, and protected groundcover.

What It Looks Like

Stocky mouse-like rodent with short tail and small ears; narrow surface runways and gnaw marks are common.

Prevention

Mow and remove weeds; keep mulch back from trunks; use 1/4-in hardware-cloth guards buried several inches; reduce dense cover.

Management

Use properly placed snap traps inside protected boxes where legal; repair habitat conditions; rodenticides carry risks and must follow label and local rules.