OG American Flag Leeks

A seedling killer caused by several different fungi working together. It hits vegetables, flowers, herbs, microgreens, and cover-crop seedlings the same way — seeds rot before they emerge, or young seedlings collapse right at the soil line. Wet seed-starting mix and poor airflow in seedling trays are the classic conditions.

Triggers: Wet soil or starting mix, poor drainage, seedlings packed too tightly, contaminated trays or media, and stagnant air all favor damping-off.

Risk fades when: Drying the soil surface and improving airflow slows new spread. Collapsed seedlings don't recover, but the rest of the tray can be saved.

Onion maggot is the most important early-season pest of onions, leeks, and garlic in northern temperate regions. Larvae feed on bulb basal plates and roots, killing plants and predisposing damaged bulbs to storage rot. First-generation damage can routinely reduce plant stands by over 50%. Three generations occur annually in northern regions; only the first is typically devastating to a young crop.

Triggers: Three generations per year in northern regions. Overwinter as pupae in soil. Adults emerge late April-May. Adult: small grayish-yellow fly resembling slender housefly. Females lay eggs at soil base of onion plants.

Risk fades when: Cool, wet weather favors onion maggot while hot, dry summer weather is detrimental to its survival, so pressure eases during midsummer heat between flights; the first (spring) generation is the largest and most damaging, and destroying culls and crop debris reduces overwintering pupae (Wisconsin Hort, UMaine Extension).

SoilStack treats this entry as allium white rot for field alert purposes, rather than Botrytis neck rot (which is mostly a post-harvest storage issue). White rot is allium-specific and causes yellowing, wilt, decay of the basal plate (where roots attach), white fungal growth, and tiny black poppy-seed-like resting bodies (sclerotia). It thrives in cool, moist soil.

Triggers: Cool moist soil favors infection. Development is optimal around 60-65°F and slows sharply above the upper 70s.

Risk fades when: White rot is markedly inhibited above 78°F and reduced by drying soil, though the sclerotia persist in soil for years.

An allium-specific rust on garlic, onion, leek, shallot, and chives. Produces orange to reddish pustules on older leaves first. Favored by cool, humid weather.

Triggers: Optimal infection is near 59°F with saturated humidity for several hours. The disease stays active in cool 57-75°F weather with dew or high humidity.

On American Flag Leek: Humid still conditions and prolonged leaf wetness.

Prevention: Water the soil, not the leaves, and maintain airflow

Risk fades when: Rust thrives in cool saturated air. Dry foliage or heat above the favorable range lowers new infection pressure.

Aphids are soft-bodied sap-sucking insects that cluster on tender new growth. Most established plants tolerate moderate populations and will outgrow damage on their own, but aphids are the most important plant virus vectors in the garden, transmitting more than 100 plant viruses including potato leafroll, cucumber mosaic, and turnip mosaic. Honeydew excreted while feeding supports sooty mold growth and attracts ants that protect aphids from natural enemies.

Triggers: Optimal development at ~75°F (green peach aphid) per UC IPM Floriculture; melon aphid develops fastest above 75°F. Many species heat-intolerant above 90°F and crash in mid-summer. Soft new growth and over-fertilization with high N favor population buildup. Females give live birth parthenogenetically most of growing season — one generation in ~1 week under optimal conditions.

Risk fades when: Per UC IPM and Clemson HGIC, populations crash in mid-summer heat (>90°F) for many species, return in cooler conditions

The main foliar disease of onions, garlic, leeks and shallots in warm humid weather. It starts on the older, lower leaves as small sunken water-soaked spots with pale centres. Those enlarge into oval lesions, several centimetres long, that turn purple to brown and develop concentric light and dark rings - a zonate bull's-eye that is the diagnostic sign. Ringed lesions that merge or encircle a leaf girdle it, and everything above wilts, collapses and dies back from the tip. Leeks show this as classic leaf-tip dieback. The fungus is a close relative of the one that causes early blight on tomatoes, and behaves much the same way: it overwinters on infected debris and cull piles, spreads by wind and splashing water, and needs persistent leaf wetness to infect. Infection often follows injury - abrasion from blowing sand, damage from another fungus, or the wounds and succulent necks left after topping at harvest. That last route is why curing matters: dry the necks before topping and cure bulbs thoroughly, or the same fungus follows the crop into storage.

Triggers: Alternaria porri overwinters on infected bulbs and crop debris and can be seedborne in onion. Spores are dispersed by rain, irrigation water, wind, and by people and equipment moving through a wet crop. Older leaves are the most susceptible, and tops become more prone as plants mature. Infection often follows injury - from other fungi such as Botrytis, or from sand and dust during windstorms. Colorado State: warm, moist weather after midseason favours Alternaria, and the pathogen enters mature leaves and bulbs through wounds and the succulent necks left after tops are removed. UC IPM uses consecutive nightly leaf wetness above 12 hours as the signal to shorten spray intervals. UMass notes that Stemphylium leaf blight looks nearly identical and needs microscopy to separate, but management is the same.

On American Flag Leek: Humid still conditions and prolonged leaf wetness.

Prevention: Water the soil, not the leaves, and maintain airflow

Risk fades when: Infection stops without free moisture on the leaf. Dry weather and good airflow halt new lesions, but existing ones stay and the leaf tissue above a girdling lesion will not recover. The disease also follows the crop into storage through wounds and wet necks, so curing is part of resolution, not an afterthought.

A good leek is a firm white shaft over three inches long and at least an inch across. Getting there takes a long season and steady conditions, and thin, pencilly leeks are almost always a sign that one of three things went short: room, food, or water. Leeks are shallow-rooted, which makes them poor competitors and quick to feel drought. USU Extension is blunt that water stress reduces both yield and plant size, and recommends keeping the soil near field capacity and watering deeply every week. They are also heavy feeders on a 120 to 150 day clock - USU side-dresses nitrogen twice, in mid-May and again in late June. Spacing matters as much: extension guidance runs from 4 to 6 inches between plants. Weeds do disproportionate damage in the first two months, when leeks grow slowly and cannot shade them out, and cultivating too deep severs the shallow roots you were trying to protect. The length of the white shaft is a separate lever entirely: it comes from planting deep in a dibbled hole, or from hilling soil around the plants two or three times as they grow. Start indoors, transplant early, and give them the whole season.

Triggers: USU Extension: leeks require regular watering with soils maintained near field capacity, moistened thoroughly to 18 inches every 7 days; water stress will reduce yields and plant size, and rooting depth is shallow. Leeks need additional nitrogen beyond planting fertiliser - side-dress in mid-May and again in late June. Spacing 4 to 6 inches apart in rows 8 to 16 inches apart; transplants need 6 to 8 weeks growth before setting out. UMN Extension: most leeks require 120 to 150 days; a quality leek has a firm white shaft more than three inches long, and most varieties are fully developed when stem width exceeds one inch; swelling at the base, called bulbing, is undesirable. UMD Extension: weed control is particularly important during the first two months when plants grow slowly, and cultivation must avoid damaging the shallow roots. Shaft length and blanching come from deep dibble planting or hilling soil two to three times during the season.

On American Flag Leek: Crowding or irregular watering limits stem expansion.

Prevention: Use proper spacing and provide steady moisture.

Risk fades when: A thin shaft will thicken if the season is long enough and the limiting factor is removed, so it is worth diagnosing which one it is. Crowded plants: thin or transplant to 4 to 6 inches. Drought: soak to 18 inches every seven days rather than watering lightly and often - the roots are shallow and dry out fast. Hunger: side-dress nitrogen in mid-May and again in late June. Weeds: clear them in the first two months, cultivating shallowly. For length rather than girth, plant transplants deep in a 6-inch dibble hole and let water wash the soil in, or hill soil around the plants two or three times. Leeks are frost tolerant, so a late crop can be left to size up.

Thrips are tiny (1/16 inch) slender insects with fringed wings that puncture and rasp leaf surfaces, leaving silver stippling with black frass dots. The biggest concern is virus vectoring: western flower thrips is the principal vector of tomato spotted wilt virus (TSWV) and impatiens necrotic spot virus (INSV), which affect more than 600 plant species. Greenhouse and high tunnel infestations can be devastating.

Triggers: Hot dry weather; greenhouse/high tunnel environments. Female lays eggs inside leaf tissue. 2 larval stages feed; 2 non-feeding pupal stages in soil/litter. Lifecycle 10-21 days. Many overlapping generations. Bridge crops (spring wheat, peach, strawberry per NC State) build populations before vegetable hosts available.

On American Flag Leek: Sap-feeding insects hide in leaf folds and scar foliage.

Prevention: Keep plants vigorous and monitor the leaf bases.

Risk fades when: Infestations are worst in hot, dry weather; cool conditions slow development and heavy rain or overhead irrigation knocks populations down sharply, so risk fades as the weather turns cool and wet and rebuilds the following spring (UK Entomology, Cornell IPM).

Onions lose fights with weeds, and they lose badly. The reason is anatomical: an onion has a shallow, sparse root system and upright, narrow leaves that never close a canopy or shade the ground. Nebraska Extension states it plainly - due to their shallow root systems, onions compete poorly with weeds. A weedy onion bed does not just look untidy. Every week of shaded, crowded growth costs leaves, and because bulb size is set by the number and size of leaves at bulb initiation, lost leaves mean a permanently smaller bulb. The window that matters is from emergence through bulb initiation; after the bulb starts sizing, weeds cost less. The catch is that the same shallow roots that lose to weeds are easily severed by a hoe. Cultivate shallowly, close to the surface, and do not hill soil up around the plants. Mulch lightly instead: it suppresses weeds and conserves moisture without root damage.

Triggers: Nebraska Extension: due to their shallow root systems, onions compete poorly with weeds; shallow cultivation is necessary and the soil should not be hilled up around the plants. Light mulch decreases weeds and conserves soil moisture. The critical period runs from emergence through bulb initiation, because per Iowa State Extension bulb size is largely determined by the number and size of the leaves at bulb initiation - leaf growth lost to weed competition cannot be recovered once bulbing starts.

Risk fades when: Leaf growth lost to weeds is not recoverable - once bulbing begins the onion makes no new leaves, so the bulb is capped at whatever top growth survived. Weed early and often rather than waiting for a big clean-up. Cultivate shallowly and close to the surface; onion roots sit just below it and a deep hoe cuts them. Never hill soil up around onions. A light organic mulch suppresses weeds and steadies soil moisture at the same time, which also helps prevent split and double bulbs.

Leafminers are tiny fly larvae that feed between the upper and lower surfaces of leaves, creating winding pale tunnels or blotchy patches. They rarely kill plants but can ruin the marketability of leafy greens grown for foliage. Allium leafminer is an emerging pest in the eastern US (first detected in Pennsylvania in 2017) that damages onions, garlic, leeks, and chives.

Triggers: Liriomyza trifolii: 1 generation in ~1 month at typical greenhouse temps, 14 days at 95°F, 64 days at 59°F (UC IPM). Adults active mid-day. Allium leafminer emerges late March-early April, second flight September-October. Broad-spectrum insecticides trigger outbreaks by killing parasitoids.

Risk fades when: Parasitic wasps usually bring populations down as the season progresses unless broad-spectrum sprays disrupt them; damage peaks in warm spring and summer flushes and eases in cooler weather, so risk fades with natural-enemy buildup and falling temperatures (USU Extension, PNW Pest Handbook, UC IPM).