Chinese Broccoli (Gai Lan)

The Delia root-maggot complex includes the cabbage maggot (D. radicum) on brassicas and root crops, the seedcorn maggot (D. platura) on bean/pea/corn seedlings, and several other species. Larvae tunnel into roots, basal stems, and seeds, killing seedlings outright or creating tunnels that ruin root crops for market. A 2021 Oregon industry survey found 100% of root crop growers reported cabbage maggot damage; 44% with 10-25% yield loss.

Where it lives: Throughout the United States, including Alaska and Hawaii

Triggers: Overwinter as pupae in soil/crop residue. Adults emerge early spring (300-600 GDD base 40°F after Jan 1 in PNW). Cool moist soils favor egg survival; soil >95°F in top 2-3 inches kills eggs. Multiple generations per year. Seedcorn maggot attracted to decaying organic matter — high risk after fresh-incorporated cover crop.

Risk fades when: Cool, wet weather favors root maggots and damage is worst in early spring, dropping to its lowest in midsummer when soil temperatures above about 95 F kill the eggs; a late-summer flight can still injure fall root crops, so risk returns as weather cools (UMaine Extension, Wisconsin Hort, UNH Extension).

A watery soft rot of stems, lower leaves, and heads. Infected tissue turns into a soft, watery decay, plants wilt and collapse, and dense white cottony growth with hard black resting bodies forms on and inside the stems. It is favored by cool, moist conditions and wet soil, and the resting bodies survive in soil for years.

Triggers: S. minor infects stems and leaves touching the soil; S. sclerotiorum also forms small mushroom-like apothecia that release airborne spores infecting upper leaves and flowers. Wet conditions and dense canopies raise risk; sclerotia persist in soil for years.

What it looks like: On cabbage-family crops, water-soaked brown patches on stems or leaves near the soil quickly turn to a soft, watery rot, and the plant wilts and collapses. Heavy white mold and small, hard, black resting bodies form on the outside and inside of the stems. Airborne spores can also rot upper leaves and flowers.

Disease or overwatering? Split a collapsed cabbage-family stem. White mold with hard black bodies on or inside it points to white mold (Sclerotinia). A plant wilting from soggy soil has a firm stem with no mold and no black bodies.

Will it recover? Rotted cabbage-family plants wilt and collapse and will not recover, and the black resting bodies keep the disease in the soil for years. Next time, avoid wet soil, rotate, and turn the soil deeply; that helps against the soil-borne form but not against spores blown in from nearby plantings.

Risk fades when: Drying conditions slow new infection. Five dry days exits the active window; established rot does not reverse and sclerotia persist in soil.

Source for the signs above: UC IPM Pest Management Guidelines: Cole Crops, Sclerotinia Diseases

A brassica-specific Alternaria disease that causes dark target-like spots on the leaves and rot in the heads of cabbage-family crops. It infects brassicas only (including brassica-family root crops like turnips and radishes via specific overrides) — it won't show up on carrots or beets.

Triggers: Cool to warm wet conditions, wind, rain, and infected seed or plant debris all favor spread and infection.

What it looks like: Pin-sized black specks that grow into round spots with rings, like a target, and a yellow halo. Shows first on older, lower leaves; spots can tear. Can also brown broccoli heads and cauliflower curds.

Disease or overwatering? Target-like rings point to Alternaria. Even yellowing or wilting after soggy soil, with no spots, points to too much water.

Will it recover? Spotted leaves don't heal. Remove badly spotted lower leaves and clear plant debris after harvest. The fungus also travels on seed and cabbage-family weeds.

Risk fades when: Moisture drives new infection and spread. Dry weather reduces new pressure, though existing infections and inoculum in the soil persist.

Source for the signs above: Cornell Vegetables, Alternaria Leaf Spot of Brassicas

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.

Where it lives: Throughout the United States

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

A cool-weather foliar disease most damaging on seedlings and young plants. It shows as faint yellow spots on the upper leaf surface with a white to gray fuzzy growth on the underside. On heads and curds it can cause dark speckling that downgrades quality. It is driven by cool temperatures, leaf wetness, and high humidity.

Triggers: Favored by cool temperatures and prolonged leaf wetness or high humidity. Most serious on seedlings, where it can kill young plants; on older plants it spots leaves and can speckle heads.

What it looks like: Irregular yellow patches on cabbage-family leaves that later turn tan to light brown, with white fluffy growth on the leaf undersides when the weather suits it. Heavily hit leaves look blighted, and in broccoli and cauliflower it can leave black streaks and patches inside stems and floret branches.

Disease or overwatering? On cabbage-family plants, yellow patches with white fluffy growth underneath point to downy mildew. A seedling pale and limp from soggy soil has no patches and nothing growing underneath. On young transplants the first spots can look like bacterial leaf spot, so check the underside for the white growth.

Will it recover? Spotted leaves do not recover, and seedlings with badly infected seed leaves and first true leaves may die. It needs cool, moist weather and carries over on weeds and in crop residue, so clear both. Next time, choose a resistant broccoli variety where one is offered.

Risk fades when: Warm, dry weather suppresses it. Three dry days with highs above 75°F exits the active window; spotted leaves stay damaged.

Source for the signs above: UC IPM Pest Management Guidelines: Cole Crops, Downy Mildew

Flea beetles are small (1/16-inch) shiny beetles that jump like fleas when disturbed. They chew small round 'shothole' or 'pinhole' damage in leaves and can destroy emerging cotyledons of broccoli or eggplant in 24 hours. Most species are host-family specific — crucifer flea beetle on brassicas, tuber flea beetle on potatoes, eggplant flea beetle on solanaceous crops.

Where it lives: Across the US

Triggers: Overwinter as adults in leaf litter and field margins. Active at mid- to late-spring temperatures. Warm winter → higher next-spring populations (NC State). Hot dry conditions amplify damage on stressed seedlings.

On Chinese Broccoli (Gai Lan): Small beetles chew shot holes in young leaves.

Prevention: Use row cover at planting and keep seedlings growing quickly.

Risk fades when: UMN: 1-2 generations in Minnesota, populations crash after mid-June

A warm-soil wilt that turns leaves a dull yellow-green, often more on one side of the plant, with downward curling and leaf drop from the bottom up. Cutting the stem shows brown discoloration of the water-conducting tissue. Unlike most brassica diseases it is favored by warm soil, and the fungus persists in soil for years.

Triggers: Favored by warm soil (roughly 80-90°F). The fungus enters through the roots and clogs the water-conducting tissue, causing one-sided yellowing and wilt. It survives in soil for many years; resistant cabbage varieties are the main defense.

What it looks like: Mostly a cabbage disease. Lower leaves yellow, often on one side of the plant, then turn brown and drop. A cut stem shows brown discoloration in the water-conducting tissue, and in time the whole plant may yellow, wilt and collapse.

Disease or overwatering? Cut the cabbage stem. Brown discoloration inside the water-conducting tissue, with yellowing heavier on one side, points to Fusarium yellows. A cabbage drooping from soggy soil yellows evenly and is clean inside. It is worst in warm soil, 75 to 85°F, and barely develops below 60 F.

Will it recover? Yellowed, wilted cabbage will not recover, and the fungus survives in soil indefinitely, so do not move soil or plant debris from that bed. Next time, grow a resistant cabbage variety and plant in spring or winter, when soil is cooler; resistance weakens as soil warms.

Risk fades when: Activity slows as soils cool below about 75°F. Seasonal pressure fades, but the fungus persists in the soil for many years.

Source for the signs above: UC IPM Pest Management Guidelines: Cole Crops, Fusarium Yellows

Hollow stem is what it sounds like — the main stem of broccoli or cauliflower develops a cavity, usually visible when you cut the head off. There are two distinct causes that look slightly different inside. Boron deficiency hollow has watery tissue that browns or blackens over time, with visible brown patches in the stem wall. Rapid-growth hollow (from excess nitrogen, hot weather, or wide spacing) has clean bright tissue inside the cavity — no browning. Boron deficiency is most common in sandy soils with low organic matter where boron leaches out easily, or in high-pH soils where boron is locked up. Excess nitrogen pushes the plant to grow faster than the central stem can keep up, leaving the cavity. Wider plant spacing (16+ inches) increases the per-plant nutrients available, which accelerates growth and increases hollow risk. The fix is the opposite: close spacing (10-12 inches), moderate nitrogen (no more than 3.5 lb N per 1000 sq ft), and a soil test for boron levels. If the test shows low boron, side-dress with a small amount — but be cautious because boron toxicity is just as serious as deficiency.

Triggers: Per MSU/UDel/WSU/UMN Extension: dual cause. Boron deficiency hollow shows brown/black discolored tissue inside (visible in stem wall). Rapid-growth hollow shows clean bright tissue inside. Sandy low-OM soils, high pH (above 7), and excessive nitrogen + wide spacing (16+ inches) all contribute. Spacing 10-12 inches reduces incidence significantly.

Risk fades when: Hollow stem doesn't make broccoli inedible — cut around the cavity. For next planting: soil test for boron, narrow spacing to 10-12 inches, moderate nitrogen, maintain pH 6.0-6.8. If boron test shows below 1 ppm, side-dress carefully; boron toxicity is just as serious as deficiency.

Slugs and snails are nocturnal mollusks that chew irregular holes in leaves and clip off succulent seedlings. They leave characteristic silvery slime trails. Hermaphroditic and prolific, brown garden snails lay around 80 eggs per month for up to six clutches per year.

Where it lives: Across the US, including watered gardens in the desert Southwest

Triggers: Active at night and early morning in damp conditions. Coastal CA and southeast — active year-round. Spring rains and dense ground cover (mulch, debris, weeds) create harborage.

Risk fades when: Hot, dry summer weather drives slugs and snails into inactivity, so risk drops sharply once conditions turn warm and dry; populations are highest in cool, wet spring and fall, and in mild-winter coastal regions they can stay active year-round (UMN Extension, Penn State, UC IPM).

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.

Where it lives: Throughout the United States

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.

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).

Wireworms are the larvae of click beetles. They live in the soil, eating seeds, seedlings, roots and tubers, and they bore small holes and tunnels into potatoes and sweet potatoes that can make them unusable. For home-garden sweet potatoes, UGA Extension says wireworms and root-knot nematodes seem to be the biggest problems. Their main food is the roots of grasses, so the worst damage comes in gardens dug out of sod or lawn within the past few years. Some kinds take several years to grow up, so a problem bed can stay a problem for years. Severe outbreaks are rare, and once you see damage no spray will fix it: the steps that work are taken before planting.

Where it lives: Found across the US; different wireworm species are the main pests in different regions

Triggers: Larvae and adults spend the winter 9 to 24 inches deep in the soil. In spring, once the soil warms to about 50 F, larvae feed within the top 6 inches; when the soil gets too hot (above 80 F) or dry they go deeper, and as the soil cools in fall they may feed near the surface again. They like moist, cool, heavy soil. Female click beetles lay eggs in grassy, weedy areas, which is why ground that was sod or lawn has the most wireworms. How long one generation takes depends on the kind: from one to six years. The corn wireworm takes up to six years, so larvae of every size are in the soil at once. In the Southeast, the southern potato wireworm has one or two generations a year and the tobacco wireworm has one.

Risk fades when: Wireworms do most of their damage in early spring, when they are near the soil surface; in summer they move deeper, and as the soil cools they may feed near the surface again (NDSU Extension). They become active and look for food as soon as the soil warms in spring (UMass Extension). In the Southeast, wireworms usually attack potatoes and sweet potatoes late in the season (Clemson HGIC; NC State Extension). The month numbers are a reading of 'spring' and 'late season' for most of the US; spring comes earlier in the South and later in the North.

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.

Where it lives: Across the US; the allium leafminer only in the Northeast and Mid-Atlantic so far

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).