Drumhead Cabbage

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.

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.

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.

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

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

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.

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

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.

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.

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.

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.

A solid cabbage head is a tightly packed ball of leaves with nowhere left to expand. When the roots suddenly take up more water than the head can absorb - classically a heavy rain after a dry spell - the inner leaves swell, pressure builds, and the head cracks open. Illinois Extension puts the cause plainly: splitting is the pressure of excessive water taken up after the heads are solid. That last clause is the whole story. Immature heads have a loose internal structure with enough give to absorb the swelling, so they do not split; only firm, mature heads do. Excess nitrogen makes it worse by pushing rapid leaf growth during head formation, which USU notes produces both loose heads and splitting. A split head is still edible - it just will not store, and the exposed inner tissue toughens and collects grit and insects the longer it sits. There is a rescue trick, and extension services do endorse it: sever the roots on two sides with a spade, or lift and give the head a quarter turn to break some side roots. It cuts water flow to the head. But it damages the plant, so the head must be harvested soon after.

Triggers: Illinois Extension: splitting is caused by the pressure of excessive water taken up after the heads are solid. USU Extension: moisture fluctuations during heading will cause maturing heads to split open, and high nitrogen applied once heads begin to form causes loose heads and splitting. The disorder is gated on maturity - immature heads have a loose internal leaf structure that flexes as cells expand, so they do not crack. Leaving a mature head in the ground past readiness raises the risk with every rain event.

On Drumhead: Large heads crack when maturity and moisture surges coincide.

Prevention: Harvest on time and avoid big drought-to-deluge swings.

Risk fades when: A split head does not heal. Harvest it immediately and use it first - split heads store poorly, and the exposed inner tissue toughens and traps grit and insects. Per Illinois Extension, cutting the roots by spading on two sides of the plant, or breaking them by lifting and twisting the head a quarter turn, reduces further splitting by cutting water flow to the head; USU describes the same technique. Both damage the plant, so the head must be harvested relatively soon afterward. Prevention for the next crop is even soil moisture, mulch, and stopping nitrogen once heads begin to form.

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.

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.

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).

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).