Napa Cabbage

A soilborne and seedborne fungus that rots the stem near the soil line and produces sunken cankers dotted with tiny dark fruiting bodies. Leaves yellow, plants are stunted, and badly affected plants topple or fail to grow. It is most damaging in cool, moist soil, and secondary rots like white mold and soft rot often follow.

Triggers: Most important in cool, moist soil. Spreads on infected seed and in crop debris; spores splash to stems in wet weather. Tiny dark fruiting bodies (pycnidia) in the stem cankers are diagnostic.

Risk fades when: Drier conditions slow splash spread. Four dry days reduces active risk; stem cankers remain.

A brassica-specific bacterial disease causing yellow V-shaped lesions that start at the leaf margins, blackened veins, and gradual whole-plant decline. Affects brassicas only (including brassica-family root crops via specific overrides). Despite the name, it's unrelated to grape or kiwi black rot.

Triggers: Warm wet weather, dew collecting at leaf margins, splashing water, and infected seed or transplants all favor bacterial spread.

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

Blindness in brassicas is when the central growing point (apical meristem) is damaged before head formation, so the plant grows vegetatively but never produces the central head you planted it for. Causes include: severe cold injury below 25°F killing the meristem, cabbage looper or cabbage worm feeding directly on the growing tip, mechanical damage during transplanting, or boron deficiency in extreme cases. The plant otherwise looks healthy — full leaves, vigorous growth — but no head forms. Some plants compensate by producing side shoots from leaf axils (lateral broccoli florets) which can be harvested. Prevention: protect from extreme cold during early growth, control cabbage worms, handle transplants carefully.

Triggers: Meristem damage from cold, pest feeding, mechanical injury, or severe boron deficiency. Plant continues vegetative growth but skips head formation entirely.

Risk fades when: Blind plants won't produce a central head. Sometimes side-shoot florets develop from leaf axils — harvest those. Otherwise the plant is a loss for head production. Compost and replant for fall harvest.

Brassicas are biennial plants from the Mediterranean — they expect a winter cool period to trigger flowering. When spring transplants experience temperatures below 50°F for 2-8 weeks during early growth, they're 'vernalized' (cold-primed to flower). When warm summer days arrive, the plant skips its head-formation stage and shoots up a flower stalk. Different brassicas need different vernalization periods: long-maturing cabbage needs up to 20 weeks of cold to flower; early Chinese cabbage and cauliflower need as little as 1 week. Plants only respond to cold once they've reached 2-15 true leaves — that's why broccoli set out too early (too small) or too cold often bolts.

Triggers: Per MSU/UMN/UDel Extension: cold exposure below 50°F for 2-8 weeks during 2-15 leaf stage triggers vernalization. Warming to >61°F devernalizes some species (cabbage, Chinese cabbage, kohlrabi) but not others. Distinguish from 'buttoning' (premature small head from too-early cold) which occurs in undersized plants. Bolting hits mature plants AFTER vernalization, driven by heat/stress.

On Napa Cabbage: Young plants exposed to cold, heat, transplant stress, or long days can send up seed stalks before good heads form.

Prevention: Favor fall planting, avoid cold-stressed transplants, and keep moisture steady.

Risk fades when: Once vernalized + stressed, brassicas commit to flowering. The current crop is done. Fall plantings (mid-summer sowing, late-summer transplant) rarely bolt because they vegetatively grow in declining day length and harvest before next year's vernalization.

A brassica-specific soil-borne disease that forms swollen, distorted roots and causes plants to stunt or wilt during the heat of the day. Infects brassicas only (including brassica-family root crops via specific overrides) — it won't affect carrots, beets, or other non-brassica root vegetables.

Triggers: Moist, acidic soils plus water movement favor the swimming zoospores that infect roots. The pathogen persists in soil for years once established.

Risk fades when: Moisture drives new infection. Dry weather reduces immediate pressure, though existing infections and soil inoculum persist for the long term.

Corn earworm is the same species as tomato fruitworm and cotton bollworm — a polyphagous caterpillar that bores into ears of corn through fresh silks, into tomato and pepper fruit, into lettuce heads, and into bean and pea pods. In sweet corn, losses can reach 50%. The species migrates north annually from southern overwintering grounds; in much of the northern US, it does not survive the winter when temperatures drop below 30°F.

Triggers: Overwinters as pupa in top 2-4 inches of soil where winter temps permit. North of I-70 (Illinois IPM): does not reliably overwinter — populations arrive via migration mid-July through September. Females prefer fresh corn silks for egg-laying; older silks rejected.

Risk fades when: In the North (above roughly 40 degrees latitude) the insect does not overwinter, so populations arrive only by summer migration and are wiped out by fall frost, resetting each year; in the South it overwinters as a soil pupa and pressure lasts longer (UMN VegEdge, Wisconsin Hort, Cornell IPM).

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.

The imported cabbageworm is the most important caterpillar pest of brassicas in much of the United States. Adults are the familiar small white butterflies with one or two black spots on each forewing, conspicuous as they flutter low over cabbage and broccoli plants on warm days. The velvety green caterpillars blend into foliage and do their damage hidden among inner leaves, often boring into developing heads.

Triggers: Adults active during daytime in warm weather. Female lays 400-1000 single eggs over 5-20 day adult life. 2-4 generations in north, 6-8 in south. Eggs hatch in 4-10 days; larvae through 5 instars in 10-14 days.

On Napa Cabbage: Caterpillars can feed inside loose heads.

Prevention: Use row cover early and scout before heads close.

Risk fades when: Multiple generations feed through fall until a hard frost; the insect overwinters as a chrysalis in garden debris, so activity ends with freezing weather and fall cleanup of crop residue reduces next year's population (Wisconsin Hort, UMD Extension).

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.

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).

Cabbage tipburn (sometimes called 'internal browning') is calcium-transport failure inside the head. Inner leaves are growing fast and demand calcium, but the closed head structure makes them poorly-transpiring — they can't pull calcium up with water flow like outer leaves can. When growth outpaces calcium delivery, the margins of those inner leaves collapse and turn brown. Often unnoticed until the head is cut open. Most common in fast-growing summer cabbage. Slower-maturing varieties and steady moisture both reduce the risk. The same calcium-cascade mechanism causes blossom-end rot in tomatoes and tipburn in lettuce — same physiology, different host.

Triggers: Closed head structure means inner leaves transpire poorly + can't pull calcium with water. Fast-growing summer cabbage most susceptible.

On Napa Cabbage: Rapid growth plus uneven water or calcium movement can brown inner leaf margins.

Prevention: Maintain even soil moisture and avoid growth surges from excess nitrogen.

Risk fades when: Once internal browning forms, the head is compromised. Cut and discard affected portions or compost. Plant slow-maturing varieties and provide steady moisture for next planting.