Dark Green Romaine Lettuce

Lettuce bolting is the plant shifting from leaf production to a flower and seed stalk. Long days and warm weather drive it: UC IPM notes the stalk "may be stimulated by high temperatures", and Michigan State University Extension adds that lettuce held at 35-41°F for 2-3 weeks early on (even as seed) and then grown at 70°F or above with 16-hour days flowers faster still. That is why spring-sown lettuce bolts in July while fall-sown lettuce rarely does. Once a central stalk emerges, leaves turn bitter within 3-5 days from chemicals called sesquiterpene lactones, and there's no way to reverse it. Plant slow-bolt cultivars, sow so heads mature before summer heat and long days, and harvest cut-and-come-again style to extend the window.

Triggers: Per MSU Extension, lettuce held at 35-41°F for 2-3 weeks early in growth (even as seed) and then grown at 70°F or above with 16-hour days bolts fastest, and UC IPM notes the stalk may be stimulated by high temperatures; heat and long days are the trigger, an early cold spell makes it come sooner. Above 68°F the plant 'devernalizes' (loses that cold head start), which is why fall-sown lettuce bolts less. Drought stress and pest pressure can also speed it up.

On Dark Green Romaine Lettuce: Heat stress and long days push the plant from head growth into flowering, which turns leaves bitter and loosens texture.

Prevention: Mulch, keep soil moisture even, use shade cloth in hot spells, and choose slow-bolting cultivars or fall sowings when possible.

Risk fades when: Bolting is irreversible once the central stalk emerges. The plant is done as a salad crop. Pull it, save seed if it's an open-pollinated variety you like, or compost. Plant fresh seed for fall harvest after midsummer when nights lengthen again — fall lettuce rarely bolts.

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.

Where it lives: Across the US except Alaska; moths fly north from the South each summer

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 most common and damaging foliar disease of lettuce. It causes light green to yellow angular patches on the upper leaf surface, bounded by the veins, with a white fluffy growth on the underside. Older, outer leaves are hit first, and severe infections make heads unmarketable. It spreads in cool, damp weather with prolonged leaf wetness.

Triggers: Cool, moist conditions with leaf wetness are required for infection. Spores are short-lived and dispersed by wind during moist periods. Most active at 50-64°F; activity falls off above about 70°F.

What it looks like: Light green to yellow angular spots on the upper side of lettuce leaves, oldest leaves first, with white fluffy growth on the underside of the same spots. With time the spots turn brown and dry up, and badly infected leaves can die.

Disease or overwatering? Turn the lettuce leaf over. Angular yellow spots on top with white fluffy growth directly beneath them point to downy mildew. Lettuce limp from soggy soil looks washed out all over, with no spots and nothing growing on the leaf underside.

Will it recover? Spotted lettuce leaves brown and dry and do not recover, and seedlings infected on their seed leaves can die. Next time, grow a resistant lettuce variety; watering at the soil, as drip does, keeps leaves drier and lowers the damage but will not stop it in cool, damp weather.

Risk fades when: Warm, dry weather suppresses the pathogen. Three dry days with highs above 70°F exits the active infection window, though infected leaves stay damaged.

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

A soilborne rot that attacks the crown and lower leaves where they touch the ground. Infected tissue turns into a brown, soft decay, the outer leaves wilt, and eventually the whole plant collapses, usually near maturity. White cottony growth and small hard black resting bodies form on the rotted crown. Wet soil and cool, moist conditions drive it.

Triggers: Wet soil conditions favor both Sclerotinia species. Cool, moist weather is needed for S. sclerotiorum to produce its airborne spores; S. minor infects crown and lower leaves in contact with the soil. Sclerotia survive in soil for 2 to 3 years.

What it looks like: On lettuce close to harvest, a brown, soft rot eats into the crown where stems and leaves touch the soil. The older leaves wilt first, then the whole head wilts and collapses. Heavy white mold and small, hard, black resting bodies, up to about 1/8 inch, form on the outside of the rotted crown.

Disease or overwatering? Check the base of a collapsed lettuce. Thick white mold and small hard black bodies on a soft brown crown point to lettuce drop. Lettuce that sags in soggy soil has a firm crown with no mold. Botrytis crown rot and Verticillium wilt can look similar, so the white mold with black bodies is the deciding sign.

Will it recover? A collapsed lettuce cannot be harvested and will not recover. Pull infected plants and take them away; the black resting bodies survive in soil for 2 to 3 years. Next time, keep the bed surface as dry as you can, make sure it drains, and avoid following beans, cauliflower, celery, endive, pepper or tomato.

Risk fades when: Drying soil slows new infection. Five dry days exits the active window, though established crown rot does not reverse and sclerotia persist in soil.

Source for the signs above: UC IPM Pest Management Guidelines: Lettuce, Lettuce Drop

Tipburn looks like a disease but it's the same mechanism as tomato blossom-end rot — calcium can't reach the fast-growing inner leaves of the lettuce head, the cells collapse at the margins, and brown necrotic patches appear. Most commonly triggered by a drought-then-rain cycle: the plant grows slowly when dry, then growth explodes when water returns and the new leaf cells outpace the calcium supply. Heat and rapid nitrogen-fed growth make it worse. Closed-head varieties (iceberg, butterhead) hide tipburn inside the head until cut open — by then it's too late. Open-leaf and loose-leaf lettuces show it earlier on outer leaves and can sometimes be trimmed. Foliar calcium sprays don't fix it because calcium can't move through phloem to inner tissue; the only prevention is steady soil moisture + slower growth.

Triggers: Per UC IPM and Cornell Vegetable Program: tipburn results from calcium-transport failure in expanding inner leaves. Triggers: drought-then-rain swings (USU), high humidity reducing transpiration (Cornell), excess nitrogen pushing rapid growth, soil pH outside 6.0-6.8. Closed-head varieties hide tipburn inside until harvest; open-leaf shows it earlier.

On Dark Green Romaine Lettuce: Rapid head growth under warm weather or irregular moisture disrupts calcium movement to inner leaves.

Prevention: Keep watering steady, avoid excess nitrogen late, and harvest before heat spikes.

Risk fades when: Foliar calcium sprays don't reach inner head leaves and won't reverse damage already done. Once tipburn shows, the head is compromised — trim outer leaves on open types or harvest early. Prevent next planting via consistent moisture, lower-nitrogen fertilization, and avoiding mid-summer heat-stressed crops.

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 soilborne rot that starts on the lower leaves touching the ground, producing rusty-brown sunken lesions that spread up into the head. Affected leaves turn slimy and a brown, dry rot can move into the heart of the plant. It is favored by warm, humid conditions and by heads sitting on wet soil.

Triggers: Rhizoctonia solani is most active in warm, humid weather. Infection begins where lower leaves contact moist soil, so dense heads and wet ground raise risk.

What it looks like: As lettuce heads begin to form, brown, sunken spots appear on the leaf midribs lying on the soil. The fungus then moves into the leaves inside the head, and soft rots often follow until the head collapses.

Disease or overwatering? Lift the lowest lettuce leaves. Brown, sunken spots on the midribs that touch the soil point to bottom rot. A lettuce limp from soggy soil droops all over with no spots on those midribs.

Will it recover? Once rot reaches inside the head, the lettuce collapses and will not recover. The fungus lives in soil indefinitely and is favored by warm, moist weather. Next time, plant lettuce where bottom rot has not been a problem, and avoid beds full of undecomposed plant residue.

Risk fades when: Drier soil and foliage slow spread. Three dry days reduces active risk, though rotted tissue stays damaged.

Source for the signs above: UC IPM Pest Management Guidelines: Lettuce, Rhizoctonia Diseases (Bottom Rot)

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.

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

Fuzzy gray mold on flowers, fruit, and wounded tissue. It thrives in cool, humid, enclosed spaces — University of Minnesota notes this is unlikely to be a problem in open home gardens and rare even in field tomatoes. It's mostly a greenhouse and high-tunnel concern, included here because SoilStack supports those growing environments.

Triggers: Develops at 60-75°F with humidity above 80%. Infection requires 5-8 hours of standing water on the plant tissue. UMN's data shows it's unlikely in open home gardens.

What it looks like: On lettuce, a soft, wet, brownish-gray to brownish-orange rot starts on the oldest leaves lying on the soil, then moves into the crown. Fuzzy gray to brown mold covers the rotted base where it is shaded by the leaves above, and flat black resting bodies can form. Badly hit plants wilt and collapse.

Disease or overwatering? Look under the outer lettuce leaves at the crown. A soft, wet rot covered in gray to brown fuzz points to gray mold. Lettuce that sags from soggy soil has no fuzz and no rotted crown. The wilt and collapse alone can look like lettuce drop, so the gray to brown fuzz is the sign to go by.

Will it recover? A lettuce crown that has rotted through will not recover; the plant wilts and collapses. Pull it. Next time, avoid bruising or breaking leaves, keep the bed surface from staying wet for long stretches, and do not set out overgrown transplants, which break and get infected more easily.

Risk fades when: Temperatures above 82°F suppress growth and spore production. That's the published threshold.

Source for the signs above: UC IPM Pest Management Guidelines: Lettuce, Botrytis Crown Rot (Gray Mold)