Problem: The edges of lettuce leaves turn brown and die back.
Diagnosis: Tip burn. Inconsistent watering contributes to the problem, and some lettuce varieties are more susceptible to tip burn than others.
Solution: Use drip tape or soaker hoses to keep the soil consistently moist. Plant resistant varieties, including 'Magenta' and 'Ermosa', a butterhead bred for heat tolerance.
Problem: Tiny green, white, or black insects are clustered on the underside of lettuce leaves.
Diagnosis: Aphids. These pests are a problem, especially when the weather is cool, because they emerge earlier and tolerate colder temperatures than many of their predators.
Solution: Use insecticidal soap to control especially bad infestations, but before you spray, monitor the problem for a few days. During most times of the year, beneficial insects quickly control aphids. Encourage a robust population of aphid enemies by growing plants with nectar-rich flowers, including cilantro, dill, and sweet alyssum.
Problem: Your lettuce is shot through with tiny holes.
Diagnosis: Flea beetles
Solution: Install a row cover over your lettuce immediately after sowing or transplanting to exclude flea beetles from your crop. Row covers also help control thrips, leaf hoppers and aphids.
Problem: Your lettuce seedlings suddenly died.
Diagnosis: Damping-off, a soilborne fungal disease that thrives in cool, damp, cloudy conditions and kills seedlings
Solution: Space plants apart to provide good airflow and reduce watering.
Problem: Mature plants rot or collapse.
Diagnosis: Fungal disease, including bottom rot or sclerotinia drop.
Solution: If you live where cold, damp weather is normal during the lettuce-growing season, space plants farther apart to encourage good air circulation and reduce watering. Plant varieties that resist fungal disease, such as 'Optima'.
Problem: Your lettuce tastes terrible.
Diagnosis: Your plants are old, bolting, or drought-stressed. Solution: Bitterness naturally occurs in older lettuce plants, but it can occur in drought-stressed young plants. Taste-test your lettuce often. To prevent bitterness, keep the soil evenly moist and harvest plants before they begin to form a conical head.
Problem: Overnight, your lettuce developed big holes and ragged edges.
Diagnosis: Slugs or snails
Solution: Remove mulch from around lettuce, because it provides the perfect hiding place for slugs and snails. Sprinkle an organic slug bait, such as Sluggo, around your plants, being sure to follow the package's directions. Set out beer traps. Simply fill a shallow plastic container, such as a yogurt cup, with beer and then sink it into the soil, leaving a 1/2-inch lip above the soil line.