Why is my lettuce dying?

QUESTION: I’m noticing my lettuce plants starting to turn brown. Why is my lettuce dying? -Kim T

ANSWER: There are many reasons a lettuce plant may start to die, which we’ve outlined here along with the signs and solutions for each. The more quickly you can work to assess the problem and find a solution, the better the chances of your lettuce recovering.

• Anthracnose: You can tell a plant is struggling with anthracnose by the watery spots that appear on the outermost leaves. Spots will expand, and when they’re mature, the center of the lesions falls out, leaving infected plants full of holes. Anthracnose is a fungal disease that’s spread through splashing water. If your plants are struggling with anthracnose, be careful not to overwater them, and always water from the base to avoid splashing water on their foliage. Remove and discard any plants that show symptoms, then work to address the conditions that allowed anthracnose to strike. Rotate crops every year and use certified disease-free seeds to lessen the risk of plants contracting anthracnose. Topping your lettuce plot with a layer of mulch can help keep this disease from spreading and also keeps water in the soil instead of letting it splash on your plants.

• Crowded growing conditions: When lettuce plants are crammed too close together, the plants won’t ever grow to be large, and their leaves will taste bitter. Thin out or reposition your plants to make sure they have 12 to 14 inches of room between head lettuces and six to 10 inches of distance between for looseleaf varieties of lettuce.

• Damping off: Damping off affects seedlings as they’re just starting out, as early as the germination stage and usually before transplanting into the garden. It’s caused by a combination of environmental factors like excess moisture or high humidity and microbes (Rhizoctonia solani, Pythium species, Sclerotinia species, and Thielaviopsis basicola) and can affect any kind of plant. Seedlings show they’re struggling with damping off with brown spots or white mold on the stem, and there may be mold visible on the soil as well.
  
  Plants that are affected gradually weaken, fall over, and then die. In some cases, seedlings won’t show any symptoms, but underneath the soil, the roots are black or brown from the disease. There is no way to treat the problem once it’s started other than planting a new batch of seeds and correcting the problems that caused damping off. Never sow your seeds into cold, wet soil. Make sure seedlings have plenty of drainage and aren’t being overwatered. Give young plants plenty of ventilation, too—once seeds have germinated, remove any plastic cover you’re using over their container. If you continue to struggle with damping off, try using a soil-free starting mix. After the first few weeks of their growth cycle, plants are no longer at risk for damping off.

• Heat stress: Most varieties of lettuce you’ll grow in your garden are at their best in cool or moderate weather and will struggle some during the hottest times of the year. When the thermostat goes up, leaf production slows down, and foliage may wilt. In extreme cases of heat, lettuce plants may start to bolt. Look for heat resistant varieties to grow during the summer, or grow lettuce during cooler parts of the year.

• Lettuce drop: Lettuce drop is a term used to describe the effects of infection by two different fungal pathogens: Sclerotinia minor and Sclerotinia sclerotioru. If your lettuce plants are infected, you may see withering foliage or rot on the plant’s leaves, stems, or in the root system. Plants will grow more slowly than usual, then eventually collapse and die. Make sure your soil offers plenty of drainage and ensure that conditions don’t stay too wet to reduce the chances of your plants getting lettuce drop. Keep leaves as dry as you can by watering plants from the base. Dig up any affected plants and destroy them, then address the moisture problem by increasing drainage in the soil or moving plants to a more hospitable location.

• Powdery mildew: This fungal disease shows up as white powder on the top and underside of leaves, or you may be able to see the black spores in some cases. In addition to the mildewy appearance, leaves may turn yellow or brown when they’re fighting against powdery mildew. This disease tends to thrive when the weather is humid, and it can be spread by wind. Treatments for powdery mildew include sulfur, neem oil, and potassium bicarbonate as well as fungicides. Often, gardeners remove and destroy plants that have contracted powdery mildew instead of treating them.

• Problems with water: Your lettuce can start to die if it’s getting either too much or too little water. The plant won’t be growing as it should, and the foliage may turn yellow or  wilt. If soil is too moist, plants can get fungal disease, and if the problem persists, the root system can be damaged by root rot. Make sure the soil provides plenty of drainage, and if you’re having to water your lettuce too often, consider adding mulch to help the soil retain water. Lettuce plants should get around an inch of water per week, between rainfall and the water you give them.

• Root nematodes: These garden pests live in the soil and attack plants by burrowing into their roots. Affected plants will display stunted growth as early as the seedling stage and may develop galls on their roots. In especially bad cases, the foliage above the surface of the soil may start to wilt, change colors, or die. Keep plants strong by making sure they get the right amount of water and fertilizer so they can fight back against root nematodes. Pull up any plants that are left at the end of the season to prevent nematodes from reproducing in their remains, and rotate your lettuce to a new spot in the garden each season to avoid nematode issues.

• Septoria leaf spot: Plants struggling with septoria leaf spot get small spots on their oldest leaves that eventually turn brown and dry out. In especially bad cases, the spots can combine to form large necrotic areas on the foliage, causing leaves to wilt and eventually, the death of the plant. Septoria leaf spot is caused by a fungus that spreads during wet, humid weather and can hide out in wild lettuces over the winter. Remove all affected foliage from your plants as soon as you notice a problem, and you can also treat plants with fungicidal sprays if necessary. Avoid splashing your lettuce plants with water, instead watering them at the base. Whenever possible, start with seeds that have been certified as disease-free to ensure your seeds aren’t infected. Practice crop rotation to lower the risk of crops being infected with septoria leaf spot.

• Sunscald: Like people, plants can be burned if they get too much sun. Sunscald can show up as pale blistered spots or brown singed areas on foliage. Plants that are too damaged from sunscald can wither and die. Prevent losing your lettuce to sunscald by hardening off your plants to introduce them to the outdoor sun gradually, and plant them somewhere that offers some shade for protection. If your property doesn’t offer a lot of shade, you can plant lettuces in the shadow of other crops or use shade cloth.

• Temperature or timing issues: If you planted your seeds when the weather was warm, it may simply be too hot outside for them to germinate. Lettuce has a 99 percent success rate for germination at 77 degrees Fahrenheit, but add just nine degrees to bring the temperature up to 86, and their nearly perfect record for germination drops to 87 percent. If you really want to plant lettuce in the summer, choose heat resistant varieties, and use mulch to help keep the soil cool.