Lettuce is a cool-weather crop that can be easy to grow, depending on which type you choose. Loose-leafed varieties are the easiest, while crispheads pose more of a challenge. Lettuces don’t like hot sun. If you’re planting a summer crop, try planting them in the shade of tomatoes or cucumbers. Lettuce plants are prone to a handful of problems that can make them wilt.
Lettuce may wilt during the day, or even die, when under heavy attack by the lettuce root aphid. In addition, the developing heads remain soft. Lettuce root aphids are small, soft-bodied insects that are yellowish or white. Look for them on the roots, along with a white wooly material. Control aphids by planting resistant species and removing Lombardy poplars from the area, because these aphids overwinter on that tree. Before replanting infested soil with another lettuce crop, till the soil deeply and let it thoroughly dry. Insecticides are ineffective against this insect, but applying imidacloprid when you plant your lettuce may help prevent an attack.
Bottom rot is a fungal infection that causes the outer lettuce leaves to wilt. Rust-colored or brown spots appear on the leaf petioles and midribs and a brownish liquid can seep from them. During warm, wet weather, the fungus may infect the leaf blades and the entire head can turn slimy brown. Lettuce is infected with bottom rot when the lower leaves touch the soil, so growing lettuce on a 4-inch-high by 6-inch-wide ridge helps prevent this disease, since ridges keep the bottom leaves off the soil. Fungicides can be applied to help control bottom rot. Infected fields should be plowed after harvest to bury debris.
Sclerotinia drop is a serious fungal disease that causes lettuce plants to collapse in just two days. Early symptoms include wilted outer leaves and the appearance of a water-soaked area on the stem, near the soil line. Leaf bases rot as the fungus grows in them. Inner leaves turn slimy. During moist weather, a white fungal mass appears over the entire head. To prevent sclerotinia drop, plant lettuce in well-drained soil and avoid watering them from overhead. Don’t crowd plants, and rotate crops with small grains because they don’t host this fungus. If you have a small garden, you can remove infected plants to keep the disease from spreading to other plants. Plow debris under the soil as soon as you’ve finished harvesting.
Fusarium wilt is a fungal infection that causes seedlings to wilt and die. A reddish-brown streak that runs from the top of the taproot to the outer layer of the crown appears in mature plants, and older lettuce heads may suffer from tip burn. Other symptoms include yellow leaves and brown or black streaking of the tissues that provide water to the leaves. Fusarium wilt can spread in soil and water that’s been contaminated. There is no way to effectively control this disease except by not planting head lettuce in infested fields for several years and by not moving soil from an infested field into another area.
A buildup of ammonium in the soil causes young lettuce plants to wilt when the soil’s wet or compacted. In addition, plants become stunted. Older leaves may yellow and turn brown. Only individual plants are affected. Other plants in the garden show no symptoms. Examine the roots to determine if you have a problem. In the early stage of toxicity, the central core of the taproot will be yellow or light brown. Later it becomes dark brown or red. Sometimes a cavity forms in the core. Lateral roots may have blackened tips and the outside surface of the root may be cracked and turn yellow or light brown. Avoid using nitrogen fertilizers that contain ammonium.
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