Sunday, December 18, 2011

Reasons for Yellow Lettuce

Regularly inspect your lettuce for yellowing spots or leaves.

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Lettuce, which is among the easiest cool-season crops to grow, is generally a hardy plant, capable of withstanding a light frost. In some temperate regions, you can cultivate this obliging annual in both spring and fall. Loose-leaf lettuce is the fastest-growing and lowest-maintenance of the four available varieties; butterhead, romaine and crisphead, or iceberg, lettuce are the other three varieties. Lettuce, while robust, is not invulnerable. A variety of factors, from improper gardening practices to disease, can cause leaves to become yellowed, or chlorotic. Correctly identifying the cause of the yellow leaves is essential to correcting the problem.

Related Searches: Mineral Deficiencies

Yellowing leaves on lettuce can be a classic symptom of nitrogen deficiency. In some regions, this is particularly likely to occur in spring, when nutrients are easily washed from the soil by rain. According to the Royal Horticultural Society, mulching around the bases of the lettuce with organic compost, such as cow manure or rotted leaves, can re-nitrogenize the soil. For a quicker solution, apply a high-nitrogen fertilizer such as sulphate of ammonia. Follow the label directions for the correct amount.
Magnesium deficiency also causes yellowing, usually between the veins of leaves. This form of mineral deficiency, more common in light, sandy soils, can also be triggered by the overuse of high-potassium fertilizers. The society advises applying Epsom salts -- or magnesium sulphate -- around the roots of the plants at the rate of 1 ounce per yard.

Overwatering

According to the website VegetableExpert, over-watering can also cause yellow and drooping lettuce leaves. As a general rule of thumb, most lettuces should be watered every six to 10 days, to a depth of 15 inches. You need to adjust watering frequencies according to the weather; rainfall may cut down the need for watering, while a string of hot, clear days necessitates more frequent irrigation. Deep but infrequent watering is more effective than frequent, shallow watering; the latter fails to promote strong establishment of roots.

Fungal Diseases

Pale yellow spots on the upper sides of mature lettuce leaves are a classic sign of downy mildew, scientifically known as Bremia lactucae. A white, fluffy growth developing two days after the onset of spots can help confirm the diagnosis. After the initial yellowing, the infected tissue usually turns brown. University of Florida researchers report that downy mildew can be treated with fungicides, including maneb and metalaxyl. If downy mildew on lettuce is a problem in your area, try to buy mildew-resistant cultivars. Avoid overhead watering, which can splash leaves, and don't harvest leaves when they are wet.
Fusarium wilt, or Fusarium oxysporum, usually begins with a reddish-brown streak running from the root to the crown of the lettuce, followed by yellowing leaves that turn brownish-black. Promptly remove infected plants and discard them in a closed container. The best defense against fusarium wilt is buying fungicide-treated lettuce seed.

Requirements

Providing proper conditions for your lettuce can help prevent infections and mineral deficiencies. According to the Ohio State University Extension, the ideal temperature for growth is between 45 degrees F. and 65 degrees F.; higher temperatures stress plants and may cause bitter-tasting lettuce. The soil should be moist, rich in organic matter and well-drained, with a slightly acidic pH of 6.0 to 6.5. To provide proper air circulation, space plants 12 inches apart.

ReferencesRoyal Horticultural Society: Nutrient DeficienciesOhio State University Extension: Growing Lettuce in the Home GardenNorth Carolina State University: Lettuce ProductionUniversity of Florida IFAS Extension: Downy Mildew of LettuceUniversity of Arizona: Lettuce Fusarium WiltVegetableExpert: Watering Your VegetablesPhoto Credit Zedcor Wholly Owned/PhotoObjects.net/Getty ImagesRead Next:

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