As pepper plants mature and upper branches shade the plant's first leaves, these lower leaves normally yellow and drop from the plant. Other types of yellowing could indicate serious trouble. Pepper plants need healthy green leaves to convert sunlight to food. When plants turn yellow, you'll need to act quickly to correct problems or lose the plant to stress and starvation.
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Like tomatoes, pepper plants evolved in warm climates and only thrive in northern temperate zones during the warm summer season. Growing peppers indoors for transplanting to the garden improves the timing of the plant's fruiting habits. Peppers need that early start in order to set fruit before nighttime temperatures rise above 21.1 degrees Celsius and stop production. Setting peppers out in the garden too soon exposes the plant's roots to cold soil and slows growth. If temperatures at night range from 10.0 to 12.7C, flowers abort and leaves turn yellow.
Healthy green pepper plants set in nitrogen-poor garden soil gradually lose vigour and turn yellow even when temperatures favour pepper growth. Nitrogen fertiliser restores the plant's green colour and stimulates strong top growth. Peppers also need phosphorous and potassium for good health, so use a balanced garden fertiliser at planting time. Extra nitrogen prevents bacterial spot disease and encourages peppers to mature fruit before other diseases affect the plants, according to MSUcares.com.
Insects feeding on plant sap drain plants of essential nutrients and in severe infestations cause the colour of leaves to change from green to yellow. Spider mite damage first shows as white stippling on pepper plant leaves. As mite populations grow, leaves turn yellow and then to a red or bronze hue. Sap-sucking aphids not only damage plants by draining sap, but also infect plants with viruses. Alfalfa mosaic virus transmitted by aphids causes mottling of pepper leaves with white or yellow patterns. Insecticidal soap sprays offer good control of soft-bodied insect infestations.
Two fungal diseases cause yellowing leaves in peppers and other solanaceous crops such as potatoes and tomatoes. Fusarium wilt enters the plant through feeder roots and plugs vessels that carry water to the pepper's leaves. Plants, at first, show signs of wilt during the day but recover at night. As the infection worsens, entire branches yellow and die. Verticillium wilt attacks lower leaves first but infects the entire plant. Peppers succumb to either disease rapidly, so plant resistant varieties to avoid trouble.
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- University of Illinois Extension: Watch Your Garden Grow -- Peppers
- MSUcares.com; Control Pepper Disease; David Ingram, et al.; October 2010
- Integrated Pest Management Center; Crop Profile for Bell Peppers in Virginia; Therese N. Schooley, et al.; May 2003
- Ohio State University Extension; Fusarium and Verticillium Wilts of Tomato, Potato, Pepper and Eggplant; Sally A. Miller, et al.