Tomatoes are a favourite warm season crop to grow for gardeners. The plant is extremely versatile and can grow in hanging baskets, containers and staked in the garden. Unfortunately, numerous cultural and natural problems can cause the plant's leaves to turn yellow and dye. It is important to get to the root of the problem to yield a healthy tomato crop.
Gardeners should examine where the tomato plant's leaves are turning yellow. Lack of sunlight at the bottom of the plant can result in yellow leaves, because the foliage needs sunlight to produce the proper nutrients for chlorophyll. Pruning some of the foliage back to allow more light can greatly help the plant to stay a natural colour of green.
All plants need a specific pH range in order to absorb the soil's nutrients. Growing a plant out of this range will prevent the plant to receive nitrogen. Nitrogen is crucial for making protein, amino acids and DNA. Tomatoes must be grown in a soil pH range between 6.0 to 6.5. To find out if tomato plants are grown within the correct range, gardeners conduct a soil pH test on the designated planting area. Gardening supply stores and nurseries both carry soil pH tests that can be used to determine if they are grown in the right range. Soil that is lower than 6.0 should be amended with lime. Alkaline soil that is over 6.5. must be amended with sulphur.
Three common fungal diseases can cause yellowing of the leaves: early blight, powdery mildew and septoria leaf spot. Fungal diseases generally occur when temperatures start to rise in the late spring. In fact, septoria leaf spot can survive the winter in the foliage and plant debris on the soil. Powdery mildew looks like a fine dusting of whitish grey powder on leaves. The fungal spores slowly turn leaves yellow before their eventual death. Dead foliage generally remains on the plant. Early blight is characterised by large yellow spots on leaves with black centres. All three fungal diseases should have their diseased areas pruned and sprayed with a fungicide.
Leaf minors cause considerable damage to tomato crops. These insects tunnel through the tomato leaves and suck out the nutrients. Because of the pathway of the dead tissue they leave in their wake, these bugs eventually prevent the tomato plant from making chlorophyll which can lead to stunted growth.
Planting tomato plants too early can cause yellowing of leaves, because phosphorous in the soil is not available. Soil that is still cool from the winter cannot transfer phosphorous to the plant. Gardeners can use plastic mulch to warm up the soil or wait until the middle of spring to plant their tomato plant.