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Just a few problems are likely to be the cause of your poinsettia leaves turning yellow and wilting. If they have been exposed to cold temperatures even for a few minutes while taking them from the store to the car. However, they usually just start to drop their leaves. So if there yellow and wilting, there is probably another cause.
Over- and under-watering is a common cause
Over-and under-watering can directly result in poinsettia leaves turning yellow and wilting. With over-watering, the lower leaves are the first to turn yellow. Check your watering frequency. Wait until the top of the soil is dry before watering your plant again. And make sure that any excess water is draining easily from the pot.
Over-watering can also result in diseases. Powdery mildew (discussed above) thrives in a warm and wet environment. Root rot, another fungal disease, is also due to a water-logged environment. As this disease starts in the roots and travels upwards, you usually don’t see any symptoms until it’s too late to save the plant.
Under-watering results in the plant having not enough water to develop fully and the leaves also turn yellow and wilt. It’s up to you to decide if you have a watering problem and which one it is.
First up are sap-sucking pests that harm the plant’s leaves and reduce the energy available to it to keep the foliage strongly growing. Next there are some fungal diseases that coat the leaves with a powder and cut down on the amount of photosynthesis available for healthy growth. And then we have your watering schedule – either over- or under-watering can have the leaves turning yellow.
Certain species of insects suck out the sap of the poinsettia leaves and other parts of the plant, including the buds, stems and flowers. This leaves the plant in a weakened state, with not enough energy to maintain healthy growth let alone to create new foliage. The leaves turn yellow and may have holes in them if the insects actually eat the plant material. After a while, the leaves wilt, and may turn brown before they fall off the poinsettia.
Several insects are common to most plants and maybe the ones causing damage. My guide, Poinsettia pests and diseases to watch out for, describes what to do about each of these sap-sucking pests that turn your leaves yellow and cause them to wilt.
Check for scale insects. There are 25 species of these, so you probably have at least one kind on your plant. You can see them most of the time as they’re around 6mm long. They have a hard waxy shell which protects their soft bodies from insecticide sprays. It’s best to attack the young nymphs rather that the grown adults.
Thrips are another common sap-sucking insect. These, however, are only about 0.1cm long and so are difficult to see unless they’re clustered in a mass. They’re black, brown or tan and the adults have wings. Look for these on the poinsettia flowers and under the leaves.
Mealybugs also suck the sap out of plants. They’re not as small as the thrips but they do hide in out of the way places on the plant. Look in the leaf sheaf and under the outer coating of the stem. They may also hide in the plant roots. They like to live in groups, so that helps your chances of finding them. And they leave honeydew behind. So search out this clear, sticky substance for evidence of their presence.
Powdery fungal diseases develop a coating on the leaves that stops sunlight getting to the green chlorophyll within them. This reduces the amount of photosynthesis that the poinsettia can carry out, thus lessening the amount of energy that the plant has. This is turn results in the leaves turning yellow, wilting and even dropping off the plant.
Powdery mildew is a common fungal disease found on many plants. It appears as patches of white powder and can be all over the plant – the topside and underside of leaves, on the flowers and buds, and all over the stems. Starting out as separate patches, they eventually all join up and cover the entire leaf and even the entire plant.
Sooty black mildew
Sooty black mildew develops in the honeydew substance left by some of the sap-sucking insects discussed above. It also stops the sunlight reaching the leaf, causing a lack of energy to keep the poinsettia’s strength up.
To treat each of these fungal diseases, go to my article, Poinsettia pests and diseases to watch out for.