Yellowing leaves can be a sign of several things, from diseases to pests to water stress and nutrient deficiencies. If you notice pepper plant leaves turning yellow, check your plants carefully to figure out what is wrong with them. If the problem is a disease, it will probably be evident from other signs, such as black spots on the leaf or wilting. If the problem is an insect infestation, it may also be indicated by tiny dots on the leaves. You can treat many insects with neem oil or insecticidal soap.
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Another cause of pepper plant leaves yellowing is fungus. Fungal diseases like bacterial leaf spot, fusarium wilt, and southern blight can all cause a general sluggishness of the plant and yellowing of the foliage. This type of fungus will often spread to the fruits, as well. To prevent this, avoid extremes of wetness and dryness. A fungicide can be used, but it is best to use one in the early stages of the disease and reapply it as necessary.
Sometimes, your plants may simply need more of the nutrients that make them green. Nitrogen, magnesium, and calcium are all important to the plants’ growth, and you can help them out by using a well-balanced fertilizer or enriching the soil with these minerals.
In some cases, your plants may just need a bit more sunlight. If they are not getting enough sunlight, they will slowly begin to yellow and die. This is not as common as other problems, but it can happen.