1. Moisture stress
The most common reason that plants’ leaves turn yellow is because of moisture stress, which can be from either over watering or under watering. If you have a plant that has yellow leaves, check the soil in the pot to see if the soil is dry.
If you believe that the problem is due to under watering, water the plant more often an a dish to recollect any water that has overflowed, so that the roots can absorb the extra water.
On the other hand, over watering can contribute to the leaves turning yellow as well. If you feel the soil and it is too wet then you know that you have been putting too much water on the plant. In this case the solution is simple in that you should not add as much water or water less frequently.
2. Plant nutrition
The yellowing of your plants can also be a good indication of their nutrition. Specifically, if there is an strange pattern to the yellowing, like if the veins on the leaves are green and the tissue is yellow then it is almost always a nutrient problem.
Common sources of nutrient issues are under-fertilizing or over-fertilizing, so it is important to use fertilizer at the labeled rate.
Frequently people tend to use too much fertilizer on their plants to make them grow faster, but what it actually does is create a toxic environment which “burns” the leaves out causing them to turn yellow.
In addition to the problems listed above, other conditions that lead to the yellowing of the leaves include infectious diseases (fungi or bacteria), poor soil, natural aging of the plant and plant destroying pests.
3. Lack of Light
Another common reason that plants’ leaves turn yellow is because not enough light is reaching the plant. This occurs because the rate of photosynthesis is limited in low light but as the light is increased, photosynthesis increases as well.
The temperature contributes to the color of the leaves as well, when it is either too hot or too cold. In terms of the cold temperatures, cold drafts on most tropical plants will contribute to the yellowing of the leaves. If it is not periodic temperature change like a draft, the leaves will most likely be brown if they are exposed to prolonged cold temperatures, especially when they are positioned near an air conditioner.
5. Normal Aging
As many plants age, the lower leaves will turn yellow and drop off. This is simply a normal part of their growth. It is especially true of foliage plants such as Dieffenbachia and Dracaena, which are popular types of houseplants. In this case, don't worry. If the plant becomes too leggy, consider trimming back the main stem to promote new growth and bushiness.
6. Fungus or virus infection
Plant leaves can also turn yellow when the plant has been affected by a fungus infection or a virus. About 85% of the plant diseases are caused by fungal or fungal-like organisms. In many cases, the plant contacts the fungus when its leaves come in contact with the soil or when the bottom leaves are splashed with tiny drops of dirt. Most fungi develop in warm and moist environments.
When a plant is affected by a fungal infection, usually, you can see the first signs on its leaves, but this rule doesn’t always apply. Not all the time the leaves of the plants affected by fungus turn yellow or entirely yellow. Sometimes the disease starts with brown spots on the lower leaves like in the case of the tomato early blight, or septoria leaf spot. Plants can also be affected by viruses. For example, the rose mosaic virus creates yellow patterns on the leaves of the roses. Sometimes, these color patterns look like a mosaic, therefore, the name “mosaic virus.”
7. Cold Draft
Cold drafts on tropical plants will often cause the leaves to turn yellow and drop. This is different from short periods of exposure to intense cold, which will cause outright browning on the foliage or pale, transparent spots to appear between veins.
If your plant is near an air-conditioner vent in summer or a drafty window in winter, move it to a less turbulent place. Keep an eye on it to see if the yellow leaves spread any further. It's also a good idea to mist tropicals that you're overwintering to increase the humidity.
There are numerous reasons why the plant leaves are turning yellow. Some can be related to a disease, pests, improper humidity, transplanting stress, not enough or too much sunlight, plant aging, etc.
In general, when some of the leaves or all the leaves turn suddenly yellow, that’s a sign that something disturbs the plant. Since the reasons depend upon so many factors, it’s up to you to determine the exact cause based on the information presented in this article.