When we’re growing pepper plants, we expect lush, green bushes or miniature trees loaded with sweet peppers and chili peppers. So when we see that our pepper plants are turning yellow, we start worrying.
If you see only a few leaves turning yellow and falling off, that’s nothing to worry about. This often happens with leaves shaded under heavy foliage of the rest of the plant. Plants will also sometimes defend against disease by dropping leaves at the first sign of infection.
However, if you notice many pepper leaves are turning yellow, called chlorosis, this is often a sign of a bigger problem.
Common causes of pepper leaves turning yellow include a lack of specific nutrients (nitrogen, magnesium, or iron), overwatering and underwatering, diseases or pest infestations, colder temperatures, and more rarely, excess chlorine.
Why Your Pepper Plant Leaves Are Turning Yellow
1. Lack of Nutrients
The most common reason for yellowing leaves is a lack of nutrients. But before you start dumping fertilizer over your peppers, you should know what type of nutrient deficiencies can cause pepper plants to turn yellow.
It’s also important to check how much you water because that can indirectly impact nutrient uptake. Soil that is too alkaline (very high pH) can also affect how well pepper plants can absorb nutrients. Nutrient availability is optimal at a neutral or very slightly acidic pH.
Nitrogen is one of the main nutrients needed for plant growth, along with phosphorus and potassium; it’s the N in NPK fertilizer numbers. Nitrogen (in the form of nitrate) can freely travel throughout the plant and is important for leaf and stem growth. Nitrogen deficiency is identified by a yellowing of lower leaves, while the new leaves are green, but often a lighter green than normal, as nitrogen is pulled out of the lower leaves and into new shoots.
Magnesium is a micronutrient only needed in but essential for photosynthesis. At the center of every chlorophyll molecule is a magnesium atom, which is what harvests energy from the sun and starts the process of converting that energy into a form plants can use. It’s also partially mobile in the plant.
A magnesium deficiency in pepper plants can be identified by a yellowing of leaves while veins remain green.
Iron is very important for photosynthesis and plant metabolism, but is mostly immobile in the plant. That means you will see it mostly in new growth. If your pepper plants have an iron deficiency, you will notice that new leaves are yellow or white with green veins while the rest of the plant looks normal.
2. Overwatering or Underwatering
As mentioned above, a lack of nutrients in the soil can turn pepper plant leaves yellow. But sometimes pepper plants can’t get the nutrients they need because of overwatering and underwatering.
Pepper roots need a moist environment to properly absorb nutrients from the soil, so underwatering or letting your pepper plants dry out can negatively impact nutrient uptake.
The opposite, overwatering, also interferes with the absorption of nutrients in two ways. Roots that are completely submerged in soggy soil can end up rotting (root rot) which can stunt pepper growth and in some cases kill your plants.
If you grow peppers in pots, overwatering can also flush out nutrients from your potting mix. This is why I like to give my container peppers a light liquid fertilizer feed every 1 to 2 weeks.
Overwatering is also often the cause of yellow leaves on young pepper seedlings. Always keep your seed starting mix moist but not soggy.
3. Disease and Pests
There are a whole host of diseases and pests which can cause pepper leaves to turn yellow. If you live in a very humid climate, like in the deep south or a tropical country, diseases are more likely to be the culprit.
If your pepper plant leaves are turning yellow with brown spots, that’s usually a sign of bacterial leaf spot. The best way to deal with bacterial leaf spot is through preventive measures.
First, bacterial leaf spot can be transmitted by infected seeds, so if you planted peppers from seeds and you noticed one variety is suffering from bacterial leaf spot while others are healthy, avoid planting those seeds and get new ones next year. The Department of Plant Pathology at Cornell advises you can also use hot water or bleach treatment on seeds, but this may impact germination. The easiest way to avoid leaf spot is by avoiding the collection of water on your pepper leaves; always water your pepper plants at the base of the plant, not over the leaves. If this becomes a serious problem in your garden, Cornell also advises avoiding planting peppers in that location for 3 years (3-year crop rotation).
Some fungal diseases can also cause pepper leaves to turn yellow, such as fusarium wilt and Southern blight. While you can use organic fungicides to treat fungal diseases, preventive measures such as avoiding hot and wet conditions (fusarium wilt) or hot and dry conditions (Southern blight) are the best defense.
Pests can also cause yellowing of pepper leaves, but pests such as aphids, thrips, and flea beetles will only cause noticeable problems during large, obvious infestations, so you will know it’s a pest problem.
4. Cold Temperatures
There is scant research on this topic, but I and other gardeners have noticed that pepper plants tend to start turning yellow in the colder months, well before a killing frost.
This seems to be a natural process, and since it happens at the end of the season, I don’t worry too much about it. But if you live in a climate with cold, but not freezing, winters, you can add cold protection like garden fleece or plastic; or you can place container peppers against the south wall of your home where the microclimate is a couple degrees warmer.
5. Excess Chlorine
In most cases, this won’t be the cause of your pepper plants turning yellow, but it’s important to mention it here. Too much chlorine in the soil can end up causing pepper leaves to turn yellow and dry up.
Excess chlorine can be caused by salt buildup from too much fertilization over the years, or it can be caused by high levels of chlorine in water. The only way to make sure this is the case is by getting your soil tested. If you suspect the problem is excess chlorine, you can flush out your pots with rainwater or any dechlorinated water (allow chlorinated water to sit for a few days to naturally dechlorinate). If your peppers are planted in the ground and there is too much chlorine or salt, you will have to gradually improve the soil by increasing drainage, adding organic matter, and/or building up raised beds with fresh compost.