The Causes & Cures of Yellow Leaves on Tomato Plants
Sometimes, the yellowing of tomato leaves is natural and not a cause of worry, but sometimes, you need to pay attention. Yellowing could indicate something as silly as too much water, but if your tomato plants are under a pest attack, it could turn ugly.
There are many things that can make tomato leaves turn yellow: under or over-watering, a nitrogen deficiency, pests, or a possible disease are among the most common. Some are easy to fix and some are more tricky.
If it's your first time growing tomatoes, mysterious problems can affect your confidence. Many people give up if their first crop fails. Don't quit! Determine the problem and learn how to fix it so that your next crop grows well. Growing tomatoes or any other plant to perfection is a matter of skill, and all skills need to be developed.
Why Do Tomato Leaves Turn Yellow?
Excess or deficient water
Learn how to gauge how much water your plant needs (see below).
Work to aerate the soil.
Virus, fungus, or bacteria
Read on for a complete list of solutions.
Below, you'll find the signs of and solutions for an infestation.
It could be an imbalance of nitrogen, minerals, alkalinity, etc.... see below for diagnoses and solutions.
Lack of sun
If you can move the plant to get more sun, then do; if most of the plant is unaffected, don't worry about it.
It's a normal stage of the growth cycle.
Don't worry about it!
What Makes the Tomato Plant Leaves Turn Yellow?
- Excess Moisture. Tomato plants need perfect soil moisture levels. The soil shouldn’t be too wet nor too dry—always maintain the Goldilocks level. Determine how much water is needed by taking your weather conditions, the soil type, and the level of mulch you use into account. In some places you'll need to water multiple times a day, but in others, watering just a few times a week is fine.
Root Rot is one of the possible outcomes if you overwater potted tomato plants. When the roots of the plant are flooded with water for extended durations they cannot breathe anymore. This lack of oxygen causes the plant tissue to die and eventually decay. The decayed sections of the root spread which leads to the death of the plant.
There are two possible causes to root rot though, one is overwatering and the other is the root rot fungus. The fungus usually lies dormant in the soil and turns active only in very moist conditions under extended durations of time (such as overwatering). Over watering just once could be sufficient to turn this dormant beast active.
- Compacted Soil. The first time I planted tomatoes, I planted seeds directly into the ground, skipping the nursery stage. This resulted in the plants turning yellow when they were around a foot tall. After some research, I found out that the soil wasn't aerated well enough and it was too compact. The plants recovered after I dug up and loosed the soil a bit, which gave them enough room to spread their roots and breathe.
- Viral, Fungal, or Bacterial Attacks. Prevention is the only guaranteed solution for these issues. If your plants exhibit any of these problems dramatically, it's best to uproot and burn them since these pathogens are likely to spread to the neighboring plants. If the issue is soil-borne, you can't re-plant in that spot without risk. Here are the most common:
Septoria leaf spot, or Septoria lycopersici, is a fungus that causes gray or brown spots surrounded by yellowed areas, and usually starts at the lowest leaves. Do what you can to reduce moisture in the area by removing affected leaves, watering the soil without wetting the leaves, and doing what you can to increase air movement to help evaporation. Be careful not to cross-contaminate. Use a fungicide.
Early blight, a fungus called Alternaria solani, appears on the lowest, oldest leaves first. It looks like little brown spots with concentric rings that form a “bull’s eye." Eventually, the leaf turns yellow, withers, and dies. Treat as you would Septoria leaf spot.
Bacterial wilt, aka Ralstonia solanacearum, is a soil-borne bacterium common in moist, humid, sandy soils. It moves quickly up from the roots to the stem. Remove and burn the affected plant so the bacteria doesn't spread.
Verticillium wilt from Verticilliurn alboatrum, a soil-borne fungus that lives in many cool Northeast gardens. It starts as yellow patches on the low leaves and progresses to brown spots and curled, dead leaves. The plant can't be saved, but the soil can be treated to prevent problems in the future.
- Pests. Pests can be controlled using predators or other natural methods. Pests are not always obvious—you may need to look closely at the stem or turn over the leaves and search to find them. I've seen white insects (mealybugs) and aphids. Soapy spray gets rid of mealybugs and ladybugs feast on aphids. You may want to search for natural ways to get rid of mealybugs if you're going the organic route.
- Nutrient Deficiencies. Macro and micro-nutrient deficiencies are a leading cause of yellow leaves in all plants, not just tomatoes. It does not necessarily mean that your soil is lacking—there are instances where the plant is simply incapable of absorbing nutrients. See below for more information.
Yellow Tomato Leaves Sometimes Indicate Nutrition Deficiency
- Absorption Difficulty (or Under-Watering): Tomato plants can absorb nutrients only through their roots. If this is prevented for any reason, then they are going to lack key nutrients. Water is the medium through which they absorb nutrients from the soil. Therefore, you need to ensure they get sufficient water, but not too much (see the point about excess moisture above).
- Imbalanced Alkalinity: Is your soil pH acidic or alkaline? Tomato plants need the right pH range for successful absorption of nutrients. You’ll need to add a little fertilizer, but don’t over-fertilize, which leads to high pH.
- Lack of Nitrogen: When there's a deficiency of nitrogen, the older leaves at the bottom usually turn yellow whereas the upper, new leaves remain bright green as though there’s no problem at all. However, you’ll notice that the overall growth rate drops and your tomato plants will be shunted. You could add urea or ammonium to the soil or any other form of manure.
- Deficiency of Potassium: Here, the leaf as a whole doesn’t turn yellow, but the area between veins turns yellowish and the leaves may wilt. You could add potash to your soil.
- Calcium Deficiency: The growing tips of the plant may turn yellow and die within a few days. This is known as blossom end rot. Adding any compound containing calcium will work wonders.
- Lack of Magnesium: This will result in stunted growth and the outer edges of the leaves may become pale and yellow. Epsom salts are a good source of magnesium.
- Sulphur Deficiency: Do the new leaves look yellow but the older foliage remains fresh and green? Does the plant suffers from stunted growth? Add sulphur.
- Zinc Deficiency: Lack of zinc leads to the area between veins turning yellow, especially in the new leaves. This often leads to a bunch of small leaves at the top (a rosette).
All of these issues can easily be prevented if you amend the soil.
Yellow Leaves on Tomato Plants – Not Always a Cause of Worry
If you observe any plant, eventually you'll see the older leaves wilt and die. Similarly, your tomato plant will also have yellow leaves at the bottom. This is a normal stage of the growth cycle. Also it could indicate a lack of sunshine due to shading by the higher leaves. As long as the plant continues to grow healthily and produce fruits, you need not worry.