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 because it could indicate something as silly as excess water to essential nutrient deficiencies, both of which are easy to fix once detected. Plus, if your tomato plants are under a pest attack, it could turn ugly.
It's not just a deficiency of nutrients that can cause foliage to lose color and deform, but also an excess of certain nutrients which leads to something gardeners refer to as a 'burn' which manifests as deformed, yellowish leaves and fruit.
If it's your first time growing tomatoes, mysterious problems can be detrimental to your confidence and 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. For more on perfect watering techniques, I suggest checking out my article on watering tomato plants.
- 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. Here are the most common:
Septoria leaf spot, or Septoria lycopersici, is a fungus that looks like 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.
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.
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.
These are just a few of the issues you might encounter. Prevention is the only guaranteed solution. If you plants exhibit any of problems dramatically, it's best to uproot and burn them since these pathogens are likely to spread to the neighboring plants. Remember, these are usually soil-borne, so you can't re-plant in that spot without risk.
- 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.