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Tomato plant leaves turning yellow

Tomato plant leaves often turn yellow due to common garden maintenance issues, including overwatering, underwatering, transplant shock, soil nutrient deficiencies, and compacted garden soil. Yellowing leaves can also be a symptom of common fungal diseases, including blight or leaf spot, as well as of pest pressure from insects, mites, and/or nematodes.

The good news is that the affected leaves can generally be easily removed, and the newer leaves that replace them are much more likely to stay green if the root cause of the yellow leaves is addressed. Here is a list of reasons your tomato plant might have developed some yellow leaves and a few tips to prevent them.

Yellow tomato leaves on plant

Maintenance issues that can cause yellowing leaves

Garden maintenance issues are a major cause of yellowing leaves on tomato plants. This includes too much or too little water, transplant shock, nutrient deficiencies, and soil compaction.

1. Watering tomato plants

Watering your tomato plant is crucial for a happy, healthy, and productive plant. Usually, if it’s a watering problem, it’s either overwatering or underwatering that is the issue. This can take the form of watering too much too often or letting the plant dry out and then giving it an “extra-large” watering all at once. Neither scenario is optimal, as tomato plants grow best with a consistent soil moisture level in which the soil contains both moisture and air pockets.

The problem with overwatering all the time is that it could lead to root rot, especially in climates where the soil is quite moist naturally. If there is too much water in the soil, it can suffocate the plant’s roots, cutting off the oxygen supply to the leaves, which will cause them to turn yellow and drop off. Saturated soil has few/no air pockets available to the roots. It is common to see ponded water on top of the soil (especially in clay soil).

The other end of the spectrum may be that you have underwatered your tomato. If the leaves turn yellow on the edges and then fall off, and you know you have missed a watering session or two, you have your answer. Tomato plants do not like to dry out and will begin to wilt at the first signs of overly dry soil.

Be sure you are watering your plants consistently, deeply, and slowly at the base of the plant. The easiest way to do this is to set up a drip irrigation system that includes hoses that run along the soil with an automated timer that turns the water on and off. Tomato plants may need watering only a couple of times a week in the spring but might need daily watering each morning during heatwaves in mid-summer.

Transplant shock - causes of yellow tomato leaves on plants

2. Transplant shock

This is a common cause of newly planted tomato plants’ leaves turning yellow. When you take the seedlings from a warm greenhouse or from inside your house and plant them in the cold ground, it can shock your young tomato plants.

You don’t need to be too worried as long as your plant is still getting new growth; snip away the yellowing leaves. Removing them will let the plant focus on new growth and recover much faster from transplant shock. Read more about when to transplant tomato seedlings outdoors.

3. Nutrient deficiency

Yellowing leaves are typical for tomato plants with nutrient deficiency. Generally, when your tomato plant lacks nutrients, it is nitrogen that is to blame. Having adequate nitrogen is crucial for leaf production and an overall healthy plant. If you notice the older leaves turning yellow and the plant has stopped growing, it is likely that not enough nitrogen is the culprit. If you’re not really sure, you could always perform a soil test to get a good profile of the nutrients in your soil and what it may be lacking.

For the fastest-acting nutrient input, use a water-soluble or liquid concentrate organic tomato fertilizer. You can also use a slow-release granular tomato fertilizer for the longer term. Lastly, a nice surface mulch of organic compost will provide a natural nutrient boost for your plants.

4. Soil compaction

If the soil around your tomato plant becomes too compacted, it will cut off the oxygen, water, and nutrients to your plant, and it will begin to suffocate. Telltale signs will be yellowing leaves and packed down the soil. This can happen from not using good soil with organic matter to help keep it aerated and from walking on the soil too much.

Fungal diseases

Fungal diseases are another common cause of yellowing leaves on tomato plants. Various fungus-caused plant diseases include blights and leaf spots. Fungal disease in general can be counteracted by ensuring lots of air circulation around the plants, using an organic surface mulch, pulling out weeds while they’re still small, and avoiding overhead watering.

1. Early blight

This fungal disease is widespread in tomato plants and is generally caused by the fungus Alternaria solani. Early blight is always a threat to anyone growing tomatoes as it hangs out in the soil, and if the conditions are right, hot and humid, it will attack your plants. This nasty fungus can affect the leaves, stems, and fruit of your tomato plant to the point that it kills it.

Early blight will make its first appearance on the older leaves that are closer to the ground. It will start as small dark spots, and as the spots grow, they will begin to look more like target rings. This fungus can move to the stem, eventually girdling the plant and killing it. It can also move to the fruit itself and cause concentric rings and have lesions with a leather-like appearance.

Start treatment by immediately removing and disposing of the affected leaves off the bottom of the plant. Then spray the plant with a natural fungicide, being sure to spray the undersides of the leaves and the stems. Also, check to ensure plants have adequate air circulation and surface mulch while also pulling out weeds and avoiding watering the leaves of the tomato plant.

2. Late blight

Late Blight is the more serious and more common blight fungi caused by Phytophthora infestans. It is usually lethal to your plants, although they generally succumb to cold temperatures first in cooler climates.

Unlike early blight, late blight prefers cool, damp weather. It will show up on your tomato plants on the stems as hard, dark brown areas that have rounded edges. If it moves to the fruit, they will develop dark brown spots; eventually, secondary infections will set in, and the fruit will turn mushy.

Start treatment by immediately removing and destroying all affected tissue. Be sure to remove any affected tomatoes off the plant. Then spray the remaining leaves and stems with an organic fungicide.

3. Septoria leaf spot

Caused by the fungus Septoria lycopersici, it is likely to affect tomato plants, as well as other plants in your garden in hot and humid weather. It starts on the underside of the lower leaves of your tomato plants. The infection begins as a small dark brown spot with a lighter-colored center and a yellow outline around the whole area.

Start by removing all affected leaves off the plant. Then treat the remaining leaves with an organic fungicide to lessen the effects on new leaves.

Pest problems

Garden pests are another common reason for tomato plant leaves turning yellow. Pests range from large caterpillars to tiny bugs and mites and even microscopic nematodes.

1. Tomato hornworms

Hornworms are not worms at all, but the caterpillars of sphinx moths, and they will decimate your tomato plants. Hornworms have big appetites and can eat leaves and stems very quickly.

You will likely see the damage the hornworm has caused before you notice the hornworm itself. It blends in perfectly with plants and can go undetected until it has caused damage to your plants. The hornworm also enjoys munching on peppers, potatoes, and eggplants.

You will notice leaves and stems being eaten and black droppings on the leaves and around the base of the plant. If caught early and removed, your plants will recover and continue to put out fruit for you.

2. Spider mites

Spider mites are tiny little insects that you may not be able to see, but you will notice their webbing on your tomato plant; eventually, the leaves will turn yellow and drop off the plant. Spider mites will kill your tomato plant if not treated quickly and properly. They are relatively easy to ward off. Keeping your plants watered and fertilized properly will make them much more resistant to a spider mite infestation.

If you do notice spider mites on your tomato plants, prune away any heavily infested leaves and wash the rest of the plant with a mild soap and water mixture.

Tomato plant leaves turning yellow

3. Root-knot nematodes

These are tiny worms that affect the root system of your tomato plants. You won’t see them since they affect the roots of your plants, but some signs that root-knot nematodes are destroying your plants are yellowing leaves, wilting leaves, and stunted growth. They cause swelling and growths that look similar to knobs on the roots (root galls).

Root-knot nematodes can be tricky to control or get rid of altogether, but not impossible. Rotating your crops is crucial so they cannot establish themselves, as well as tilling your soil will stop them from becoming entrenched in the dirt. You may also want to try companion planting with marigolds, such as the popular ‘Nema-Gone’ Marigolds For Nematode Protection from Burpee.

Preventing yellow leaves on tomato plants

Here are some tips to prevent problems with yellow leaves (and other common issues) when growing tomato plants:

  • Practice good hygiene. Always disinfect tools, stakes, cages, and anything else you are using in your garden to ensure you are not introducing bacteria to your plants.
  • Use disease-resistant seeds. There are so many seeds available that are disease resistant and can help better the chances of healthy plants. Keep in mind that these plants are disease resistant but can still be infected if preventative measures and proper gardening practices are not implemented.
  • Assess plants regularly. Performing regular assessments on your tomato plants will help you catch a problem and treat it before it kills the plant or spreads to your other plants.
  • Wait for the ground to warm up. After a string of warm days, it is tempting to get your plants in the ground and kick off your garden. Resist the urge if the temperature is still dropping at night. Planting your tomatoes in cold soil may make them more susceptible to diseases and stunt their growth.
  • Rotate crops. Change up the location of your plants every years. This will ensure that pests and disease don’t become established in the soil and that the soil doesn’t become depleted of certain nutrients. Tomato plants are commonly rotated on a 4-year cycle (which works very well if you have four raised beds to move them around from year to year).
  • Give them space. Not crowding your tomato plants together will give them good air circulation and help to keep the foliage dry and disease-free.
  • Water properly. Assess regularly how your plants are doing in terms of water. Is the soil always wet, or does it seem to be dry shortly after you water? Another thing to consider is the time of day when you are watering your tomato plants. Giving them a good drink in the morning will help them dry out before the sunsets.
  • Prune lower branches. Removing the lower branches and then adding a straw mulch will help distance the tomato plant’s leaves and the soil. There may be fungus left in the soil from the year before, and when it rains, that soil will splash up onto the lower branches. This will give the fungus a great start in decimating your plant. Removing the lower branches and putting a mulch down will prevent any splashing while it rains and while you water your tomatoes.

Yellowing leaves on your tomato plant shouldn’t induce panic, but the root cause does need to be figured out so you can treat the problem and prevent any future issues. It is a common problem that even the most skilled gardeners face.

Mary Jane Duford
Mary Jane Duford

Mary Jane Duford is a quintessential Canadian gardener. An engineer by trade, she tends to an ever-expanding collection of plants. In her world, laughter blooms as freely as her flowers, and every plant is raised with a dash of Canadian grit.

Mary Jane is a certified Master Gardener and also holds a Permaculture Design Certificate. She's also a proud mom of three, teaching her little sprouts the crucial difference between a garden friend and foe.

When she's not playing in the dirt, Mary Jane revels in her love for Taylor Swift, Gilmore Girls, ice hockey, and the surprisingly soothing sounds of bluegrass covers of classic hip-hop songs. She invites you to join her garden party, a place where you can share in the joy of growing and where every day is a new opportunity to find the perfect spot for yet another plant.

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