Whether you have cared for your tomato plant from seed or you’ve hand-picked the right transplant from the garden centre, a bit of panic may set in when you spot the leaves of your tomato plants turning from green to yellow.

Especially as beginners, it’s hard to know what is normal and what isn’t. And if we find more and more leaves turning yellow, we rightly become alarmed. Still, sometimes it’s difficult to identify the problem and even harder to know what to do about it

1.TRANSPLANT SHOCK CAN CAUSE LEAVES TO TURN YELLOW

When you first transplant your tomatoes into the ground, especially in the early spring when nights aren’t warm yet, tomatoes will go through a transplant adjustment period. In this week or two following transplant, you’ll notice your once-vibrant green leaves lightening in colour. But, if you look at the newer leaves at the top, they are young, healthy, and growing.

WHAT TO DO ABOUT IT? Yellow lower leaves should be cut off anyway (see below), so as long as you see healthy, vibrant leaves at the top of the plant, cut off the yellow leaves at the stem. They won’t benefit the plant and will likely only serve as a gateway for the disease.

Transplant Shock Prevention: Some varieties of tomatoes are more susceptible than others, but waiting to plant the tomato plants until the optimum time will help to prevent transplant shock. Wait until the nighttime temperatures stay in the 50s.

There are a number of reasons why tomato plant leaves turn yellow, most of which are easily rectified. Below are the most common causes for yellowing tomato leaves and what you can do about the issue.

2.NUTRITIONAL DEFICIENCIES–

If you only see a few yellow tomato leaves toward the bottom of the plant, you usually have nothing to worry about. This normally means these leaves aren’t getting the nutrients they need from the soil or they aren’t getting enough sunshine. Most often this occurs on older plants that are bearing fruit.

It could be something as simple as a lack of nitrogen in your soil. If this is the case, check the nitrogen level by taking a soil test to determine exactly what, if any, nutrients are lacking so you can treat accordingly.

Feed tomatoes at planting time and monthly throughout the season, as tomatoes have hearty appetites. Follow the directions carefully and beware of overfeeding, which can cause lush plants at the expense of fruit.

3.PESTS

A number of pests can come badly on plants, frequently causing yellow tomato leaves. Insecticidal soap or horticultural oil is good for treating smaller pests such as:

1.Aphids
2.Thrips
3.Spider mites
4.Whiteflies
5.Larger tomato pests like hornworms and cutworms can be picked off by hand, or controlled with applications.

4.WATERING PROBLEMS

Too much or too little water can cause yellow tomato leaves. Soak tomatoes thoroughly once every five to seven days, depending on weather and soil type. Let the soil dry between watering and never allow the soil to remain soggy.

Water tomato plants carefully at the base of the plant and keep the leaves as dry as possible. Watering early in the day is best.

5.DISEASES

Early Blight: Most Common Cause of Yellow Tomato Leaves

Early blight is the culprit every year in my garden, and it’s easy to spot when you know what you’re looking for. Caused by a soil-borne fungus, early blight travels from the soil to the lower leaves. At the earliest stage of infection, these lower, older leaves will begin showing irregularly shaped yellow splotches that progress into brown spots with a yellow “halo” around them. The splotches appear almost like a target with a brown centre.

Early blight tomato yellow spots on leaves
As the disease progresses unchecked, this entire stem and leaves turn yellow and then brown, and finally, they shrivel up completely.

PERVENTION

Keep plants spaced out well (3 feet minimum) to allow airflow between the plants. Wet, humid conditions exacerbate early blight.

Mulch heavily in the entire tomato area, creating a barrier between the soil and the tomato leaves.

As the young plants grow, cut off lower leaves completely, especially if they are touching the ground (even if they’re healthy). Leave a 12-18″ gap between the ground and the lowest sets of leaves.

When irrigating, chose a drip or soaker hose method to dispense water to the root zones of the plants (view my choices here). Aim not to let the leaves get wet, which will allow the disease to spread more easily. If you must use overhead watering, do so at the beginning of the day so the water can evaporate quickly. At the end of the season, remove all plants and destroy them; do not compost. Rotate crops next season.

2.Fungal Diseases

Fungal diseases are a common reason for yellow leaves on tomato. For example, early blight is evidenced by yellow leaves and small spots or lesions that grow larger, eventually taking on a bulls-eye appearance. The fruit is usually unaffected unless the disease is severe. Late blight, on the other hand, is a more troublesome disease that starts on the upper leaves. You can recognize late blight by the large, oily-looking lesions on both leaves and stems.

Fusarium wilt, -which usually shows up on warm weather, typically causes yellow tomato leaves on one side of the plant, often beginning with the older, bottom leaves. Growth is stunted and the plant likely won’t produce fruit.

These and other fungal diseases can be treated with a fungicide containing chlorothalonil. Water properly. Allow space between plants to provide ample air circulation, and prune thick growth, if necessary.