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Why Are My Tomato Leaves Turning Dark Green

Healthy-looking tomato plants are often the pride and joy of a garden. These bountiful plants produce some of the most versatile vegetables, growing various tomatoes, from succulent cherry tomatoes to larger slicing tomatoes. The exotic colors of some heirloom varieties can make your average gardener very proud. 

So, it’s no wonder gardeners tend to worry when their tomato plants’ leaves darken. Even a few shades of off-color can signal that something is seriously wrong, and some common problems can put every tomato plant in the plot at risk. 

Understanding what causes tomato leaves to darken is critical to getting a good harvest, but many things can change the color of your tomato plants’ leaves. 

The most common reasons for dark leaves are severe wilt and blight diseases, which can spread between tomato plants without much effort. Bacterial speck is one of the more common cause of darker colored leaves on tomato plants.

However, those aren’t the only reasons your tomato leaves might go dark. 

Sometimes, a couple of shades darker is nothing more mysterious than sunlight causing excess chlorophyll production, but it can still be concerning if you haven’t seen it before. 

Here’s what you need to know about darkening tomato leaves, what the different colors mean, and what you can do to address dark leaf problems. 

What Causes Tomato Leaves To Darken

There are a few different reasons your tomato leaves might darken, and it’s important to tell what color they’re turning to help you figure out what’s going on. 

Let’s talk about the different dark colors tomato leaves turn and what each means. 

Dark Green Leaves

Dark green leaves are usually one of two things, intense sunlight causing more chlorophyll production than normal or an early stop on the way to a different color. 

Identifying what is happening can be difficult at times, but you’ll generally know within a week of identifying dark green leaves whether they will stay dark green or turn brown or black over time. 

Excess chlorophyll isn’t a bad thing for tomato plants. In fact, with enough water and supporting nutrients in the soil, chances are you will get more flavorful tomatoes from plants with dark chlorophyll. 

However, you should watch these plants closely, especially if they turn dark early in the growing season. That can signify that they’re getting too much sun and will burn later in the season when sunlight is hotter and more intense. 

Purple Leaves

Purple tomato leaves might sound unusual or a sign of an exotic heirloom, but usually, it’s a sign of a soil problem. 

The green color in plant leaves and stems isn’t the main color of the plants. That green comes from photosynthetic chlorophyll. So, if your plants aren’t green, or the green is going away, they have lower chlorophyll concentrations than normal, which means less energy production and slower growing plants. 

Purple typically means that your tomato plants aren’t getting enough nutrients, specifically potassium. More fertilizer, or a quick potassium boost like banana leaf tea, will help your tomatoes return to their normal color. 

Dark Brown Or Black Leaves

Black and brown colors are typically strong indicators of disease. In some cases, your tomato’s leaves might wilt and then turn a light brown if they aren’t getting enough water, but if increasing the amount of water you give them doesn’t quickly improve things, you’re probably dealing with wilt, blight, or bacterial speck. 

All these diseases are serious, and only a couple of the most common diseases of tomatoes are curable. Blight-ridden tomatoes should be removed from your garden and disposed of before they can infect the other plants, and you should replace your soil or relocate your garden for next year. 

Diseases That Can Turn Your Tomato Leaves Dark

There are a lot of plant diseases out there, and a fair few will specifically target your tomatoes and other garden vegetables. Here are some of the common diseases that turn tomato plant leaves dark when infected. 

Bacterial Speck

Bacterial speck looks exactly how it sounds. This disease causes small black specks on your plant’s leaves. The specs are more prominent on the bottom of the leaves at first and then become more noticeable on top. 

This disease can also affect mature tomato fruits, and discolored fruits aren’t safe to eat. 

Treatment usually involves:

Bacterial Canker

Bacterial canker is another communicable bacterial disease that mainly targets greenhouse tomatoes, spreading through contact with the plants or with tools or gloves that have been exposed to diseased plants. 

Removing the plants is usually the recommended treatment for bacterial canker. While there are some treatment options out there, this disease spreads aggressively, so it’s usually best to remove the plants before the disease spreads. 

Bacterial canker is identified by appearance. The disease makes leaf veins turn dark before other parts of the plant wilt and darken. Eventually, white spots will appear on diseased tomatoes themselves. 

Late Blight

Late blight is one of the most dreaded diseases in gardens worldwide. It strikes late in the season after you’ve already put a lot of work into your plants; it isn’t curable, kills plants quickly, and spreads very easily between plants and through the soil. 

Late blight can kill an entire field of vegetables in days or weeks and often lingers in that soil for years before going away. 

Blight is characterized by dark spots, rapid wilt, and dark patches along the plant’s stems. 

Septoria Leaf Spot

Septoria leaf spot has to be pretty severe before it causes tomato leaves to turn dark; turning yellow is a more common reaction to septoria leaf spot. 

However, if your tomato leaves turn yellow and dark brown or black before wilting and falling off the plant, you might be dealing with Septoria leaf spot.