Tomato Leaves Turning Black? Here’s Why it Happens!

Are your tomato leaves turning black or brown and sending you into a panic? Gardening can sometimes feel like a constant battle against pests and diseases. Unfortunately, tomatoes are especially prone to attack.

Crop rotation, integrated pest management, and disease-resistant varieties are not always enough.

One specific problem you may see on your tomato plants is the leaves turning black. Let’s take a look at some of the more common reasons for your tomato leaves turning black as well as some tips to improve the health of your tomato plants. 

The Long Answer

Tomato leaves turn black when the plant is stressed by temperatures, disease, or herbicide injury. Here are the most common causes and how to prevent them.

Frost

Tomatoes are warm-weather crops with no frost tolerance. Frost damage is the most common reason tomato plants’ leaves turn dark green or black. Frost damage occurs in the early spring or late fall in most North American regions but can sometimes be a problem for high-elevation mountain gardeners in midsummer. Wait until the plant has thawed to determine the full extent of frost damage. 

Frost damage can have various appearances, depending on the following:

• Specific weather conditions (humidity, wind, or precipitation)
• The size and age of the tomato plant
• Leaf health and overall plant stress
• Tomato variety
• Topography and elevation

After a heavy frost, the entire tomato plant may be dark green or black. A light frost may only damage the tops and sides of a tomato plant, leaving the interior leaves green and the fruit undamaged. Just a touch of frost may leave black spots on only the most exposed leaves.

If frost is mild and there are still green leaves and stems, tomato plants can recover and continue growing. When a newly planted tomato experiences light frost in spring, check if the main growing point has been frozen. If it is still green, there is a chance the plant will continue to grow with only a minor setback. 

Prevention

Frost damage can be prevented by planning, planting at the right time, and paying attention to the weather forecast!

Wait to plant your tomatoes until after the last spring frost. Even when planted after the last “predicted” spring frost, it is worth watching the forecasted nighttime lows after transplanting tomatoes in the spring. If temperatures are predicted to dip near or below freezing, take action!

Frost damage can be prevented by:

• Waiting to plant until the risk of frost has passed
• Using row covers
• Using a water wall to provide insulation for your tomatoes
• Using cloches on extremely cold nights
• Building a hoop house for young tomato plants

Plan to have row covers, frost blankets, or old sheets on hand at planting time to cover plants if frost (or near-frost) is predicted. Planning ahead is so much easier than frantically rummaging for old sheets or covering plants with the light of a headlamp!

Also, consider a water wall for your younger tomato plants. These plastic circular tubes sit around a tomato plant; when the pocket-like sides are filled with water, they act like an insulator, keeping the temperature around the tomato plant much warmer. They look a little strange, but they can be a godsend in cooler temperatures.

If you just planted your tomatoes, while they’re still small, you can use a hoop house over them to provide extra protection from the elements. These can be as simple as two pieces of rebar with an arch of PVC pipe hooked onto them, or they can be much more complex. Adding a sheet of plastic, frost blanket, or floating row covers over the hoop house can not only insulate, but will also keep pests at bay. Be forewarned that it will also keep pollinators away, so you’ll need to hand pollinate until you remove the cover!

Early Blight

Early blight is a plant disease found throughout North America. It affects tomatoes and other nightshades, as well as some weeds. Symptoms include:

• Dark yellow leaves
• Brown or black leaf edges
• Dark lesion spots on the stems
• Bottom leaves of the plant are usually the first affected
• Leathery, dark spots may form on the fruit
• Clear concentric rings visible in fruit lesions 

Warm, wet weather can cause the rapid spread of early blight among your tomato plants. Humid regions tend to see more early blight compared to drier western regions. The pathogens that cause early blight (Alternaria spp.) are found in the soil and enter plant tissues when the soil splashes on the stems and leaves during rain or overhead watering. The pathogen also spreads by wind, contact with equipment, tools, or even a gardener’s hands that have come into contact with an infected plant.

Prevention

The best way to fight early blight is prevention. Choose tomato varieties that are bred with some disease resistance. Always choose healthy tomato starts when purchasing plants.

Limit soil splash on the tomato plants by using mulch or landscape fabric around the base of the tomato plant since the pathogen can be found in soil. Remove any tomato branches that are very close to or touching the soil.

In humid climates, increase the spacing between your tomato plants. This encourages more airflow.

Do not work amongst tomato plants when the leaves are wet from watering, rain, or even morning dew. Moisture encourages disease to spread easily. Sanitize tools between pruning plants to limit the spread of disease.

Clean up tomato plant debris at the end of the season, removing as much plant material from the garden as possible to limit the disease overwintering on infected plant tissue. Do not compost infected plants, and always rotate the spot you are growing tomatoes in year after year.

Late Blight

Late blight was the cause of the Irish potato famine, and the disease is still with us today. Late blight (Phytophthora infestans) affects both tomatoes and potatoes in the Solanaceae family. Not all regions of the United States have late blight regularly, though some may see it more years than not. As a wind-blown pathogen, it depends on prime conditions to spread far and wide.

Late blight can cause tomato leaves to turn brown to black and can be extremely damaging to tomato crops. Late blight spreads quickly, especially in cool, wet weather. It starts as dark brown blotches on leaves. The blotches do not follow the leaf veins. Later, it can spread to the rest of the plant.

All types of tomatoes are affected, though some varieties are more resistant than others. A serious case of late blight in your garden will result in completely dead plants that look like they have frozen.

Prevention

Follow the same cultural controls as for early blight to prevent late blight. Especially avoid working in tomatoes or potatoes when the leaves are wet since disease spreads easily under wet conditions (even if the plants look healthy).

Always purchase disease-free certified potatoes and healthy tomato plants if buying starts. Don’t allow volunteer tomato or potato plants in your garden since they may be growing from tubers or plants that may have been infected the prior year.  

Herbicide Injury

Unfortunately, gardens that are adjacent to roadways, public right-of-ways, utilities, railroads, or conventional agricultural fields are at risk for herbicide injury. Herbicides are chemicals designed to kill plants. Since your garden plants are probably not “round-up ready,” they are susceptible to injury or death from herbicide contact. 

Most herbicides are commercially applied by spraying. The sprays are light enough to “drift” on the wind if conditions are windy enough. Herbicide applicators should not spray when winds are over 5-10 miles per hour, but it still happens and may cause drift into unwanted areas, including your garden.

Herbicide injury may be difficult to deduce on most plants, but in tomatoes, it commonly looks like:

• Warped, twisted, or stunted leaves and stems
• Curled leaf appearance
• Leaves may stay green or turn yellow, brown, or black.
• Uneven symptoms due to the wind patterns that occurred during the herbicide drift
• The smell of pesticides strongly during the day

If you see an applicator spraying chemicals upwind from your location, this can be a significant indicator of herbicide drift occurring in your garden. 

Prevention

Troubling as herbicide drift is, there is little most gardeners can do to prevent it from happening. Being proactive can be the best path to take, and using row covers may be enough to prevent some of the herbicides from contacting your plants. (Be aware that the chemical may still be present on the row cover, so use caution or PPE when removing).

Proactive prevention can also be as simple as starting a conversation with those who spray herbicide: neighbors, local utilities, or private companies. Educate them about the location of your garden, and your concern for herbicide drift, and request they spray responsibly and mindfully. You probably won’t change their mind about the chemicals they use, but you can advocate for their responsible use of chemicals. 

Posting “do not spray” signs around your property can be an effective deterrent too. Be proactive about mowing long grass, weeds, or brush (on your own property) that needs to stay clear for road sight lines or adjacent to utilities to discourage spraying in that area.