If you notice black spots on your tomato plants, you are not alone. Many gardeners find that the fruit, leaves, or stems of their tomato plants turn black during a growing season.
You are probably wondering what causes tomato plants to turn black. I wondered the same thing, so I did some research to find out. It turns out that the location of the black spots can help to determine the source of the problem.
So, why is your tomato plant turning black? Black spots on the tomato fruit itself can be caused by blossom end rot, buckeye rot, anthracnose, bacterial spot, or sunscald. Black spots on tomato leaves can be caused by early blight, late blight, bacterial speck, and sooty mold, among other things. Black spots on tomato stems can be caused by bacterial wilt, alternaria stem canker, or tomato pith necrosis.
Let’s examine the many causes of black spots on tomato plants, along with some ways you can prevent this from happening in your garden.
Why Is My Tomato Plant Turning Black On The Fruit?
There are many different causes of black spots on tomato fruit: nutrient deficiency, bacteria, viruses, and mold are all possible reasons.
Blossom End Rot
Blossom end rot affects tomatoes, but it also peppers and eggplants. Blossom end rot first manifests as sunken, decaying areas on the bottom of a tomato fruit. These areas sunken areas on the fruit eventually turn dark brown and then black.
For more information, check out this article from Wikipedia on calcium deficiency and blossom end rot.
Blossom end rot is caused by a calcium deficiency in the plant. This can occur for several reasons:
- There may be insufficient calcium in the soil. You can treat this problem with lime (calcium carbonate) or other calcium supplements.
- There may be too much nitrogen in the soil. This could be caused by excessive use of manure or nitrogen rich fertilizers. You can prevent this problem by using fertilizers with lower nitrogen content. On a fertilizer package, N-P-K is clearly stated, for example, 10-10-10. The first number is the percent by weight of nitrogen – look for a lower number to get less nitrogen. For more information, check out my article on over fertilizing plants and my article on low-nitrogen fertilizers.
- There may be a pH imbalance in the soil. If soil pH is too low or too high, a tomato plant will have trouble absorbing calcium through its roots, even if there is plenty of calcium in the soil. You can treat this problem by correcting pH with either lime (to raise pH) or sulfur (to lower pH).
- There may be inconsistent watering (too little followed by too much). Try to avoid watering before or after rainstorms, and check the soil with your fingers for dryness before watering.
Before adding any amendments to your garden, you should do a soil test to determine if your soil has nutrient or pH imbalances. For more information, check out my article on how to do a soil test.
Buckeye rot is a fungal disease that affects the fruit of the tomato. It is caused by the fungus phytophthora parasitica. Buckeye rot also affects pepper plants.
Buckeye rot starts out as brown spots (lesions) on the tomato, especially if the fruit was touching the soil. The spots eventually get larger, and darker brown or black rings appear.
The lesions caused by buckeye rot tend to stay smooth and firm as the disease progresses. With time, the entire fruit will rot. If the conditions are moist, a white fungus that looks like cotton may grow on top of the lesions.
Buckeye rot is spread by rain and by water on the surface of the soil. For more information, check out this article from Clemson University on Buckeye Rot and other tomato disorders.
Anthracnose is a fungal disease that damages tomato fruit, but not leaves. It is caused by the fungus Colletotrichum coccoides. Anthracnose can also affect peppers and eggplants.
When infected tomatoes ripen, you will first see small circular dents on the fruit. Later, the center of these circular dents will get darker and larger.
In humid weather, you may see pink spores on top of the dents. Water from rain or irrigation will spread these spores to other plants. For more information, check out this article from Clemson University on Anthracnose and other tomato disorders.
Bacterial spot is caused by the bacteria Xanthomonas vesicatoria. Interestingly, this disease attacks green tomatoes, but not red ones. It can also affect pepper plants.
Affected plants will display raised spots (scabs) on the fruit. You may also see small spots with irregular shapes on the leaves.
This disease thrives in wet weather, and often spreads after a rainstorm. Fruit is infected when insects or animals puncture the skin of the fruit, but leaves can be infected without such damage occurring.
For more information, check out this article from Clemson University on Bacterial spot and other tomato disorders.
Sun scald occurs when tomato fruits are exposed to excessive amounts of direct sunlight. Normally, the leaves of a tomato plant will block or absorb the light from the sun, preventing much of it from getting to the fruit.
However, if a tomato plant has suffered from defoliation or poor nutrition, there may not be enough leaves to protect the fruit from the sun.
Sun scald first appears as white, tan, or gray blisters on the tomato fruit. Normally, sun scald occurs on the sides of the fruit, and it happens to fruits on the side of the plant that gets the most sun (or has the fewest leaves).
Eventually, black mold will grow in the sun scald patches, which explains the black spots you see on the fruit. For more information, check out this article from Wikipedia on sun scald.
Why Is My Tomato Plant Turning Black On The Leaves?
The problems that can cause black spots on tomato leaves are numerous. Let’s look at some of the common ones you might see.
Early blight can be caused by two types of fungi: Alternaria tomatophila and Alternaria solani. It can also affect potatoes. You will first see small brown spots, usually on older tomato leaves (lower on the plant).
Eventually, the spots will get larger and you will see concentric circles forming, almost like a bullseye pattern. You may also see black, leathery spots on the tomato fruit.
Generally, early blight develops in warm temperatures, from 59 to 80 degrees Fahrenheit. High humidity levels (90% or greater) also encourage growth of early blight, since it is a fungus.
For more information, check out this article from Clemson University on Early Blight and other tomato disorders.
Late blight can infect both tomatoes and potatoes. In fact, it was responsible for the Irish potato famine of 1840. It is caused by the fungus Phytophthora infestans.
Late blight can affect all parts of the tomato plant. On the leaves, the disease first appears as small, dark spots.
Eventually, these spots get larger, and white mold will grow on the edges of the spots. You may also see dark, shiny, firm lesions on the tomato fruit. Lesions can grow on tomato stems and quickly kill them.
For more information, check out this article from Clemson University on Late Blight and other tomato disorders.
Black sooty mold is a dark mold that looks like soot (from a chimney) that covers leaves and possibly stems of tomato plants. You can tell your plant has black sooty mold if you can scrape away the mold or wash it away with water.
For more information, check out this article from Mississippi State University on black sooty mold.
Black sooty mold is caused indirectly by aphids. When aphids feed on tomatoes (or other plants), they suck the juices out of the leaves and stems.
After they digest the juices, aphids leave behind honeydew. This sticky, sweet waste product provides the perfect energy for sooty mold. As sooty mold grows, it spread to cover leaves and stems wherever the aphids have left honeydew.
To prevent sooty mold from affecting your tomato plants, take steps to get rid of aphids in your garden. One way is to spray them with a combination of water, dish soap, and alcohol. For more information, check out my article on how to get rid of aphids.
To learn more about sooty mold, check out this article from Wikipedia on sooty mold.
Septoria Leaf Spot
Septoria Leaf Spot is caused by the fungus Septoria lycopersici. It usually affects older leaves on the tomato plant (the ones closer to the ground). Septoria leaf spot can also affect the stems of tomato plants.
The first sign will be many small, circular spots with dark edges on leaves. You will also see smaller black areas in the center of the circular spots. These black areas produce spores to spread the fungus.
Leaves that are severely affected may turn yellow and fall off the plant. Rain or overhead watering that wets the leaves will help Septoria Leaf Spot to spread.
In extreme cases, a lack of leaves due to Septoria Leaf Spot will lead to sun scald (see the description earlier in this article). In this way, Septoria Leaf Spot will indirectly affect tomato fruit.
Fusarium wilt is caused by the fungus Fusarium oxysporum. The first sign you will see is wilted “lazy” leaves on the lower part of the tomato plant.
The leaves may eventually turn yellow, brown, and black as necrosis sets in. The stems will also show browning if they are cut lengthwise.
For more information, check out this article from Clemson University on Fusarium Wilt and other tomato disorders.
You can also check out this article from Wikipedia on Fusarium Wilt.
Verticillium wilt can be caused by 6 different species of Verticillium fungi. As with fusarium wilt, the first sign is wilted leaves, and possibly wilted stems.
Often, the disease will affect only one side of a plant, moving up through the stems instead of around the stem. Eventually, you will see yellowing leaves (chlorosis) followed by browning or blackening and death of plant tissue (necrosis).
For more information, check out this article from Wikipedia on Verticillium Wilt.
Tomato Spotted Wilt Virus
Tomato Spotted Wilt Virus is spread to tomato plants when the thrip, a tiny insect, feeds on plants already infected with the virus. Some of the first signs are stunted growth, dark spots on leaves, and purple veins.
The younger leaves (higher up on the plant) may twist and curl as the dark spots on the leaves grow larger. A plant that survives may go on to bear fruit with dark, black spots all over it.
Tobacco Mosaic Virus
Tobacco mosaic virus causes mottling and black areas on the leaves of tomato plants. The virus is spread by plant sap, and there is no known cure. It can affect tomatoes, peppers, eggplants, and potatoes.
Infected plants should be removed and destroyed. Also, be sure to avoid working with your plants after touching or using tobacco products.
For more information, check out this article from the University of Massachusetts on tobacco mosaic virus.
Herbicides can also damage tomato plants and cause damage or death of leaves. If you live near a commercial farm, this is more likely.
Leaf mold is caused by the fungus Passalora fulva. The mold first appears on older leaves, lower on the plant and close to the soil.
These lower leaves are more likely to get wet from overhead watering or splashing rain. They are also more likely to stay wet, since higher leaves on the plant prevent sunlight from drying out the lower leaves.
Leaf mold starts out as pale green or yellow spots on the top of the leaf. These spots get larger and turn darker, eventually becoming brown or black.
The spores of leaf mold can be spread by rain, wind, or tools, so be sure to wash and sterilize your equipment between plants.
For more information, check out this article from Clemson University on Leaf Mold and other tomato disorders.
Why Is My Tomato Plant Turning Black On The Stems?
There are a few tomato problems that can cause black spots on the stems. Let’s take a closer look at those.
Bacterial Wilt is caused by the bacteria Ralstonia solanacearum. This bacteria gets into tomato plants through injury sites in the roots made by insects or when transplanting seedlings.
Warm and moist climates will promote the growth of bacterial wilt. For more information, check out this article from Clemson University on Bacterial Wilt and other tomato disorders.
Alternaria Stem Canker
Alternaria Stem Canker is caused by the fungus Alternaria alternate. It lives in tomato seeds and seedlings, and is spread by airborne spores.
Alternaria Stem Canker causes brown or black spots on the stems of infected tomato plants. The leaves of an infected plant may also turn yellow or brown, with lower leaves affected first.
Alternaria spores can even cause respiratory infections and asthma in humans, so be careful if you find it in your garden. For more information, check out this article on Alternaria Stem Canker from Wikipedia.
Tomato Pith Necrosis
Tomato Pith Necrosis is caused by many species of the Pseudomonas bacteria. These soil-borne bacteria infect tomato plants on cloudy, cool, moist days.
The early signs of Tomato Pith Necrosis are black, necrotic spots on the stems. Eventually, these spots become larger and grow along the stem.
In time, the stems will become hollow. Parts of some tomato leaves may also turn black as a result of Tomato Pith Necrosis.
How To Prevent Tomatoes From Getting Diseases
There are many diseases that could cause your tomato plants to turn black. Luckily, there are also some steps you can take to prevent these problems.
Choose Disease Resistant Tomato Varieties
One of the best ways to avoid disease in the first place is to choose disease resistant tomato varieties. The description of a tomato variety in a catalog will indicate resistance to common diseases such as Fusarium Wilt.
Use Crop Rotation
Crop rotation means that you do not plant the same crop in the same part of your garden for two years in a row. Preferably, you rotate crops on a 3 or 4 year cycle. This helps to prevent diseases from establishing a foothold in your garden.
Tomato plants become infected with many of the same diseases as peppers, eggplants, and potatoes (nightshade family). Thus, you should avoid planting any of these crops in the same area two years in a row (that is, don’t plant potatoes right after tomatoes in the same spot in your garden the next year).
Do Not Wet The Leaves On Your Tomato Plants
Water your tomato plants close to the soil, not from above. This prevents water from splashing soil up onto the lower leaves, which is where many diseases start.
It also prevents the leaves from getting wet, which helps to discourage the growth of mold and fungus.
Leave Enough Space Between Tomato Plants
If tomato plants do become infected, it is easier to prevent the spread of disease if your tomato plants have adequate spacing.
Plant tomatoes at least 18 to 24 inches apart, and try to prune wandering branches so that the leaves or branches from neighboring plants do not touch. Leaving enough space between plants also means that splashing water from rain is more likely to infect only one plant, rather than multiple plants at once.
Remove The Lower Leaves Of Your Tomato Plants
Many diseases start in the soil and make their way onto tomato plants when water splashes soil onto the lower leaves. In light of this fact, it is a good idea to remove the lower leaves once your tomato plants are established.
Remove And Destroy Infected Plants
Once a plant becomes infected, there is likely no cure. In most cases, the safest course of action is to pull up the plant and destroy it by burning.
This might seem like a waste, but composting the plant presents the danger that a disease will survive the winter in your compost pile. Then, it can come back with a vengeance to haunt your garden in later years.
By now, you have an idea of just how many bacterial, viral, and environmental threats there are to tomato plants. Hopefully, taking some of the preventative measures listed here will help to prevent black spots anywhere on your tomato plants.
I hope you found this article helpful – if so, please share it with someone who can use the information.