Black spots on tomatoes are caused by a range of tomato related diseases and disorders. One common cause among home growers is blossom end rot. There are other causes of dark spots on tomatoes such as Bacterial Spot and Bacterial Speck. Using the right methods and approaches, spots on tomatoes can be effectively controlled or prevented.
Table of Contents
Introduction
The tomato is botanically known as Solanum lycopersicum. There are over two dozen Solanum lycopersicum diseases and disorders. According to the Cornell University Department of Plant Pathology’s Tomato Disease Identification Key, the majority of these diseases and disorders affect tomato plant leaves.
This feature discusses the causes of black spots on tomato fruits. These are spots occurring on the outer membrane, skin or surface of the tomato fruit. They may also cause damage that penetrates the flesh of the tomato.
In many cases, apart from making the tomato fruit unmarketable, spots on tomato skin makes the tomato vulnerable to severe attack by pests and other diseases.
Black spots that appear on tomatoes are symptomatic damage of bacteria, fungus, oomycetes (plant pathogens) and viruses. Disorders which occur in tomatoes may also cause black spots although these are not classified as diseases. Tomato plant cultivation and production methods often bring about these undesirable disorders.
The following are potential causes of black spots on the surface of tomato fruits. We take turns to explain each of the causes and provide some possible intervention or prevention methods.
Tomato Black Spots Caused by Blossom End Rot
A well known cause of black spots on tomatoes is blossom end rot. Blossom end rot (BER) is an example of a physiological disorder and is not a disease. This makes it relatively easier to prevent and control compared to tomato diseases. BER appears on the blossom end or bottom part of the tomato as a brown or black spot. The spot looks water-soaked and delicate. It typically starts off small before continuing to enlarge, sink into the fruit and cause decay.
Although ripened tomatoes seem most affected, BER can also affect green tomatoes which are yet to ripen. Furthermore, according to the Michigan State University Extension, blossom end rot affects the first tomatoes of the season the most.
Tomatoes affected by BER cannot be recovered. Often the affected tomato becomes watery and unusable. In best cases, tomato black spots caused by BER are an unappealing dry dark spot or scab.
Causes of BER
Two factors are primarily responsible for blossom end rot in tomato fruit. A lack of calcium and drought which itself is a lack of moisture associated with inconsistent irrigation or hand watering.
Blossom end rot is particularly a big problem among home growers due to tomato cultivation and production approaches that favor the causes of BER. Unlike commercial operations where cultivation and production is professional and systematic, inconsistent watering is expectedly common among hobby growers
Tomato plants rely on water for the mobility of calcium from the soil to the fruit and also within the plant itself. According to The Alabama Cooperative Extension System, even brief lapses in water supply can trigger calcium deficiency leading to BER.
In addition, Epsom Salts are popular in the home gardening community. However, use of Epsom Salt may contribute to the development of black spots on tomatoes. According to the Yard & Garden Report (August 2016) by North Dakota State University, Epsom Salts interfere with the calcium absorption of tomatoes. They suppress calcium uptake from the soil instead promoting better uptake of magnesium, one of the two minerals found in Epsom Salts. It is the subsequent calcium deficiency that leads to blossom end rot.
Preventing BER
An obvious way to end or prevent black spots on tomatoes is to incorporate measures that eliminate or reduce the risk of BER developing on your tomato fruits.
- The first and simplest way of preventing black spots on your tomatoes is to water them frequently and evenly. This means avoiding periods of drought no matter how brief. This ensures even and continuous flow of calcium to the tomato fruit reducing the risk of blossom end rot developing.
- The University of Arkansas System, Division of Agriculture recommends adding lime to the soil to achieve a pH level of 6.0 to 7.0. In many cases, application of lime to the soil should follow a professional soil test.
- Egg shells are rich in calcium. These can be crushed and sprinkled around the base of the tomato plant. However, egg shells would need time to decay first and incorporate into the soil before benefiting the tomato plant. The University of Minnesota Extension has also tried using powdered milk in water as a fast and effective remedy against calcium deficiency. This method is already popular among some growers who strongly recommend this option.
- Grow tomato cultivars that are resistant to blossom end rot. These include Manalucie Tomato and Ropreco Paste Tomato varieties among others. Some heirloom tomato varieties easily succumb to conditions that trigger BER. These varieties should be avoided.
- Practice mulching your tomatoes as much as possible. This preserves soil moisture preventing borne dryness favored by BER. Best mulching is done using organic materials such as wood chips, straw and peat moss to reduce the risk of introducing some other diseases to your tomatoes.
Tomato Black Spots Caused by Bacterial Spot
Apart from BER, another cause of black spots on tomatoes is a bacterial infection known as Bacterial spot. Bacterial spot appear on tomato skin as crowded small black spots which may be brown or dark. Bacterial spot also affects the stems and leaves of the tomato plant. In severe cases leaves turn yellow and die.
Causes of Bacterial Spot
Bacterial spot comes from a number of bacterial pathogens such as Xanthomonas vesicatoria and Xanthomonas gardneri. The pathogens are initially foreign microorganisms which are introduced to a clean garden through contaminated seeds or transplants.
Bacterial spot enters the plant through natural openings such as stomates or lacerations on the plant or fruit. Black spots on the tomato only appear later.
Preventing Bacterial Spot
Tomatoes that have been attacked by Bacterial spot and showing black spots as in the image above can not be recovered. They become unattractive and unmarketable produce. The immediate action to take is to remove the affected plant root-and-branch to prevent the bacteria from spreading.
- Only plant seeds from a trusted source to prevent importing contaminated seed. In addition, as a precaution, soak seeds in water at a temperature of 50°C or 122°F for some 25 minutes to eliminate the pathogens.
- Properly dispose of infected plants. Debris of infected plants should not be composted. Burning or burying is most effective.
- Practice crop rotation by rotating a garden or field with a non-host crop. Crop rotation should be for at least one year.
- Keep the tomato plant leaves dry by avoiding overhead watering. Wet leaves easily facilitate the spread of pathogens that cause black spots on tomatoes.
Bacterial Spot Explained (Video)
This video shows Bacterial spot as a cause of black spots on ripened tomatoes.
Tomato Black Spots Caused by Bacterial Speck
Bacterial speck is another potential cause of black spots on tomatoes. Tomatoes infected with Bacterial speck exhibit almost similar symptoms to that of Bacterial spot. Infected tomatoes develop small black or dark specks embedded in the fruit. In some cases the specks come together and kill fruit tissue. For this reason, Bacterial spot is often easily confused for Bacterial speck. However, one has spots, the other specks which may not be easy to tell apart.
Bacterial speck occasionally attacks tomato fruit. In most cases the disease devastates tomato plant leaves and stems. Leaves develop crowded black spots whilst turning yellow in part or whole. When black spots are observed on tomatoes, there is a need for closer inspection to avoid mistaken identity of the disease.
Causes of Bacterial Speck
Bacterial speck is caused by a bacterium called Pseudomonas syringae pv. tomato. The disease is promoted by low temperatures and high humidity. The pathogen is known to lay dormant on plant leaves and foliage causing no harm until favorable conditions are availed. Once it has infected the tomato, the symptoms will only then appear as black spots on tomatoes and tomato foliage.
Bacterial speck is introduced into an environment through contaminated seed and transplants. Furthermore, windy rains easily and effectively carry the pathogen to other plants causing a deadly spread.
Bacterial speck appears to contaminate just about everything it comes into contact with. This includes soil, trellises, stakes, weeds, tools and through aerosols according to the University of Massachusetts Amherst Center for Agriculture, Food and the Environment. In addition, the bacterium can also remain potent whilst hosted on the surface of non-host plants.
Preventing Bacterial Speck
- Only plant certified seeds. Use same heat treatment of seeds as Bacterial Spot to kill the pathogen
- Practice crop rotation.
- Disinfect tools and all materials.
- Properly dispose of infected tomato plants through burning.
- Use pesticides. However, this may be ineffective in severe cases.
Other Causes of Spots on Tomatoes
Apart from the above examined causes of black spots on tomatoes, there are other diseases that can cause spots on tomato fruit. These diseases do not generally cause dark or black spots on the tomato.
For example, Bacterial canker typically causes light spots as shown above. As tomatoes can suffer diseases in combination, the dark spots seen in the image are from another infection such as Bacterial spot.
Other harmful spot-causing diseases on tomatoes include Anthracnose, Early Blight, and Wilt Virus. Some of these diseases tend to cause obvious harm to plant foliage such as leaves and stems without causing any noticeable damage on the fruit itself.
Conclusion
Black spots on tomatoes are a result of physiological disorders as well as disease. Tomato disorders such as blossom end rot cause big black spots on the bottom of tomatoes while diseases such as Bacterial spot and Bacterial speck tend to bring about small dark spots on the skin of the tomato. Damage caused by these disorders and diseases on the fruit can not be reversed. Prevention is the most effective way of dealing with black spots on tomatoes. Planting verified seeds or treated seeds, sanitization of tools and materials used in the garden as well as crop rotation are effective methods of preventing the development of black spots on tomato fruit.
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