What Do Black Spots on Leaves Mean?

There is no single cause of black spots on a plant's leaves. This is important to remember as the treatment options will vary accordingly. The treatment for one cause of black spots may not work on another. Moreover, it is possible the treatment may exacerbate the problem and result in the death of the plant.

Another important factor we need to consider is the individual plant's care. Some people treat all of their plants the same, but each plant has their own specific care needs. This is especially important in terms of the amount of light and irrigation they receive. Since plants are indigenous to different parts of the world and the various ecosystems therein, their needs differ. Each individual species will also have their own care requirements.

While the level and quality of care we provide the plant is important, it is not the only reason a plant can develop black spots. However, ensuring we meet the plant's care needs is the best way to prevent it happening. We look at the causes, treatment and prevention of black spots in plants in greater detail below.

Plants are grouped into different categories according to various characteristics. One such characteristic is their lifespan. Some plants only grow for a short time, while others can last for years or even centuries. Generally speaking, plants are grouped into the following categories:

• Annual plants: plants which will germinate, grow and then die within one single growing season.
• Biennial plants: plants which take two years to complete their entire life cycle.
• Perennial plants: although some people mistakenly think these plants will renew every year, it is more generally a term for plants which require more than two years to complete their life cycle.

You my not be aware what type of plant you are caring for. If you have had an annual plant for more than one growing season and dark spots appear on its leaves, it is likely coming to the end of its natural life cycle. If the same occurs after two years, it may be a biennial plant. If you know your plant is a perennial and is within its minimum lifespan, we need to consider other reasons for black spots on its leaves.

Black spots may also be a result of too much sun exposure. As stated above, different plants require different levels of care. The amount of sunlight they receive is one vital factor. Some might assume that plants from tropical climates will need a lot of sun, but this is not always the case. Many plants indigenous to tropical areas thrive in shaded areas, such as those below rainforest canopies.

Some people believe that watering plants during periods of intense sunlight can lead to them burn. This is a widely held belief, but there is little scientific evidence to support this effect on smooth-leaf plants. Although leaves with fine hairs may see some effect, the bigger problem is that hot sun will evaporate the water more quickly.

We can distinguish overexposure to sunlight as a cause of black spots by looking at the overall quality of the leaves. If the leaves are dry and brittle with black spots, it is likely due to sun. This is not the case if the leaves are supple and appear to have sufficient moisture.

If we think the black spots on our plant leaves are due to sunlight, we need to consider its location. Research the amount of the sun the plant needs and place it somewhere according to these guidelines. For example, if you are keeping an indoor plant outdoors, you will need to bring it inside.

One of the most worrying and common reasons why black spots appear on plant leaves is due to a fungal infection. In fact, one such fungal infection is commonly known as ‘black spot’. This is a disease which is caused by various fungi and it is one of the biggest killers of roses. These black spots are very particular. The spots have fringed edges and grow to around a half inch in diameter.

Various fungi can result in infections, but not all fungi present in the same way. The following are some different types of plant fungus:

• Black spot: some fungi will cause the black spots, although we can also see yellow coloration or even purple hues in the spots themselves. These are commonly caused in overly humid conditions.
  
  
• Rust: while this is not actual iron oxide, some fungi present in a manner which closely resembles it. Orange colored bumps appear over the upper surfaces of leaves. They can grow into various patterns and tend to be more common in cooler environments.
  
  
• Powdery mildew: white spots can appear on plant leaves due to fungi, but it is important to differentiate these from other issues such as insect eggs. The fungi which result in this problem are common in crop plants, but they can appear on many we have in the home. Unusually, it can also develop in drier areas.

Although not always the case, fungi tend to proliferate in wet and humid conditions. For this reason, we can usually differentiate this from too much sun as the plant leaves will likely be moist or even saturated. We need to be careful if we spot the signs of plant fungus and act quickly. Not only may the plant itself be taken over and die, but some can spread easily and harm neighboring plants.

Although fungal infections are more common causes of black spots on plant leaves, they are not the only one. We can also see bacterial and viral diseases which affect plants. The latter appears to more commonly affect agricultural crops^[1], although they can affect houseplants also. Bacterial plant infections can be treated by antibiotics for plants. Viral infections cannot be cured, but the plant may be managed to help fight against the virus.

As with the amount of sunlight a plant requires, irrigation is a vital factor. When the plant is not given enough water it can develop black spots. This can happen in combination with too much sunlight as the heat will increase evaporation. For example, we may provide sufficient water for a plant during winter, but once the days become warmer, the plant will need more irrigation.

Too much water can also be detrimental to your plant's survival. As seen in the above section, excessive moisture can promote fungal diseases. It can also cause the plant's roots and other systems to function improperly. It is for this reason many plants require drainage holes in the bottom of their containers.

We can generally tell the difference between too little and too much water by the dryness of the plant leaves. If the black spots appear on dry and desiccated leaves, it likely has insufficient watering. With black spots on wet limp leaves, it means the plant is probably being overwatered.

Another reason why a plant might have black spots on its leaves is due to pests. Various pests can infest a plant's leaves and result in black spots. They include:

• Spider mites: these miniscule pests can be seen with a magnifying glass. They often leave wispy webs underneath the plant leaves, but their effect on the leaves can result in a patterning on the topside. These can range in color from white to black, although they may be somewhere in the middle.
  
  
• Mealybugs: these are larger pests which can infest a plant. Adult males develop wings so they can transfer from plant to plant more easily. They feed on plant sap and express a sticky fluid known as honeydew. This can lead to sooty black spots on the plant leaves. Different mealybug species attack different types of plants.
  
  
• Aphids: another plant pest which produces honeydew is the aphid. They also produce black moldy spots on the leaves. They can vary greatly in appearance, size and color.

Preventing pests on plants can be tricky. Even if we provide all the correct care for the plants, an infestation can take place relatively easily. For this reason, we may need to use a suitable pesticide on our plants to both kill the existing pests and prevent further infestations.

Although each cause of black spots on a plant's leaves will need to be treated accordingly, there are certain general tips we can use to help the plant. If we keep the affected leaves, they can lead to spread of the problem. In turn, this can lead to the death of the plant.

1. Remove affected leaves:
   
   To ensure the health of your plant, the first thing you should do is prune all infected leaves as quickly as possible. This is especially important if there is a fungal or bacterial infection as it can spread through direct contact. Furthermore, ensure the tools you use are disinfected every time you cut a new leaf. You can use a liquid solution made up of one-fifth bleach and four-fifths water.
   
   
2. Treat the infected areas:
   
   To avoid cutting all the infected leaves of the plant, especially those that already have a yellowish appearance, we can try to recover them using remedies to prevent the disease from spreading. The simplest thing is to use fungicides, but make sure to use one with an organic compound base.
   
   
3. Rake and discard affected leaves:
   
   If your plants are indoors, we advise you to always keep the soil as aerated as possible and to remove all infected leaves. Place these in a well-sealed bag to throw them away and to prevent agents such as wind or animals from spreading the fungi again. Also, remove any affected topsoil as fungus and bacteria can grow in this area.

Generally speaking, you will best prevent your plant developing black spots by taking good care of the plant. We need to read up on the specific needs of each plant species and treat them accordingly. In this regard, here are some basic guidelines to help protect your plants:

• Keep the plant in an airy and dry area, unless it is a species that does not tolerate currents or expressly requires a lot of environmental humidity.
  
  
• Provide a substrate rich in nutrients that give the plant strength to fight against fungi and diseases. Worm castings is a great option as a fertilizer as it is ecological and very rich in healthy microorganisms.
  
  
• Always water your plants adding water to the substrate and trying not to wet their leaves or flowers. These parts tend to be susceptible to fungus or disease if kept wet for too long. Most plants only need water in their substrate or soil.
  
  
• Make sure the substrate is light and offers good drainage. If you have the plant in a pot, make sure the container has drainage holes and that you remove any excess water about 20 minutes after watering from the saucer below.

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