Leaf Spot Disease: Causes, Treatment, and Prevention

Leaf spot is the name given to a group of plant diseases caused by fungi, bacteria, or other microorganisms. Leaf spot most frequently attacks fruit trees and vegetables, but can also affect indoor plants. 

The spots on the leaves can be small or large. Leaf spot interferes with the plant's photosynthesis. If left untreated, leaf spot can eventually severely damage the plant’s foliage.

Can plants recover from leaf spot? Yes, as long as you notice the problem early. Keep reading to find out how to treat leaf spot and help your plant recover.

What Is Leaf Spot?

Leaf spot is a disease caused by a variety of pathogens, which manifests itself in spots and lesions on a plant’s leaves. There are several varieties of leaf spot, and knowing how to treat leaf spot disease on your indoor plants requires finding out what type of leaf spot you’re dealing with. The symptoms can differ significantly depending on the cause.

So, what is the cause of leaf spots? Let’s see what causes this plant disease.

What Is Leaf Spot Caused by?

Leaf spot is caused by different pathogens that have made their way onto your plant’s leaves. The two main types of leaf spot are either fungal or bacterial, with 85% of plant diseases caused by fungi. Spots can also appear on the leaves due to physical damage from various environmental factors or viruses.

• Leaf spot fungus: Leaf spot can be caused by various fungi that infect particular host plants. For instance, Sercospora leaf spot often attacks beet plants, while black leaf spot is common on roses. Leaf spot fungus spreads via airborne spores or through contaminated water. 
• Bacteria: Bacteria, like Xanthomonas or Pseudomonas, can attack a variety of plants. Bacterial leaf spot can be spread by contaminated water or soil.
• Environmental stress: Bacteria and fungi often attack weakened or stressed plants. Nutrient deficiency, overfertilization, or overwatering can stress the plants, making them vulnerable to infections and diseases. Watering or misting with chlorinated tap water, sunburn, or low humidity can also cause leaf spots.
• Viral causes: Leaf spot caused by viruses is rare in houseplants. Viral infections are often transmitted by insects (aphids and whiteflies).

How to Tell If Leaf Spot Is Fungal or Bacterial

Telling fungal and bacterial leaf spot apart isn’t always easy. Knowing which type of leaf spot is affecting your plant is the first step in finding the right remedy. Once you know their symptoms and find out which leaf spot type your plant is suffering from, you’ll be able to lessen the damage.

So, how can you tell the difference between a fungal leaf spot and a bacterial leaf spot?

Fungal Leaf Spot: Symptoms and Causes

Wet, humid conditions can cause fungal leaf spot, such as brown leaf spot, caused by Septoria. While usually not fatal, this disease looks unsightly and weakens your plants, making them more susceptible to pests and other issues.

What Are the Symptoms of Fungal Leaf Spot Disease? 

Fungal leaf spot symptoms include brown or black circular spots on the plant’s leaves. The leaf tissue looks wrinkled and damaged, the lesion’s shape is usually circular with dark or yellow margins, and it can appear on any part of the leaf. 

What Causes Leaf Spot Fungus?

Fungal leaf spot can be caused by different fungi that thrive in warm, humid, and wet conditions. Tropical plants can be affected in the wild or indoors if the humidity is too high. Leaf spot is spread via windborne spores or infected water or soil. Overwatering and a lack of air circulation around plants, like Philodendrons, Monstera, Calatheas, and more, make them susceptible to leaf spot disease. 

Bacterial Leaf Spot: Symptoms and Causes

Bacterial leaf spot can damage your plants or even kill them if left untreated. Knowing why it appears will help you prevent it.

What Does a Bacterial Leaf Spot Look Like?

The difference between bacterial vs fungal leaf spot is relatively easy to see, since leaf spots, caused by bacteria, are usually watery in the center and have a more irregular shape than fungal leaf spots. To determine if you’re dealing with bacterial leaf spot symptoms, you can check the position of the spots (near the leaf’s veins, sometimes, with a yellow halo around the spot). Bacterial leaf spot eventually causes leaf tissue to rot, giving the spots a mushy look and an unpleasant smell.

What Causes Bacterial Leaf Spot?

Bacteria cause this kind of leaf spot. Prolonged wet conditions outdoors trigger the disease in plants. While indoors, a lack of air circulation, too many plants grouped closely together, or a sick plant can cause bacterial leaf spot to spread and infect your greenery.

How to Treat Leaf Spot

How do you get rid of leaf spot disease? Now that you know what is causing leaf spots on your plant’s foliage, it’s time to choose the correct leaf spot treatment. The approach you need will depend on whether the issue is fungal or bacterial.

How Do You Treat Fungal Leaf Spot?

The first step is to remove all the affected leaves. Move your plant into a dry location, far from other plants. Use a sulfur or copper-based fungicide if a large area of your plant’s foliage is affected. If there are only a few spots, you can try using natural methods first. Neem oil contains several antifungal compounds that help keep fungal growth in check. It can help control a minor fungal issue and is completely organic.

Another natural method for treating leaf spot is to spray your plant with a home-made spray. One home remedy is to mix a small amount of baking soda (bicarbonate of soda), horticultural soap, and water, and spray the foliage. This makes the environment hostile for fungi, reducing their growth.

How Do I Treat a Bacterial Leaf Spot?

Leaf spot caused by bacteria or viruses doesn’t have a confirmed cure, so you’ll have to get rid of a severely damaged plant. If only a few leaves show spots, control the damage and prevent it from spreading further by removing all the affected leaves.

Leaf Spot Disease: Prevention

Preventing leaf spot is better than waiting until your plant needs leaf spot disease treatment. Since fungal and bacterial leaf spot often infects already weakened or stressed plants, good plant care is essential for leaf spot prevention:

• Watering and misting correctly: Water and mist your plants in the morning, leaving time for the water to evaporate from the soil and leaves before evening. This is especially important during cold spells in autumn and spring. Reduce watering in winter.
• Don’t let water gather on the leaves: Avoid splashing water over your plant’s leaves; moisture trapped in leaf axils or rosettes can cause rotting and fungal issues. Try bottom watering your plants or use self-watering pots for sensitive varieties. Also, remove the bottom leaves of the plant if they come in contact with the moist soil.
• Keep the soil clean: Ensure your plant’s soil is clean of debris.
• Provide space for better air circulation: Placing plants apart helps control excessive humidity and promotes airflow. 
• Monitor your plant’s growing conditions: Provide dry and stable conditions for your indoor plants. Fertilize regularly during the growing season to prevent nutrient deficiency, but remember that overfertilizing can cause issues; always dilute the fertilizer when feeding houseplants.
• Use clean tools: Clean pruning and propagation tools and disinfect pots before use, especially if you often reuse planters and containers. 

Leaf Spot: FAQs

Q: Is a leaf spot contagious?

A: Fungal leaf spots are caused by a fungus that grows on leaves. It eventually produces spores that spread to nearby foliage, so the sooner you remove the plant parts with leaf spots, the more chances of preventing the spores from affecting other leaves. Bacterial leaf spots can spread bacteria to plants nearby, too. So, both types of leaf spot are contagious.

Q: Can you save a plant with a leaf spot?

A: Yes, you can usually save a plant with leaf spots if you notice and treat the issue early. You’ll need to remove all the leaves affected with leaf spot. Pruning a part of your plant will help it survive.

Q: Can overwatering cause brown spots?

A: Yes, in some cases, overwatering can cause brown spots. Check whether the soil is too moist, and look for other signs of overwatering, like yellowing leaves or a bad odor from the soil.

Q: Does bacterial leaf spot spread?

A: Bacterial leaf spot can spread to other plants with splashed water, plant debris, or infected tools or soil. Bacterial leaf spot can spread quickly in favorable conditions, which include high humidity and wet foliage.

Q: Should I remove leaves with a leaf spot?

A: Yes, remove and dispose of all the leaves and stems with leaf spots to prevent the disease from spreading to other plants.

Q: Will a leaf spot go away on its own?

A: No, if you notice leaf spots on your plant, you should remove all the affected leaves, adjust the plant’s environment, and treat it against the disease. If left unattended, too much of your plant’s foliage may be damaged by the disease, causing the plant to die.

Q: Does neem oil help with Septoria?

A: Yes, neem oil is usually effective as a natural fungicide. It is used to prevent and minimize damage from Septoria leaf spot, which is a fungal disease. In severe cases, use a copper-based fungicide.

Conclusion

While leaf spot diseases can be caused by fungal, bacterial, or viral infections, the symptoms and outward appearance of the spots vary. Distinguishing between bacterial and fungal leaf spot makes it easier to overcome the problem and help your plant recover quickly. 

Removing the foliage with leaf spots is the best way to stop the disease. Keeping your plant strong and healthy with the right care will protect it from recurring leaf spot issues.