Why Does My Plant Have Spots? A Guide to Fungal & Bacterial Diseases

Quick Disease Diagnostic Chart

Hello, plant friends! Anastasia here. Finding spots or strange coatings on your plant can be worrying. When the cause isn’t a simple watering issue or a pest, you might be dealing with a fungal or bacterial disease.

These diseases are almost always caused by an environmental problem: too much moisture in the wrong place. Use this chart to identify the visual symptoms and understand the core issue.

The Disease Triangle

Plant diseases don’t just appear from nowhere. They require three things to take hold, a concept known as the “Disease Triangle”:

1. A Vulnerable Host: Your plant.
2. A Pathogen: A microscopic fungal spore or bacterium, which are always present in the environment.
3. The Right Environment: This is the key. Pathogens need a specific environment to activate and infect a plant. For almost all common houseplant diseases, that environment is excess moisture and stagnant air.

As a plant owner, you cannot eliminate pathogens from the air, but you have complete control over the environment. By keeping leaves dry and providing good airflow, you can prevent diseases from ever starting.

Symptom #1: Leaf Spot Disease

• What it looks like: Distinct, often circular, brown or black spots on the leaves. A key sign of many fungal leaf spots is a yellow “halo” around the dead, brown center.
• Why it happens: Fungal spores land on a leaf that has been wet for several hours (from misting, overhead watering, or high humidity) and begin to grow, killing the plant tissue.
• The Fix:
  1. Isolate: Immediately move the plant away from all others.
  2. Prune: Use sterile scissors to remove all affected leaves. Dispose of them in a sealed bag.
  3. Improve Airflow: Move the plant to a less crowded spot. A small fan can work wonders.
  4. Change Your Watering: Water the soil at the base of the plant, not the foliage. Water in the morning so any splashes can dry quickly.
  5. Treat (If Necessary): For persistent infections, a copper-based fungicide can be effective. Follow the label directions carefully.

Symptom #2: Powdery Mildew

• What it looks like: A thin, white, powdery coating that looks like someone dusted the leaves with flour.
• Why it happens: This fungus thrives in high humidity and stagnant air, making it a common indoor plant issue.
• The Fix: Treatment involves isolating the plant, improving air circulation, and using a fungicide. Because this is such a common problem, we have a dedicated guide with step-by-step instructions.
• 👉 Read the Complete Powdery Mildew Guide

Symptom #3: Rust Fungus

• What it looks like: Small, raised pustules on the undersides of leaves that are orange, reddish-brown, or yellow. If you wipe them, they leave a colored, dusty residue on your finger.
• Why it happens: Like leaf spot, this fungus needs water on the leaves to take hold.
• The Fix: The treatment is identical to leaf spot disease. Isolate, prune affected leaves, keep foliage dry, improve airflow, and use a fungicide if the problem is severe.

Symptom #4: Root & Stem Rot

• What it looks like: This is the most advanced and deadly disease state. The plant is wilting, lower leaves are yellowing and dropping, and the base of the stem is soft, black, and mushy.
• Why it happens: This is the result of prolonged overwatering and waterlogged soil. It starts in the roots and moves up the stem.
• The Fix: This is a plant emergency that requires surgery, not just treatment. The healthy top portion of the plant must be cut off and propagated. The rotted base must be discarded. For a detailed walkthrough, see our full guides.
• Read the Complete Root Rot Guide 👉
• Read the Complete Mushy Stem Guide 👉

Best Practices for a Healthy Collection

• Promote Good Air Circulation: This is the #1 defense against fungal disease. Avoid crowding plants. A small, oscillating fan set on low can dramatically reduce the risk.
• Water the Soil, Not the Leaves: Keep foliage as dry as possible. Misting is not recommended if you are concerned about fungal issues.
• Sterilize Your Tools: Clean your pruning shears with rubbing alcohol or a bleach solution between plants to avoid cross-contamination.
• Quarantine New Plants: Always keep new plants separate for at least a month to ensure they haven’t brought any diseases or pests into your home.
• Remove Dead Debris: Clean up any fallen leaves or dead stems from the soil surface, as this material can harbor fungal spores.

Is it a fungal or a bacterial disease? How can I tell?

Is the disease contagious to my other plants?

Can I just wipe the spots off the leaves?