5 Reasons For Black Spots On Mint Leaves [Complete Treatment]

Just imagine, what is the value of a mint plant if the leaves aren’t healthy and good-looking? 

When you notice black spots on your mint leaves, as a mint grower I understand your concerns. 

And you will be happy to know; you can get rid of this problem easily. To fix the problem, you have to know, what are the main culprits behind this problem.

There are 5 reasons for black spots on mint leaves, i.e. Mint Rust, Mint Aphids, Overwatering, Humidity, And Fertilizer Burn.

In this article, I have tried to address all the reasons for the black spot of mints with their simplest solutions. So that you don’t have to go through much trouble. 

So, without any delay let’s get things under the way-

Why Are Black Spots On Mint Leaves?-Problems & Solution

1.Mint Rust

Mint Rust is a very common disease of mints and other plants of herb families including basil and oregano. Mainly, Peppermints and Spearmints are very prone to mint rust.

It causes small black spots on the underside of the mint leaves. The leaves start to turn yellow with black spots on them. As the disease advances the whole leaves start turning black.

It’s very important to treat mint rust because it weakens the plant. As a result, Many other fungal problems can start happening. Even fungal spores can be on harboring on top of your mint leaves that look like black balls.

Control Measures

First, you need to remove the black-spotted leaves from the rest of the plants. If necessary prune the infected branch. Throw them in the garbage. 

Then you need to apply a fungicide. You need to choose a fungicide that has safe and nontoxic active ingredients for the mint leaves. Otherwise, the leaves won’t be edible (our pick: Bonide Copper Fungicide). Be sure to check on the label when it is safe to harvest the leaves after fungicide application.

Mint Rust easily attacks when the leaves stay wet in damp conditions. So, always water your mint plants from the base. Water during the morning so the leaves can dry in the sun. 

If you are wondering how to treat mint fungus at home then try these homemade recipes below:

#Recipe 1:- Copper Spray Recipe

Ingredients:

  • Half a gallon of water
  • 30 ml of copper sulfate
  • 5 tablespoons of hydrated lime (Calcium hydroxide)

Steps:

  • Start by adding water and hydrated lime in a plastic bucket. Do not use any metal bucket. Because the ingredients react with metal.
  •  Stir the mixture slowly with a plastic spoon.
  • While stirring, start adding the copper sulfate into the container. You have to keep stirring because the hydrated lime settles down easily.
  • Shake the mixture very well before spraying. 
  • Wash the sprayer thoroughly after spraying because this mixture is very corrosive.
  • Don’t store this mixture for future use.

#Recipe 2:- Garlic Pepper Spray Recipe

Ingredients:

  • 1 small bulb of garlic
  • 2 tablespoons of oil
  • 4 hot peppers
  • 5 Liter of water

Steps:

  • Blend the garlic and pepper to make a paste
  • Add the oil to the paste and let it steep overnight
  • Strain the paste to remove all the solids.
  • Now add 5 liters of water to it and spray it on both sides of your mint leaves with black specks.

Note: Use the homemade recipes in a small amount to a leaf first. If the leaf discolors then add some water to the solution to dilute. Then try again. If there’s no reaction then start applying to the whole mint plant.

Preventive Measure

Mint Rust occurs in humid conditions where airflow is less. Since mints are bushy herbs, they need enough space around them. Occasionally prune off some old leaves from time to time for better air circulation.

Keep the mints weed-free. Always clean your garden tools after working. Put away other infected plants from your mints.

2.Mint Aphids

Mint Aphid is a serious pest of herbs. It causes black spots on mint leaves. They are very small insects but their damage can even kill your mints. They feed on the leaf and transmit a fungal disease called sooty mold. This sooty mold makes black powdery spots on your mint leaves and turns them inedible.

Control Measures

A strong spray of water can wash them off in case of light infestation. But mint aphids usually attack a massive scale.

So, It is very important to kill these mint aphids to keep the mint leaves from turning black. Because they cause other severe fungal and viral diseases to the plants. And some of them are fatal.

We grow mints only for the leaves. So, We have to apply the best insecticide that is safe for mint leaves. But now the question; what is the best insecticide for mints? Here is our recommendation-Monterey Garden Insect Spray

Also, You can spray neem oil and insecticidal soap to control the mint pest (our pick: Natria Insecticidal Soap)

Here are some natural and homemade recipes written below that can kill these mint aphids

#Recipe 1:- Three-In-One Insect Spray Recipe

Ingredients:

  • 1 bulb of garlic
  • 1 onion
  • 1 teaspoon of dry pepper 
  • 1 tablespoon of liquid dish soap
  • Half a gallon of water

Steps:

  • Blend the onion and garlic together 
  • Add the dry pepper and let this sit overnight
  • Strain the mixture with a fine cloth
  • Now add water and liquid dish soap and mix thoroughly
  • Spray it on both sides of your mint leaves. 
  • Hot pepper can irritate the skin so be careful while spraying this solution. 

#Recipe 2:- Neem Oil Recipe

Ingredients:

  • 1 tablespoon of neem oil 
  • 2.5 liters of water 

Steps:

  • Mix both the ingredients
  • Spray on the underside of the mint leaves with black spots.
  • You can spray Neem oil once a week to prevent aphid attacks.

# Recipe 3:- Insecticidal Soap Recipe

Ingredients:

  •  2 tablespoons of liquid dish soap
  • 1 liter of water

Steps:

  • Mix water and liquid soap together. 
  • Spray this solution twice a week regularly. 
  • This will help repel the aphids. 

Preventive Measures

Wondering how to prevent aphids?

You can place shiny Aluminum foil under your mint plant. You can get digital insect traps or row covers to protect your mints from aphids. Don’t apply insecticides during high temperatures. This can burn the leaves and turn them black.

3.Overwatering Issues of Mints

Mints have a sensitive root system. When they are overwatered, black spots are seen near the roots. Overwatering also brings other fungal diseases like root rot and powdery mildew. These are more reasons why the mint leaves have black and white spots on them.

Control Measures

Remove the black mint leaves from the plant. Then you have to set a good watering schedule. Because improper watering can kill the mint seedlings. To avoid giving your mints too much water, check the soil moisture with a moisture meter (Our pick: Atree Soil Soil Tester Kits with Moisture, Light, and PH Test for Garden)

Water at 3 or 4 days intervals. During winter water once a week. But whenever you feel the soil has dried up, give a light watering.

Mints thrive in light and loamy soil. If you are growing mints in pots use a good herb-potting mixture (our pick: Espoma Organic Potting Mix). Make sure the pot is large enough for root growth. Keep enough drainage holes in them. 

4.Heat And Humidity Issues

Mint flourishes in full sun but they have a limit. Exposure to high heat can burn your mints and turn them black. They require 5-6 hours of sunlight and a temperature around 70-85°F. 

Mints also don’t grow well in high humid areas. The humidity evaporates the water very fast. As a result, the plants feel stressed and start turning black.

Control Measures

Provide the mints with some shade in the afternoon during extreme heat. Then again, heat always comes with high humidity. So, the soil will dry out more in these conditions. In this case, you need to increase some watering by checking with a moisture meter.

If you are growing mints indoors then you can get a humidifier to control the humidity in your room. It will help you solve the moisture problem. It will also prevent more leaves from turning black.

5. Fertilizer Burn of Mint

Mints require fertilizer for their healthy and steady growth. But going overboard is not a good idea. You see, overfertilization adds too many nutrients to the soil. Overfertilization hurts the mint roots and creates black spots on mints. It also encourages other bugs and insects to attack. 

An easy way to check over-fertilization is by using a pH meter. If the pH value goes above or below 7, it means your mints are suffering from fertilizer burn. 

Control Measures

Your mints need fertilizer once or twice a year. Avoid using just any chemically toxic fertilizer.

Apply the best all-purpose, slow-release herb-fertilizer to your mints(our pick: Jobe’s Organics Herb Plant Food). Always follow the label directions while applying.

Frequently Asked Questions (FAQs)

Can I eat mint with black spots?

If fungal disease or pests are causing the black spots on mint leaves, then you can’t eat them. They are unhealthy and they spread a foul smell. But if it’s due to water or heat issues, then you can eat them after treating them properly.

Can Black Mint Leaves Come Back?

If you remove all the infected black leaves properly and treat the plant correctly then, the black spots are very unlikely to come back.

How do I know when mint leaves go bad?

Spoiled mint leaves will turn soft, wilting, and discolored. And they will give a pungent smell. The appearance itself will show that the leaf has gone bad.

Conclusion

Getting rid of black spots on your mints takes some care. But when your plant shines with charming green leaves everything is worth it.

Prevent them from Mint Rust and Mint Aphids, apply the right dose of fertilizer, and water the right amount. This is all they need.

In this article, I tried to explain how to treat the black spots on mints with all the necessary information. I hope you found the solutions to your problem. You can treat the black spots on your oregano and basil plants in the same way too.

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