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Black Spots on Mint Leaves (Causes and Treatment)

Who doesn’t love the soothing scent and look of a healthy mint? That’s why it’s more frustrating and painful to see black spots appearing on its aromatic foliage.

Pest infestation and diseases such as mint rust, leaf blight, verticillium wilt, and leaf spot diseases are the most common causes of black spots on mint leaves. To correct the problem, water your mint when the top 1-2 inches of potting soil feels dry. Also, make sure the plant is in a well-ventilated area. To combat pest infestations, use neem oil or insecticidal soap.

Herein, I’ll explain the common causes of these black blemishes and what you can do about them.

Black Spots on Mint Leaves
Black Spots on Mint Leaves

What Causes Black Spots on Mint Leaves?

[1] Mint Rust 

Mint rust is the most common cause of black spots on the leaves of many mint species. The fungal disease manifests itself in late spring or early summer.

That is when the conditions are favorable for the fungus Puccinia menthae, which causes mint rust.

The disease first manifests itself as tiny dusty spots. They are bright orange to yellow in color. These spots lend a dusty appearance to the backs of mint leaves.

If the problem is not addressed, your mint will become weak and ugly. As a result, your mint plant is more vulnerable to leaf blight and fungal leaf spots. The leaf spots will have turned black by fall.

Common symptoms of mint rust include:

  • Paling and distortion of new shoots in spring
  • Dusty orange spots on stems and foliage in spring or early summer. 
  • Yellow spots followed by the appearance of black spots in fall
  • Affected leaves die and drop off

Control and Management of Mint Rust

I believe the first line of defense against any fungal disease is following good sanitation. 

  • You must not handle your mint plants when wet
  • Avoid wetting the leaves
  • Remove dead plant matter immediately
  • Sanitize your cutting instruments every time you use them on your plants

For non-chemical control, heat treatment is a viable option:

  • Dig up your mint in early fall before the black spots show up
  • Wash them well and heat-treated for around 10 minutes in hot water at 111ºF (44ºC)
  • After 10 minutes, remove and rinse in cold water before replanting as usual

Chemical treatments include fungicides based on azoxystrobin, tebuconazole, and myclobutanil (Check the latest price on Amazon here). The odds of eradicating the problems are high if you rotate these treatments. 

[2] Mint Leaf Blight

Mint leaf blight is another fungal problem that causes black spots on the leaves. The most common cause of leaf blight is a severe outbreak of mint rust.

The fungus Cephalosporium spp. causes it, and it is typically spread through water splashes.

During wet winters, the leaf blight fungus thrives. It can, however, strike in the spring if the weather is cool and wet.

It primarily affects the leaves of mint but can spread to the stems.

The symptoms begin as irregular black spots on the lower foliage. It then splatters black spots all over the leaves. The infected leaves will quickly die and fall off.

The affected stems may also die and begin to rot. Mint leaf blight typically spreads via cuttings and newly-homed plants.

Mint Turning Black Due To Leaf Blight
Mint Turning Black Due To Leaf Blight

Control and Management of Mint Leaf Blight

Mint leaf blight is a difficult disease to control, let alone eradicate. As a result, you must remove infected mint rhizomes and plants as soon as possible. Before disposing of them, make sure they are properly bagged.

Cultural control measures can also help. These include avoiding:

  • Wetting the leaves
  • Overhead irrigation
  • Overwatering
  • You must also keep the mint leaves dry

Heat-treat the healthy rhizomes to avoid further spreading of the mint leaf blight

For chemical controls, I recommend fungicides labeled for leaf blight on indoor plants. The most effective products often contain tebuconazole, propiconazole, or trifloxystrobin.

[3] Verticillium Wilt

Verticillium wilt is a fungus that primarily affects peppermint. It can, however, affect any variety of mint, causing widespread leaf spotting and dieback.

This fungal disease is caused by the soil-borne fungus Verticillium dahlia.

Verticillium wilt infiltrates the growing medium. The spores remain dormant for years before germinating and flourishing in hot, wet spring or summer weather.

It infects the roots first, then moves on to the upper parts of the mint plant. As a result, black spotting is often first noticed on leaves closer to the soil.

The following are the most common symptoms of verticillium wilt:

  • Wilting of some or all mint plants. You’ll witness this during hot weather from spring through fall. It’s due to water stress resulting from root damage.
  • Rapid shriveling and yellowing of primarily lower foliage. These leaves may eventually turn black and drop off prematurely.
  • Stunted, twisted, or curled up upper mint leaves
  • Stem dieback
  • Black or brown spots on the leaves and sometimes stems

Control and Treatment of Verticillium Wilt

  • Verticillium wilt spreads through contaminated soil. It’s vital not to touch the leaves after handling the soil. Clean your shoes and cutting tools after each use.
  • Sterilize your growing medium before repotting or planting
  • Control weeds in outdoor gardens
  • There are rarely any chemical fungicides for indoor control of verticillium wilt. Thankfully, the occurrence of this problem is rare. 

[4] Anthracnose

Anthracnose is another fungal disease that causes black spots on mint leaves. It spreads quickly in wet, warm conditions.

Poor aeration, overwatering, and a lack of light can all exacerbate the fungal problem.

Anthracnose appears first as tiny, sunken water-soaked spots on lower and older foliage. Lesions range in color from yellowish orange to brown.

The disease can also affect new shoots, stems, and younger leaves.

As the infection spreads, the brown spots often expand, darken, and coalesce. Eventually, your mint’s leaves turn completely black and defoliate. Other symptoms include:

  • Brown or black dead spots on leaf stalks, stems, and leaf blades
  • Affected leaves curl up to give a twisted appearance
  • Heavily affected leaves turn black, shrivel, and drop
  • Distorted growth of foliage, stems, and shoots

Control and Treatment of Mint Anthracnose

Prevention is better than cure when it comes to dealing with mint anthracnose. Good sanitation practices can go a long way:

  • Prune out and discard infected parts immediately
  • Clear up dead plant matter and debris
  • Don’t splash irrigation water on leaves
  • Mulch outdoor mint plants
  • Sanitize cutting tools after every use
  • Do not touch wet mint plants

For control, it’s best to spray using a copper-based fungicide (Check the latest price on Amazon here).

[5] Powdery Mildew

Powdery mildew is probably something you’ve seen if you’re a gardener. It is a fungal disease that coats the leaves of your mint with a grayish or white powdery substance.

Powdery mildew is a common problem for mints in hot, humid weather.

The disease first appears as small yellowish patches on the lower leaves. They will eventually turn white or gray. If left unchecked, the mint plant will completely cover itself.

This inhibits photosynthesis, causing mint leaves to turn black or develop black spots. This also reduces the vigor and growth of your plant.

Woolly aphids are known to spread powdery mildew spores. They thrive in moist environments. However, spores spread more quickly in low humidity.

As a result, the fungal disease frequently spreads aggressively in the fall and spring. This is due to the large difference in temperature between night and day. 

Powdery Mildew on Mint Leaves

Common powdery mildew symptoms include:

  • A gray or white, powder-like fungal growths on shoots and leaves. They may be seen on backsides, uppers, or both surfaces of the foliage.
  • Affected leaves turn yellow, brown, reddish, or black
  • Mildew growths turn darker as they age

Control and Treatment

Spraying with baking soda soapy solution. To prepare the mixture, combine:

  • ½ tablespoon of a mild non-detergent soap
  • 1 tablespoon baking soda
  • A gallon of water

Combine the ingredients and vigorously shake them together. Apply once a week to your mint until the powdery mildew issue is resolved.

Wetable sulfur or copper-based fungicides can also be used to spray your mint plant. After 7-10 days, spray again. Six spraying sessions should be sufficient to eradicate the pest.

[6] Stem and Stolon Canker

Stem and stolon canker is a fungal disease caused by the fungus Rhizoctonia solani. Most strains of the fungus are soil-borne and spread through water splashes.

The fungal disease affects primarily rhizomes and stems of your mint. It first manifests as reddish-brown lesions on stems and stolons.

With time, these lesions will develop into sunken cankers and turn darker.

Eventually, the disease will spread to the foliage. It’ll start with lower leaves, causing black spots, stunting, and distortion. Affected leaves usually turn yellow and may defoliate.

Stem and stolon canker can wreak havoc on your mint’s nutrient supply. It will show signs of withering and shriveling.

Control and Treatment

Good plant care can boost the survival chances of your mint plant

Apply copper-based fungicide during the growth period. Make sure to treat your mint plants before repotting or transplanting, as well.

[7] Black Stem Rust

Black stem rust is a fungal disease that causes the appearance of rust-colored lesions on the stems. It’s usually not a serious problem for mint plants. But it can spread rapidly in wet, cool conditions.

As with mint leaf rust, black stem rust affects especially peppermint and spearmint. It’s exacerbated by overhead watering and wetting of leaves.

This causes water to settle for long on foliage, causing the rust spores to germinate.

You’ll most likely see black stem rust in closely planted mint plants. The disease thrives in poorly aerated areas.

Mint Black Stem Rust
Mint Black Stem Rust

The most common symptoms of black stem rust include:

  • Black or reddish-brown cankers on stems of mint plants
  • The fungus is most active during periods of cool and wet weather.
  • The stems of your plant may become girdled. Leaves, shoots, and other parts above the affected area will start wilting and eventually die.
  • Rust-colored spots coating the backs of leaves. In later stages, these spots turn black and cover almost the entire leaves.

Control and Treatment

If you have garden mints, thin them out to improve aeration. Air circulation may aid in drying out the spores and halting their spread.

In the event of severe black stem rust, remove and discard mint plants.

If the infection is minor, simply remove and dispose of infected stems and leaves.

Some cultural changes can also be beneficial. Avoid wetting the foliage by watering directly at the growing medium. Irrigate early in the morning and expose the plants to plenty of sunlight.

When all else fails, I recommend using chemical fungicides. Use a fungicide containing chlorothalonil, azoxystrobin, myclobutanil, or propiconazole if you must spray your mint plants.

[8] Septoria Leaf Spot

Septoria leaf spot is a foliar fungal disease of Septoria species. Moderately warm temperatures and prolonged humid conditions favor the fungal problem.

The fungus Septoria lycopersici is to blame for the black spotting on most mint plants’ foliage.

The leaf spots on the lower (older) foliage will be the first thing you notice. They begin as yellow patches and progress to circular spots with dark margins and gray centers.

These spots are usually yellow-rimmed and can grow to be about 1/8-inch (3 mm) in diameter.

The fungus spores are visible as small black spots in the center of the lesions. They survive the winter on infected plant debris. They may also survive on weeds in gardens nearby.

Septoria leaf spot symptoms include:

  • Seriously affected leaves turn yellow, then brown, and eventually fall off. Defoliation usually starts from the mint plant’s base then progress upwards
  • Black to dark brown angular leaf spots, most found in between veins of leaves
  • Small black specks of fungus spores on the undersides of foliage
  • Severe leaf loss may result  in stunted growth

Control and Treatment

  • The fungal leaf spot spreads aggressively in humidity levels beyond RH70%. So, you’ll want to perk up air circulation. Also, cut back on irrigation and relocate it to a spot with less humidity.
  • Avoid handling or working your mints plants when their leaves are wet
  • You must steer clear of water splashing and overhead watering
  • Remove any weeds around your plant to prevent reinfection next season
  • Schedule an early blight fungicide spray. Apply at an interval of between 7 and 10 days.

(Source: Oklahoma State University)

[9] Mint Pests Causing Black Spots

Aphids (Myzus persicae)

It is worth repeating that some woolly aphids spread fungal diseases such as powdery mildew. This, in turn, causes the development of black spots on the foliage.

Peach aphids (Myzus persicae) are also known to cause black spots on mint leaves.

These tiny soft-bodied bugs are frequently found on the backs of leaves. They are typically yellow or green in color. However, it can also appear black, red, brown, or pink.

Look out for these symptoms of an aphid infestation:

  • Black sooty mold on foliage and stems
  • You may see some ants enjoying the honeydew
  • Heavy infestation results in leaf yellowing, stunting, and distortion
  • Shoots, especially new ones, are usually small and distorted
  • Black necrotic spots on the foliage
Control and Management of Aphids on Mint
  • Trim off and discard infested parts to control population
  • Use a forceful stream of water to dislodge aphids off your mint plants
  • Apply insecticidal soap or neem oil spray thoroughly. It should cover your entire plants to get rid of aphids. Spray after every 7-10 days to clear the infestation.
  • Although I don’t highly recommend it, you can use a chemical insecticide to control aphids on your mints.

Cutworms (Agrotis spp)

Symptoms: They take seedlings or transplants and cut them at the base. Cutworm larvae typically hide in the growing medium or plant debris. They cause deep cuts in the foliage, resulting in dead black spots.

Cutworms
Control and Management
  • Use foliage sleeves or foil collars to protect young plants
  • You can pick up and destroy by hand most visible cutworm larvae
  • Apply diatomaceous earth around the base of your mints
  • You can also use an insecticide or insecticidal spray

Thrips (Frankliniella occidentalis)

Symptoms:
  • Foliage spotted with thrips black feces
  • Silvery, coarse stippling on the leaves
  • Severely affected leaves may turn yellow and die
  • Distorted or stunted leaves due to heavy infestation
  • Black sooty mold arising from honeydew
  • Leaf drop
Thrips
Control and Management 
  • Remove thrips by blasting them with a strong water jet
  • Insecticide or pest sprays work best for a large thrip population. Use a systemic insecticide like acetamiprid or imidacloprid
  • Use blue sticky traps to trap thrips

Spider Mites (Tetranychus urticae)

Symptoms: A large infestation of two-spotted spider mites can emaciate your mint plant. That’s because they reduce its ability to photosynthesize. They invade in hot, dry conditions.

  • Wispy webbing found primarily on leaf undersides
  • Black spots on the leaves
  • Heavy infection may cause yellowing and dropping off of leaves
Treatment
  • Quarantine affected plants immediately
  • Wash your plant under a cold shower to get rid of spider mites
  • Spray with sulfur-based pesticide or insecticidal soap

How to Prevent Black Spots on Mint 

  • Avoid overhead irrigation – Instead, use drip irrigation or self-watering pots
  • Avoid water splashing, leaf wetting, or handling wet leaves
  • Practice good sanitation – Wash pruning instruments with bleach or alcohol solution after each use.
  • Remove weeds around your mint plants
  • Promote ample air circulation around your mints plants
  • Avoid overwatering

Can I Consumer Mint with Black Spots?

Short answer: no. As the problem can be a disease or pest, it’s best to eat only after the plant has properly healed.