Why Are There Black Spots on Mint?

Last Updated on November 24, 2022 by Stephanie

Who doesnt like the relaxing smell and appearance of a fresh mint? This is why its so frustrating and painful to see dark spots appear on its fragrant leaves.

Pest infestations and diseases like the leaf blight, mint rust, verticillium wilt and the leaf spot disease are among the most commonly cited causes of dark marks on leaves of mint. To fix the issue make sure to water your mint once the top 2 inches of soil feel dry. Make sure that the plant is placed in an area that is well ventilated. To fight pests make use of neem oil as well as insecticidal soap.

In this article, Ill discuss the most common causes for the black spots and how you can do to treat them.

What Causes Black Spots on Mint Leaves?

[1] Mint Rust

Mint Rust is the most frequent reason for black spots appearing in the leaf of a variety of mint species. The fungal disease is most noticeable in the latter half of spring or in early summer.

This is when the conditions are favorable to this fungus Puccinia methae that causes mint rust.

The illness first manifests in tiny, dusty spots. They range from bright yellow to orange in hue. They give an appearance of dust to the mints back leaves.

If the issue isnt taken care of the mint will grow fragile and ugly. In the end, your mint plant will be more susceptible to fungal and leaf blight leaf spots. The spots on the leaves will be into black by the time fall arrives.

The most common symptoms of mint rust are:

  • The distortion and paling of the spring shoots
  • The stems are covered with dusty orange spots and leaves in spring or the beginning of summer.
  • Yellow spots, followed by black spots in the fall.
  • The leaves affected die and fall off

Control and Management of Mint Rust

The first protection against any fungal infection is to follow the proper sanitation.

  • It is not advisable to touch your mint plants if they are they are wet.
  • Do not let the leaves get wet.
  • Remove dead plant matter immediately
  • Cleanse your cutting tools each time you work with the tools on plants

To control non-chemical issues the use of heat is an option

  • Find your mint in the early autumn, before the black spots appear.
  • Clean them thoroughly and then heat-treated for about 10-15 minutes with hot water, at 111oF (44oC)
  • Within 10 minutes take it off and wash in cold water prior to replanting like normal

Chemical treatments consist of fungicides based on azoxystrobin, tebuconazole as well as myclobutanil (Check the most current prices at Amazon right here). The chances of eliminating the issues are very high when you alternate the treatments.

mint leaves with oil

[2] Verticillium Wilt

Verticillium Wilt is a fungus which is primarily affecting peppermint. However, it can be a problem for any mint variety and cause extensive leaf spotting and dying back.

The fungal infection is caused through the soil-borne fungus Verticillium dahlia..

Verticillium wilt invades the growing medium. The spores are inactive for a long time before they germinate and begin to flourish in humid, hot spring or summer temperatures.

The infection begins with the roots and then spreads to the upper part that are part of mint. In the end, the black spots are usually first observed on leaves that are closer to the soil.

These are some of the frequent symptoms of verticillium wilt

  • The wilting of some or all mint plants. It is common in hot weather from spring to the fall. This is due to stress in the water caused by damage to the root.
  • Rapid shrinking and yellowing of the primarily lower leaves. The leaves could eventually turn black , and fall off in a hurry.
  • Twisted, stunted or curled up mint leaves
  • Stem dieback
  • Brown or black spots appear on the leaves , and occasionally stems

Control and Treatment of Verticillium Wilt

  • Verticillium wilt is spread through contaminated soil. It is important not to touch the leaves after working with the soil. Clean your footwear and cut tools after each use.
  • Make sure your growing medium is sterilized prior to you plant or repot your garden.
  • Control weeds in outdoor gardens
  • There arent many chemical fungicides that can be used indoors to control the spread for verticillium-wilt. Fortunately, the incidence of this issue is extremely rare.

[3] Mint Leaf Blight

Mint leaf blight is a different fungal issue that can cause black spots on leaves. The most frequent reason for leaf blight is an intense outbreak of mint rust.

A fungus called Cephalosporium spp. is the cause, and is usually spread by water splashes.

In winters with a lot of rain the blight of the leaves fungal growth thrives. However, it can occur in springtime if the weather is cold and humid.

It is most commonly found in mint leaves but it can also affect the stems.

The signs appear as uneven black spots appearing on the lower leaves. Then, the spots spread to create black spots across the leaves. The affected leaves will soon end up dying and then fall off.

The stems affected by the disease may start to die and begin to turn brown. Mint leaf blight is typically spread through cuttings and newly-homed plants.

Control and Management of Mint Leaf Blight

Mint leaf blight is a challenging disease to manage and even eradicate. Therefore, it is imperative to eliminate infected mint rhizomes as quickly as you can. Before you dispose of them ensure that they are appropriately sealed.

Cultural control measures may also aid. This includes avoiding:

  • The leaves are wetting
  • Overhead irrigation
  • Overwatering
  • Also, ensure that the mint leaves are dry

The healthy rhizomes should be treated with heat to stop the spread from the mint leaf disease

To combat chemical pests I suggest fungicides that are labeled for the treatment of leaf blight on indoor plants. Most effective products typically include propiconazole, tebuconazole or trifloxystrobin.

[4] Anthracnose

Anthracnose is a different fungal disease that creates the mint leaves to develop black spots. It is easily spread in humid and warm climates.

Insufficient aeration, excessive watering and the absence of sunlight can all contribute to the fungal issue.

Anthracnose is first seen as small, sunken spots of water soaked spots on the lower and older leaves. The color of the lesions varies from brownish to yellowish orange.

The disease may also be a problem for new shoots, stems and leaves that are younger.

As the infection progresses the brown spots usually grow, become darker and then become a coalescence. Then, the leaves of your mint become completely black and begin to defoliate. Other signs are:

  • Dead spots of black or brown color on stems, leaf stalks, and leaf blades
  • The affected leaves curl up, giving the appearance of twisted leaves
  • The leaves that are severely affected become black, shrivel and fall
  • Growth of foliage is distorted stems, shoots, and leaves

Control and Treatment of Mint Anthracnose

Prevention is always better than cure in fighting mint anthracnose. A good sanitation practice can make a difference:

  • Remove and clean up the affected areas immediately.
  • Clean up the dead plants and other debris
  • Do not splash irrigation water onto leaves
  • Mulch outdoor mint plants
  • Cleanse cutting tools following each use
  • Avoid touching wet mint plants

To control the problem, its recommended to use an fungicide containing copper (Check the most current cost at Amazon right here).

[5] Powdery Mildew

Powdery mildew is likely something youve encountered in your garden if youre one of them. Its a fungal disease that covers the leaves of your mint plant with the appearance of a white or grayish powdery substance.

Powdery mildew is a typical issue for mints in hot, humid conditions.

The first signs of the disease are tiny yellowish patches on lower leaves. They eventually become grey or white. If not checked, it will fully overflow with itself.

This hinders photosynthesis, which causes mint leaves to become black or to develop black spots. It also decreases the growth and vigor of your plant.

Woolly aphids have been known to transmit the spores of powdery mildew. They thrive in humid environments. However, they spread faster in humid environments.

In the end, the fungal disease is often spread rapidly during the spring and fall. It is because of the huge variation in temperatures between day and night.

Common symptoms of mildew with a powdery appearance are:

  • Gray or white powder-like fungal growth on the shoots and leaves. They can be found on the backsides, uppers or both surfaces of leaves.
  • The affected leaves change color from yellow to brown, reddish, or even black.
  • Mildew growths become darker as they get older.

Control and Treatment

The baking soda solution can be sprayed with soap as a spray. To make the mix make a mixture of:

  • 1/2 teaspoon of mild soap that is not detergent.
  • 1 teaspoon baking soda
  • One gallon of drinking water

Mix the ingredients and vigorously shake them together. Use once per week on the mint till issue with powdery mildew is solved.

Copper-based fungicides or Wetable sulfur can be used for spraying the mint plants. Within 7-10 days of the initial spray, you can spray again. Six spraying sessions are enough to eliminate the pest.

[6] Stem and Stolon Canker

Stem and Stolon canker is a fungal infection that is caused by the fungus Rhizoctonia solani. The majority of strains are soil-borne and can be spread via splashes of water.

The fungal disease is affecting the rhizomes and the stems of your mint, mainly. The first signs are brown-reddish lesions on stems as well as stolons.

As time passes, the lesions will turn into sunken cankers, and then turn darker.

In time, the disease can spread to the leaves. It will begin with lower leaves and cause stunting, black spots and distortion. The affected leaves typically change color and can defoliate.

Stem and stolon cankers could wreak havoc on the mints nutritional supply. The plant will begin to begin to show signs of wilting and shrinking.

Control and Treatment

A good plant care routine can increase the chances of survival for your mint plant

Use a copper-based fungicide throughout the growing season. Be sure you apply the fungicide to your plants prior to transplanting or repotting.

[7] Black Stem Rust

The black stem rust is an infection caused by fungal bacteria that results in an appearance of dark rust lesions on stems. Its not usually a major issue in mint plant. However, it can be a major problem in cool, wet conditions.

Like mint leaf rust, the black stem rust can affect peppermint as well as spearmint. The problem is exacerbated by excessive watering and the wetting of leaves.

The water settles over the foliage for a long time and causes bacteria that cause rust to grow.

It is common to find black stem rust on mint plants that are closely planted. The disease is prevalent in areas that are poorly air-conditioned regions.

The most frequent signs of black stem rust are:

  • Cankers of black or reddish-brown color on the stems of mint plants
  • The fungus is at its most active during dry and cool weather.
  • The plants stems could be in a state of being girdled. Shoots, leaves, and other components that are above the affected region begin to wilt and then end up dying.
  • Rust-colored spots that cover the leaves backs. Later on, the spots become black and cover the majority of leaf.

Control and Treatment

If youre a gardener you can thin them out to increase the aeration. The air circulation can aid by drying the spores, and stopping their spread.

If you notice extreme black stem rust, take mint plants out and throw them away.

If the problem is not serious it is best to remove and discard the infected leaf and stems.

Certain cultural adjustments can be beneficial. Beware of soaking the foliage applying water directly to the growth medium. In the early morning, water your plants as well as expose plants to lots of sun.

If all else fails I suggest applying chemical fungicides. Choose a fungicide that contains chlorothalonil, azoxystrobinor myclobutanil or propiconazole when you have to apply a spray to your plants.

[8] Septoria Leaf Spot

Septoria leaf spot can be described as a fungal foliar disease that affects Septoria species. Warm temperatures moderately and long-lasting humid conditions favour the fungal issue.

A fungus called Septoria Lycopersici is responsible for the black spots that appear on a majority of mint plants leaves.

Leaf spots that appear on the lower (older) leaves are your first point of see. They start as yellow patches , and then progress to circular spots that have dark edges and gray centers.

They are typically yellow-rimmed, and they can approximately 1/8 inch (3 millimeters) in diameter.

The spores of fungus are visible as tiny black spots within the middle of the lesions. They can survive winter months on plant debris that is infected. They can also live by eating weeds that grow in the gardens around.

The symptoms of septoria leaf spot include:

  • The leaves that are severely affected become yellow, and then turn brown then fall off. The process usually begins at the base of the mint plant, and then move up
  • Dark brown to black leaf spots, mostly within veins on leaves
  • Small black specks of fungusspores on the foliages undersides
  • A severe loss of leaf could cause a slowing of growth

Control and Treatment

  • Fungal leaf spots can spread rapidly in humid conditions that exceed RH70 percent. Therefore, it is important to increase the circulation of air. Also, reduce the amount of irrigation and move it to a location that has less humidity.
  • Do not handle or work your mint plants when their leaves are damp.
  • Avoid water splashes and the overhead watering
  • Get rid of any weeds that surround your plant to avoid the spread of disease in the future
  • Plan an early spray of fungicide against blight. Spray at intervals of between 7 and 10 days.

(Source: Oklahoma State University)

[9] Mint Pests Causing Black Spots

Aphids (Myzus persicae)

It is important to note that woolly aphids can transmit fungal diseases, such as powdery mildew. This is what causes the formation dark spots that appear on the foliage.

Peach Aphids ( Myzus persicae) are also believed to cause the formation of black marks on leaves.

The tiny, soft-bodied insects are often located on the back of leaves. They usually are either green or yellow in color. However, they can appear red, black or brown. It can also appear pink.

Be on the lookout for these signs of an aphid problem:

  • The black sooty mold that appears on the leaves and stems
  • There are insects taking in the honeydew
  • Infestation that is severe causes the yellowing of leaves, stunting and even distortion
  • Shoots, particularly those that are new tend to be tiny and often distorted
  • Necrotic spots of black on the foliage
Control and Management of Aphids on Mint
  • Remove and trim off the infested areas to reduce the population
  • Make use of a powerful flow of water wash the aphids from your mint plants
  • Apply insecticidal soap or oil spray in a thorough manner. The spray should be applied to all plant to eliminate Aphids. Spray every 7-10 days to eliminate the problem.
  • While I wouldnt recommend it, you could employ a chemical insecticide to get rid of aphids from your mints.

Cutworms (Agrotis spp)

The symptoms: They harvest transplants or seedlings and cut them off at the bottom. Cutworm larvae usually hide in the medium of growth or in plant debris. They can cause deep cuts to the foliage, which result in the formation of dead spots in black.

Control and Management
  • Use foil collars to shield young plants.
  • You can grab and eliminate by hand the majority of visible larvae of cutworms
  • Place diatomaceous earth at each mints base
  • It is also possible to use an insecticide spray or insecticidal spray

Thrips (Frankliniella occidentalis)

Symptoms:
  • Foliage is spotted by thrips and black feces
  • The silvery, coarse stippling is visible on the leaves
  • The leaves of those with severe damage can change color and die.
  • The leaves are stunted or distorted because of the heavy infestation
  • Mold that is black and sooty, arising from honeydew
  • Leaf drop
Control and Management
  • Get rid of the thrips with a blast of an intense water jet
  • Pesticides or insecticides work best when there is a large population. Use a systemic insecticide like acetamiprid or imidacloprid
  • Utilize blue sticky traps to catch the thrips

Spider Mites (Tetranychus urticae)

Signs: A massive infestation with two-spotted spider mites could cause a severe emaciation of your mint plant. This is because they decrease the plants ability to photosynthesise. They are at risk when it is hot and dry.

  • Wispy webbing is found mostly on the leafs undersides
  • The leaves are black with black spots.
  • A prolonged infection can cause yellowing and the dropping of leaves
Treatment
  • Quarantine affected plants immediately
  • Clean your plant in an icy shower to eliminate spider mites
  • Spray with sulfur-based pesticides or soap for insecticides

How to Prevent Black Spots on Mint

  • Avoid overhead irrigation. Instead, you can use watering drips or containers
  • Avoid splashing water or leaf wetting when handling wet leaves
  • Make sure you wash your pruning tools with bleach or alcohol solution after every use.
  • Get rid of weeds around the mint plants
  • Make sure that there is ample air circulation around your mint plants
  • Beware of overwatering

Do I Consumer Mint with Black Spots?

The short answer is no. Since the cause could be a pest or a disease and it is recommended to consume only when the plant has fully recovered.

Stephanie

Stephanie

Went from an inexperienced gardener to a half-decent one over 10+ years. I cover anything from general indoor plant guides and lawn care, to succulents and flowers. Super happy to share my tips and tricks with you :)