Why Do Potatoes Turn Black? (3 Key Things To Know)


It is a little disappointing to see potatoes turning black, whether it is at harvest time, during storage, or after cooking.  It begs the question of what causes black potatoes in the first place.

So, why do potatoes turn black?  Potatoes turn black in soil due to diseases (such as Blackleg, Early Blight, or Late Blight).  Potatoes turn black in storage due to Hollow Heart (caused by a lack of oxygen) or mold (caused by storing wet potatoes).  Potatoes turn black after cooking due to oxidation of an acid found in the tubers.

Of course, you can avoid these diseases and problems by taking some precautions when growing, storing, and cooking your potatoes.

In this article, we’ll take a closer look at some of the reasons that potatoes turn black.  We’ll also mention some ways that you can prevent black potatoes in the field, in storage, and after cooking.

Let’s begin.

Why Do Potatoes Turn Black?

There are several reasons that potatoes may turn black or gray, including:

  • Plant Diseases – if a potato turns black while still underground, then disease is a likely culprit.  Some potato plant diseases can cause potato tubers to turn black, including Blackleg, Black Scurf, Early Blight, Late Blight, Pink Rot, and Soft Rot.  Potato diseases may be caused by bacteria, viruses, or fungi.
  • Improper Storage – potatoes may turn black inside due to Black Heart, which is caused by a lack of oxygen during storage.  Storing potatoes when they are still wet after harvest can also allow black mold to grow on the outside.
  • Cooking & Air Exposure – potato tubers that are otherwise healthy and properly stored can still turn black after cooking and air exposure.  Potatoes are more likely to turn black after cooking if they are steamed or boiled before peeling.  This happens due to oxidation of an acid (ferri-chlorogenic acid) in the potatoes after they are cooked and left exposed to the air.

Let’s take a closer look at each of these causes of black potatoes, starting with plant diseases.

potato late blight
Late blight and other potato plant diseases can cause tubers to turn brown, gray, or black inside or outside.

Black Potatoes Caused By Plant Diseases

Potato plants can succumb to various diseases.  Some of these diseases will cause the tubers to turn black on the outside, inside, or throughout.

Some diseases that cause black potato tubers include:

  • Blackleg
  • Black Scurf
  • Early Blight
  • Late Blight
  • Pink Rot
  • Soft Rot

Here is a little more information on each of these diseases:

Blackleg Of Potato

Blackleg is caused by the bacteria Pectobacterium atrosepticum.  It causes black streaks on the stems of infected potato plants.

Blackleg of Potato
Blackleg of Potato can cause black streaks on the stems and dark spots on the stem end of a tuber.
Image courtesy of user:
Amy Charkowski via:
Wikimedia Commons: https://commons.
wikimedia.org/wiki/File:
Blackleg_of_Potato_Stem_Wilt.png

Blackleg causes slimy decay of a potato plant in wet weather and shriveling in dry weather.  However, it can also cause sunken, dark black spots on the stem end of a potato tuber (where it connects to the plant).

Blackleg does well in wet conditions at temperatures below 65 degrees Fahrenheit (18 degrees Celsius).  Avoid planting potatoes in cool, wet soil if you can help it – wait for the soil to warm up, dry out, or both.

Use certified disease-free seed potatoes, since the disease can survive from one generation of potatoes to the next.  Also, provide air circulation when you store your potatoes.

This disease spreads more easily in wind and rain.  You can learn more about Blackleg of potato in this article from the University of Massachusetts Extension.

Black Scurf Of Potato

Black Scurf is caused by the fungus Rhizoctonia solani.  It looks like black areas on the surface of the potato tuber that rise above the skin (“dirt that you cannot remove by washing or rinsing”).

These black spots are where the Rhizoctonia solani fungus survives the winter to infect plants again the next year.

According to the Oregon State University Extension, the same fungus can also cause Rhizoctonia stem canker in potato plants.  This disease can reduce potato yield by creating lesions on stolons (which move nutrients to the tubers).

Blackleg does well in wet conditions and at temperatures between 41 and 77 degrees Fahrenheit (5 to 25 degrees Celsius).  Avoid planting potatoes in cool, wet soil if you can help it – wait for the soil to warm up, dry out, or both.

Also, use certified disease-free seed potatoes, since the disease can survive from one generation of potatoes to the next.  Also, provide air circulation when you store your potatoes.

You can learn more about Blackleg of potato in this article from the University of Idaho Extension.

Early Blight Of Potato

Early blight is caused by the fungus Alternaria solani.  At first, it looks like small black or brown spots on the stems and leaves of the plant.

early blight on tomato leaf
Early blight on a potato plant starts off as small black or brown spots with yellow edges on the stems and leaves.

According to the Oregon State University Extension, Early Blight can also cause sunken black or brown spots on potato tubers themselves.  Eventually, these spots infect the inside of the potato, leaving the tissue brown and corky.

Early Blight does well in wet and humid conditions and develops at temperatures between 59 and 82 degrees Fahrenheit (15 to 28 degrees Celsius).  The ideal temperature range for growth is 82 to 86 degrees Fahrenheit (28 to 30 degrees Celsius).

To avoid the spread of this disease, use certified disease-free seed potatoes, since the disease can survive from one generation of potatoes to the next.  Also, provide air circulation when you store your potatoes.

You can learn more about Early Blight of potato in this article from the North Dakota State University Extension.

Late Blight Of Potato

Late blight is caused by the water mold Phytophthora infestans.  At first, it looks like pale green spots with a yellow border on the leaves of a potato plant.

late blight potato leaves
Late blight can affect the leaves, stems, and tubers of a potato plant. It is far more deadly than early blight.
Image courtesy of user:
Rasbak via:
Wikimedia Commons:
https://commons.wikimedia.org/wiki/File:
Phytophthora_infestans_potato_
Parel,_aardappelziekte_Parel.jpg

According to the Oregon State University Extension, the stems of infected plants may turn black or brown.  A brownish dry rot also appears in the tubers of infected plants, opening the way for soft rot that can cause blackening.

Late Blight does well in cool, wet conditions.  The ideal temperature range is between 60 and 70 degrees Fahrenheit (16 to 21 degrees Celsius).  Late blight is far more damaging than early blight, and it can also affect tomatoes.

The disease can spread on the wind by spores.  It can also survive the winter underground in infected tubers.

To avoid the spread of this disease, use certified disease-free seed potatoes, since the disease can survive from one generation of potatoes to the next.  Also, provide air circulation when you store your potatoes.

You can learn more about Late Blight of potato in this article from the University of Minnesota Extension.

Pink Rot Of Potato

Pink Rot is caused by the bacteria Phytophthora erythroseptica.  It can grow on both potatoes and tulips.

pink rot potato
Pink Rot of Potato causes infected flesh to turn pink at first. It will turn black when exposed to air for 15 to 30 minutes. It can also affects stems of potato plants.
Image courtesy of user:
Robert Cating via:
Wikimedia Commons:
https://commons.wikimedia.org/wiki/File:
Pink_rot_of_potato_cuased_by_
phytophthora_erythroseptica.jpg

According to the University of Massachusetts, Pink Rot starts at the stolon end of a potato tuber.  Infected flesh turns pink and then black when exposed to air for 15 to 30 minutes.

It can also affect the stems of potato plants, starting at the base and moving up.

Pink Rot does well in wet soil, and it can cause more damage by spreading through damaged potatoes that are stored in areas with high humidity.

Pink Rot can infect potato tubers whenever they come in contact with the bacteria: harvest, storage, or cleaning.  Potatoes are more likely to be infected if they are cut or bruised, and if they are harvested early at an immature stage.

You can learn more about Pink Rot of potato in this article from the Michigan State University Extension.

Soft Rot Of Potato

Soft Rot is caused by the bacteria Pectobacterium carotovorum.  It can grow on many crops, including all varieties of potato.

Soft Rot first appears as small tan spots on the tuber’s skin (often as a result of bruising due to rough handling during harvest or transport).  Later, these spots turn black after air exposure.

Soft Rot can also cause the stems of potato plants to rot and become soft.

Soft Rot does well at temperatures between 70 and 80 degrees Fahrenheit (21 to 27 degrees Celsius).  However, it can survive from 32 to 90 degrees Fahrenheit (0 to 32 degrees Celsius).

Soft Rot can infect potato tubers whenever they come in contact with the bacteria: harvest, storage, or cleaning.  Potatoes are more likely to be infected if they are cut or bruised, and if they are harvested early at an immature stage.

You can learn more about Soft Rot of potato in this article from the University of Maine Extension.

The table below gives a comparison of various potato diseases that can make tubers turn black.

Potato
Disease
CauseSymptoms
BlacklegBacteria:
Pectobacterium
atrosepticum
Black streaks
on stems &
sunken dark
black spots
on stem end
of tuber
Black
Scurf
Fungus:
Rhizoctonia
solani
Black raised
spots on skin
of tuber that
cannot be
washed off.
Early
Blight
Fungus:
Alternaria
solani
Small black
or brown
spots with
yellow edges
on stems
& leaves
Late
Blight
Water Mold:
Phytophthora
infestans
Pale green
spots with
yellow edge
on leaves;
stems turn
black/brown;
brown dry
rot in the
tubers.
Pink
Rot
Bacteria:
Phytophthora
erythroseptica
Flesh turns
pink, and
then black
after 15 to
30 minutes
of air
exposure.
Soft
Rot
Bacteria:
Pectobacterium
carotvorum
Small tan
spots on
tuber skin;
later turn
black after
air
exposure.
This table gives a comparison of various potato
diseases that can make tubers turn black.

Black Potatoes Caused By Improper Storage

Potatoes may also turn black if they are not stored properly.  One of the common reasons for black potatoes in storage is Black Heart.

According to the University of Massachusetts Extension, Black Heart takes hold when a potato experiences a lack of oxygen during storage.  The tuber starts to die inside and turns black.

According to the North Dakota State University Extension, Black Heart can happen as the potato plant is growing or during storage.  Compact or wet soil (which limits available oxygen) makes Black Heart more likely.

High temperatures also increase the chances that black heart will occur (due to faster respiration of the tuber).

To avoid this problem, plant potatoes in soil with good drainage and avoid over watering them.  Also, don’t wash potatoes before storage (you can brush off the dirt if you want, but don’t damage them by cutting or bruising).

(You can learn more about the right kind of soil for potatoes (and how to prepare it) in my article here.)

Note: Brown Center and Hollow Heart can also cause dark flesh or hollow cavities inside a potato tuber.

Storing potatoes when they are wet (after harvesting from wet soil, during rain, or washing) can also promote fungal and bacterial infections that can cause tubers to turn black.

Black Potatoes Caused By Cooking & Air Exposure

If your potatoes were fine when you harvest them (or bought them) and fine when you took them out of storage, they can still turn black after cooking.

The important questions are: why does this happen, and how can you prevent it?

Why Do Cooked Potatoes Turn Black?

Cooked potatoes contain an acid that reacts with oxygen in the air (oxidation).  The acid is called ferri-chlorogenic acid (which contains iron).

Oxidation of ferri-chlorogenic acid in potato tubers happens after cooking by frying or boiling.

frying potatoes cut
Potatoes that are cut and fried or boiled may turn black after air exposure, due to oxidation of ferri-chlorogenic acid.

According to the University of Illinois, potato blackening after cooking is more severe with a higher ratio of chlorogenic acid to citric acid in the tuber. (This gives us a hint as to how we can prevent blackening of potatoes during cooking – more on this below).

This same effect can also cause the water you used for cooking potatoes to turn black.

How Do You Keep Cooked Potatoes From Turning Black, Gray, Or Brown?

There are a some ways to keep cooked potatoes from turning black, gray, or brown.

One way is to add a little acetic acid (found in vinegar) to the water before you cook your potatoes.  However, this may prevent the potatoes from becoming tender.

Citric acid (found in lemon juice) added to water before cooking may also help to prevent potatoes from turning black.

Meyer Lemon
Citric acid (found in lemons) or acetic acid (found in vinegar) can help to prevent cooked potatoes from turning black.
Image courtesy of user:
Debra Roby via:
Wikimedia Commons: https://commons.
wikimedia.org/wiki/File:Meyer_Lemon.jpg

Why Do Potatoes Turn Black Inside?

Potatoes turn black inside due to certain diseases (late blight, pink rot, or soft rot).  This is more likely after air exposure, which can happen due to an injury during harvesting or transport (cuts, insect damage, etc.)

Potatoes also turn black inside due to improper storage.  Black Heart occurs inside a potato tuber due to a lack of oxygen, and is more likely in wet soil and high temperatures.

Can You Eat Potatoes When They Turn Black?

If your potatoes turn black after cooking, you can still eat them.  The reason is that darkening of potatoes is a natural reaction caused by the reaction of ferri-chlorogenic acid with oxygen in the air (oxidation).

However, you might want to avoid eating any potatoes that have turned black due to diseases.  That way, you can avoid ingesting fungus and bacteria that may harm you.

You might be able to make an exception if you cut away the diseased part of the tuber.  For example, the University of Wisconsin-Madison suggests that the clean parts of a tuber from a plant infected with late blight are safe to eat.

Conclusion

Now you know why potatoes turn black, along with when and how it happens.  You also know how to prevent it from happening in the first place.

I hope you found this article helpful.  If so, please share it with someone who can use the information.

If you want to read some of my most popular posts, check out the “Best of GreenUpSide” page here.  Enjoy!

~Jonathon

jonathon.david.madore

Hi, I'm Jon. Let's solve your gardening problems, spend more time growing, and get the best harvest every year!

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