What Is Potato Blight? (Late Blight Symptoms, Causes, & Prevention)


When you grow potatoes, it is important to do everything you can to avoid the most common diseases. One of these diseases is potato blight (also known as late blight).

So, what is potato blight?  Potato blight (late blight) is a devastating disease that can destroy an entire potato crop. It is caused by an oomycete called Phytophthora infestans, which causes brown or black spots on leaves, stems, and tubers. Late blight can also affect tomatoes and other plants in the nightshade family.

Of course, you can prevent potato blight by digging up and destroying tubers and plants that may be infected. Watering plants from below (instead of wetting the leaves) also helps.

In this article, we’ll talk about potato blight: what it is, what it looks like, and what causes it. We’ll also talk about how to prevent the disease and what to do if it affects your plants.

Let’s get started.


The Complete Guide To Growing Potatoes Cover

The Complete Guide To Growing Potatoes


A complete reference and an ultimate guide that teaches you everything you need to know about potato selection, planting, care, harvest, and storage.


What Is Potato Blight?

Potato blight is a disease that can damage or destroy an entire potato crop. It caused the Irish Potato Famine in the 1840s, which caused mass starvation and emigration. There was an outbreak as recently as 2009 on the U.S. east coast.

potato late blight
Potato blight (or late blight) is caused by the oomycete Phtyophthora infestans. It can affect leaves, stems, and tubers.

Potato blight can spread to other plants via spores in water or by air. It can also spread to tomatoes or other plants in the nightshade family.

Potato blight is caused by an oomycete called Phytophthora infestans (which is like a fungus). Among other symptoms, it causes brown or black spots on leaves, stems, and tubers.

What Are The Signs Of Potato Blight?

Usually, the early signs of potato blight will appear on the leaves. First, you will see small, light or dark-green water-soaked spots on leaves.

late blight potato leaf
Usually, signs of potato blight appear on leaves first: light or dark-green water soaked spots that later turn brown, gray, or black.

Usually, these spots appear on the lower (younger) leaves first, at the edges or tips. Later (especially in cool, moist weather) the lesions will expand and become gray, brown, or black water-soaked lesions on the leaves and possibly on the stems.

Sometimes you will also see white spores on the bottom of leaves (it looks like mildew). These spores can travel via water and air to infect other plants.

Later on, you might see purple to brown, sunken, and dry lesions on tuber surfaces. The tissue inside tubers also turns brown and granular.

late blight potato tuber 2
The insides of tubers may turn brown and granular after infection with potato blight (late blight).

Often, potato blight infection opens the door for bacterial soft rot or pink rot (which are other diseases that affect potatoes).

How Do You Treat Potato Blight?

Currently, there is no cure for potato blight once it appears on your plants. Your best bet it to remove and destroy infected plants, along with any tubers or plant material in the soil that may be infected.

potato plants
You cannot treat potato blight, so you should remove any infected plants and destroy them to prevent further spread of the disease.

The University of Wisconsin suggests pulling up the entire infected plant, including roots and tubers. Put it in a sealed plastic bag without holes and leave it in the sun for a few days until the plant and disease are killed.

Then, put the bags out in the trash, rather than trying to compost them.

Does Potato Blight Stay In The Soil?

The pathogen that causes potato blight can overwinter in soil by surviving in living plant tissue (such as an infected potato tuber). However, it can also survive in soil or plant debris via oospores (thick-walled overwintering spores).

late blight potato tuber
Potato blight can stay in the soil over the winter, sometimes by surviving inside of infected tubers.

Should I Dig Up Potatoes With Blight?

You should dig up potatoes with blight and dispose of them (do not compost them!). Potato blight can survive the winter inside of infected tubers, either in the soil or in storage.

Baby Potato Plant
Volunteer potato plants that come from infected tubers can spread late blight to other plants later in the season.

Potatoes that are not dug up in the summer/fall can sprout and grow as “volunteer” potato plants in the following spring. If the tubers were infected, then those plants can also be infected, and the disease can spread to other plants as usual (by water and wind).

Can You Eat Potatoes That Have Blight?

Potatoes that have late blight will often be susceptible to other diseases. To avoid eating rotten or unsafe potatoes, just throw them away.

potatoes soil
Do not try to store or preserve potatoes with late blight. Just throw them away, without trying to compost hem.

Don’t try to can or preserve potatoes that have blight. Even if you cut away the obvious bad parts, there can still be pathogens present.

Can You Reuse Soil After Potato Blight?

Do not reuse soil after potato blight if you are growing any crops in the nightshade family (potato, tomato, etc.), since they are susceptible to late blight.

alfalfa
Use crop rotation with a plant (like alfalfa) that does not get late blight and will restore nitrogen to the soil.

However, you can use this soil to grow other plants on a crop rotation schedule with potatoes (such as clover, alfalfa, etc.)


The Complete Guide To Growing Potatoes Cover

The Complete Guide To Growing Potatoes


A complete reference and an ultimate guide that teaches you everything you need to know about potato selection, planting, care, harvest, and storage.


Can Blight Spread To Other Plants?

Potato blight can spread to tomatoes or other nightshade plants, and vice versa. You can find blight-resistant tomato plants here.

tomato early blight
Tomato plants can also get late blight, just like potatoes. The disease is just as devastating for tomato crops as it is for potatoes.

However, plants in other families will be safe from late blight, including:

  • Brassicas – this grouping includes crops such as broccoli, cauliflower, cabbage, and Brussels sprouts.
  • Cucurbits – this grouping includes crops such as cucumbers, melons, and squash.
  • Legumes – this grouping includes crops such as peas, beans, peanuts, clover, and alfalfa. Clover and alfalfa would be a good choice if you have livestock you need to feed.
  • Root Crops – this grouping includes crops that grow in the ground, such as carrots, turnips, radishes, and beets.
broccoli
Broccoli (in the brassica family) is one crop you can rotate with potatoes to reduce the chance of late blight and other diseases.

How Do You Prevent Late Blight In Potatoes?

There are several steps you can take to help prevent late blight in potatoes in your garden.

First, use certified seed potatoes. These seed potatoes are bred specifically for planting and growing new potato plants.

sprouted potato
Use certified seed potatoes, which are disease free and without sprout inhibitors (such as clorproham).

They are also inspected to be free from diseases, such as late blight. In addition, certified seed potatoes are never treated with sprout inhibitors (like clorproham), which are chemicals that prevent sprouting and stunt growth.

When you order certified seed potatoes for growing, choose from blight-resistant potato varieties. Remember that potatoes can also get early blight, but late blight is the more devastating disease (for both potatoes and tomatoes).

Here are some potato varieties with late blight resistance:

  • Baltic Rose
  • Chieftain
  • Desiree
  • Elba
  • Kennebec
  • Nicola
  • Soraya
  • Upstate Abundance
  • Yukon Gem
Kennebec potatoes
Kennebec is one potato variety with some late blight resistance.

Once you have your certified seed potatoes from resistant varieties, choose a good spot for planting. Do not plant seed potatoes in areas that tend to stay wet, since water is one of the main vectors for spreading potato blight.

If you cut your seed potatoes into smaller pieces, give them time to heal up (scab over) before planting them. A couple of days should do it, and this will help to prevent disease by giving the cut tuber pieces a chance to recover.

cut russet potato with eyes
If you cut your seed potatoes into smaller pieces, give them a couple of days to heal up and scab over before you plant them. This reduces the chance of rot and disease.

In the spring, plant as early as possible to get ahead of late blight. Of course, you still need to pay attention to frost dates (you can check them here).

When you irrigate your plants, water them from below to keep the leaves as dry as possible. For example, use drip irrigation, with lines set up around the base of your potato rows.

drip irrigation emitter
Use drip irrigation for potato plants instead of overhead watering. This will help to prevent late blight, which thrives in cool, wet conditions.

This is a better alternative to spraying leaves with a sprinkler or hose, since wet leaves and stems contribute to the growth of late blight, especially during cool weather.

If you must water from above, do it during the day, not at night. Otherwise, the leaves and stems will stay wet at night, during cooler temperatures, which can leave an opening for late blight.

Keep all of your gardening tools and equipment clean during the season. Clean with soap and water or alcohol to destroy diseases on the tools. This will help to prevent the spread of any diseases that may be present.

shovel
Keep your garden tools and equipment clean by washing. This will prevent the spread of diseases like late blight.

In the spring, pull out and destroy any volunteer potato plants immediately. Although the plant might look healthy, the tuber underground can harbor late blight over winter, and the plant can spread the disease to others in your garden.

baby potato plant
Pull up and destroy any “volunteer” potato plants that you did not grow yourself. Late blight can survive inside of infected tubers over the winter.

Also, take the time to walk through your garden during the season. Keep an eye out for infected plants. Pull out and destroy any infected plants immediately (do not compost them, since this still leaves the risk that late blight will come back via your compost pile).

At the end of the season, dig up all tubers from infected (or suspected) plants. Destroy the tubers, rather than trying to compost them or salvage parts of them for eating or canning.

If you find any infected potato tubers, dig them up and dispose of them, rather than preserving them or composting them.

Finally, monitor late blight outbreaks in your area with USA Blight.


The Complete Guide To Growing Potatoes Cover

The Complete Guide To Growing Potatoes


A complete reference and an ultimate guide that teaches you everything you need to know about potato selection, planting, care, harvest, and storage.


Conclusion

Now you know what potato blight is and what it looks like. You also have an idea of what to do to salvage your crop or prevent the disease from spreading.

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

You can learn about seed potatoes here.


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~Jonathon


Jon M

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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