What Is Damping Off Of Seedlings? (Plus 14 Prevention Tips)


Whether you start seeds indoors or directly in soil, there is always the threat of damping off – which can kill your seeds and seedlings. This can set back your growing season before it even begins.

So, what is damping off of seedlings? Damping off is a plant disease caused by pathogens (like mold or fungi). It can seeds or seedlings by causing mushy or wilted seedlings and brown roots. Damping off is more likely in wet, cool soil. It affects weak, slow growing seedlings first. Damping off spreads by soil, water, wind, or insects.

Of course, there are some ways to prevent damping off so that you can avoid this problem in the first place. In this article, we’ll start by taking a closer look at damping off of seedlings, along with the symptoms and how to prevent it.

Let’s get started.


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What Is Damping Off Of Seedlings?

Damping off is a horticultural disease caused by soil-borne pathogens. These pathogens kill and weaken seeds or seedlings (so damping off can occur before or after germination).

The pathogens (fungi or mold) that cause damping off live in the soil itself. However, these pathogens may spread by other means, such as wind, water, or insects.

According to the North Carolina State University Extension, some common damping off pathogens include:

  • Alternaria
  • Botrytis
  • Fusarium
  • Phytophthora (causes late blight in potatoes and tomatoes)
  • Pythium
  • Rhizoctonia
  • Sclerotinia
  • Sclerotium
fusarium wilt on tobacco plant
Here we can see symptoms
of fusarium wilt on a tobacco
plant. Fusarium is one of many
pathogens that can infect seedlings
to cause damping off.
Picture courtesy of:
Wikimedia Commons: https://commons.
wikimedia.org/wiki/File:Fusarium_wilt_
symptom_tobacco.jpg

These pathogens thrive in cool and wet conditions, which you often see in spring when you first plant out your garden. You may also see damping off indoors, since you often start seeds in moist soil in a warm area.

The pathogens that cause damping off can spread easily between adjacent seedlings to infect an entire seed tray – whether there are separate cells or not.

empty seed tray
Damping off can spread between seedlings in any seed tray, whether there are separate cells or not.

According to the University of Vermont, slow-growing and weak plants are more susceptible to damping off. So, using healthy seeds you bought (or saving seed from healthy plants only) is essential.

In certain situations, damping off can even cause root rot or crown rot in more mature plants.

root rot
Root rot causes roots to turn brown and mushy, even in mature plants.

For more information, check out this article on damping off from the University of Minnesota Extension.

What Are The Symptoms Of Damping Off?

There are several symptoms to look for to identify damping off in seeds or seedlings. For example, you may observe:

  • Seedlings do not emerge from the soil – this means pathogens have killed the seed before it could even germinate, or very soon after the seed sprouted open. This can happen if seeds were infected with pathogens from a prior year (for example, if you saved seeds from an infected plant).
  • The cotyledons (first leaves that appear on a seedling) are soft, mushy, and have a gray or brown color – this means that pathogens have invaded the young plant, which does not yet have the defenses to stop this attack (perhaps it is weak due to lack of nutrients or water, or has weak roots).
  • The leaves on a seedling wilt and turn greenish-gray or brown
  • Seedling stems become thin and water-soaked
  • Roots are brown or nonexistent – this is much more likely if the soil or growing medium is kept too wet for too long (root rot). It can also happen if you use infected, untreated soil in seed trays. Root rot can affect older plants too, so be careful about over watering! (For more information, check out my article on over watering your plants).
  • Fluffy white “cobwebs” appear on plants in high humidity – this is a sign that humidity levels in the air are too high. This can happen in a humidity dome, especially if you forgot to clean it before using it (or did not clean it well enough).
pumpkins seeds
If your seedlings never emerge from the soil, then they could be infected with damping off. Of course, the seeds could also be too old to germinate.

In short: when damping off appears, your seeds may never turn into seedlings. Even if they do, they will look weak, sick, and discolored.

They will eventually die, since there is no way to cure damping off. Even worse, the pathogens that cause damping off can easily spread through potting mix, shared water, or even through the air to infect nearby seedlings.

tomato seedling
These seedlings look healthy, but damping off will cause soft, mushy or wilted leaves, along with thin stems and brown rotten roots. White “cobwebs” may also appear.

Why Are My Seedlings Damping Off?

Your seedlings are damping off due to the presence of at least one pathogen that causes this disease. These pathogens could come from the soil that you are using for starting your seeds.

If any damping off diseases were in your garden last year, they can cause damping off this year. This is especially true if you use soil from the garden in seed trays or pots (more about how to sterilize soil later).

sandy soil
Pathogens that cause damping off can live in the soil or on infected plant matter.

Damping off pathogens could also come from the seeds themselves. They might even fly in on the wind (for example, fusarium spores use this method to spread)l.

Another vector for damping off is insects. For example, fungus gnats can spread damping off diseases to your plants.

The pathogens that cause damping off can survive for a long time in soil or plant debris. In a greenhouse or outdoors, any irrigation or rain that causes splashing can move these pathogens from the soil to your seedling trays or pots.

(You can learn how to prevent mold in your greenhouse here).

greenhouse
A greenhouse stays at high humidity, so it could harbor pathogens that cause damping off.

Remember that slower plant growth can make damping off worse. This is because damping off tends to affect younger plants (seedlings) that do not yet have the defenses of older plants.

Some causes of slow plant growth include:

  • low light
  • over watering
  • over fertilizing (burning caused by too much salt)
  • cool soil

There are ideal temperatures for seed germination, which vary by what you are trying to grow. Generally speaking, if you are germinating many seeds in the same area, a temperature of around 75 to 85 degrees Fahrenheit (24 to 29 degrees Celsius) is within the optimal range for most plants.

For more information, check out this article on seed germination times and temperatures from the University of California.

Can Seedlings Recover From Damping Off?

Unfortunately, seedlings cannot recover from damping off. Young plants do not have the defenses necessary to fight off the pathogens that cause damping off.

There is no cure for damping off once seedlings are infected. The best thing you can do to fight damping off is to work hard at prevention.

tomato seedling
There is no cure for damping off, so work hard at prevention to protect your seedlings.

Give your seeds the optimal growing conditions so they can quickly mature and develop defenses against pathogens.

Once a seedling is infected, work quickly to remove the infected seedling and soil. That way, you can (hopefully) prevent the disease from spreading any further.


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How Does Damping Off Spread?

Damping off generally spreads by one of these methods:

  • Soil – the pathogens that cause damping off are soil-borne, so an infection can spread easily through an entire tray of seeds (or across trays) if they are all in the same area. 
  • Water – even if your seeds are in separate cells within a tray, damping off pathogens can move from the soil of an infected plant to the water used to irrigate the seedlings (for example, if you are watering trays from below).
  • Air – Many pathogens that cause damping off can spread by spores in the air, carried by the wind. This is more often a problem outdoors or in greenhouses, but it can happen indoors if you use a fan to circulate air for your plants.
  • Insects – some insects can also carry pathogens that cause damping off (for example, fungus gnats).
  • Humans – we can be our own worst enemy when it comes to damping off. Failure to clean seed trays, containers, tools (trowels, etc.), humidity domes, and other gardening equipment can come back to haunt us later.
water barrel
The water you use for irrigation can spread the pathogens that cause damping off.

As you can see, there are lots of ways that damping off can spread. Luckily, there are also lots of ways to prevent it.

So what are the steps you can take to prevent damping off? Let’s get into that now.

How Do You Prevent Damping Off Diseases?

There are many methods you can use to prevent damping off diseases from starting and spreading among your seedlings. Some ways to prevent damping off include:

  • Clean all of your containers, pots, trays, and gardening tools thoroughly between uses. This also includes humidity domes, trowels, and anything else you might use to germinate seeds or transplant seedlings into the garden. You can use a mixture of vinegar and water for an initial cleaning, and then use alcohol for a separate cleaning to be sure that everything is sterile.
  • Give your seedlings plenty of space. If possible, use a separate small pot for each seedling. At the very least, use a seed tray with separate compartments (cells) instead of one big open container. Even if one or two plants are infected, this will prevent the spread of damping off diseases through the soil to other plants.
  • Thin your seedlings when necessary. Having too many seedlings too close together encourages the spread of disease. It also causes competition between seedlings, which slows growth and prevents them from developing defenses against pathogens that cause damping off. For more information, check out my article on thinning seedlings.
  • Use fresh, sterile potting mix with good drainage (you can buy it online or at a garden center). Do not reuse old potting mix (compost it instead!) or garden soil, since both of these can harbor the pathogens that cause damping off. The exception is if you sterilize the garden soil (see below).
  • Sterilize your garden soil by fire. This tip came to me from one of Greenupside’s readers, “G”, who generously shared some advice by email. When G was younger, the family garden was having a problem with damping off. So, G’s father started a fire, then put pieces of metal roofing on a grill over the fire. They piled soil on top of the metal, a little at a time, to heat it up until it was steaming hot. There was no damping off the next year. Thank you for the idea, G! (You can also heat up in small batches on the stove or in the oven – just watch it carefully if indoors).
  • Sterilize your garden soil by solarization. This means putting a plastic tarp over the soil and letting the sun heat it up until any pathogens are killed. This method can take several weeks, and it probably won’t work until the days are longer and temperatures are warmer. You can learn more about sterilizing soil here.
  • Use warm water to irrigate seedlings. This will prevent the soil from getting cold, which delays seed germination, slows seedling growth, and encourages the growth of the pathogens that cause damping off.
  • Water seedlings from below. This will prevent stems and leaves from getting wet, which will go a long way in preventing mold or fungus on the seedlings. To water from below, put water in a large tray, and then lower the seedling containers (or tray) into the water deep enough to wet the soil.
  • Allow the soil to dry between waterings. Constantly wet soil is a perfect place for damping off pathogens to grow and spread.
  • Ensure adequate temperatures for seedlings. You can do this with a heating mat, which provides warmth from below (you put containers or seed trays on top of the heating mat). You can get more ideas for keeping soil and seedlings warm here.
  • Give your seedlings enough light. When starting seeds indoors, you will need some overhead lights (LEDs or others) to provide enough light. Keep in mind that some plants (long day) need over 12 hours of light per day. For more information, check out my article on how day length affects plant growth.
  • Provide good air circulation for your seedlings, especially in a humid greenhouse or indoors. You can do this with a fan indoors, or you can open a window or door on a greenhouse. If you are using a humidity dome, you can open the vents to allow some fresh air to get inside. Note: this can cause damping off to spread if it is already present.
  • Do not use fertilizer on your seedlings until the first true leaves form. This will prevent the salt in fertilizer from burning your young plants. For more information, check out my article on over fertilizing your plants.
  • When starting seeds outdoors or transplanting seedlings to the garden, avoid planting in low wet spots in your yard. For more information, check out my article on how to make soil drain better.
seed tray
Using trays with separate cells for each seedling goes a long way towards preventing the spread of damping off pathogens through the soil.

Of course, you can always try soilless systems, such as hydroponics or aquaponics. This will completely eliminate the possibility of damping off pathogens spreading through soil.

For more information, check out my article on growing plants without soil using hydroponics, aquaponics, or aeroponics.

aeroponics towers
Why not try hydroponics, aquaponics, or aeroponics to eliminate soil and the chance of damping off spreading through the soil?

Conclusion

Now you have a much better idea of what damping off of seedlings means, and what symptoms to look for. You also know how to prevent damping off by using good practices in your seed germination and transplanting.

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


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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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