What Is Damping Off Of Seedlings? (Plus How To Prevent It)


Whether you start seeds indoors or directly in soil, there is always the threat of damping off killing your seeds and seedlings.  This can put a damper on your growing season before it even begins.

So, what is damping off of seedlings?  Damping off is a plant disease caused by pathogens (such as mold or fungi) that kill seeds and seedlings.  Damping off is more likely in wet, cool soil and affects weak or slow growing seedlings.  Damping off pathogens can spread 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.  Before we get to that, let’s start by taking a closer look at what damping off of seedlings is, along with the symptoms.

What Is Damping Off Of Seedlings?

According to Wikipedia, damping off is a horticultural disease cause by soil-borne pathogens that kill or weaken seeds or seedlings before or after germination.

This means that 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 or insects.

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

  • Alternaria
  • Botrytis
  • Fusarium
  • Phytophthora
  • 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 from Wikimedia Commons: https://commons.wikimedia.org/wiki/File:Fusarium_wilt_symptom_tobacco.jpg

These pathogens thrive in cool, wet conditions, which we often see in the spring when first planting out our gardens.  The pathogens that cause damping off can spread easily between adjacent seedlings to infect an entire seed tray.

According to the University of Vermont, slow-growing and weak plants are more susceptible to damping off.

In certain situations, damping off can even cause root rot or crown rot in more 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 when identifying damping off in seeds or seedlings.  For example, you may observe:

  • Seedlings do not emerge from the soil – this means that pathogens have killed the seed before it could even germinate, or very soon after the seed sprouted open.  This could happen if seeds were infected with pathogens from a prior year.
  • 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.
  • The leaves on a seedling wilt and turn greenish-gray or brown – this means that the young plant does not yet have the defenses to stop this attack by pathogens.
  • 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).  This 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, your seeds may never turn into seedlings, and 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.

Why Are My Seedlings Damping Off?

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

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 come in on the wind (for example, fusarium spores), or on insects such as fungus gnats.

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.

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 (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 85 degrees Fahrenheit (29.4 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?

No, 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. 

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

The best thing you can do to fight damping off is to work hard at prevention.  Give your seeds the optimal growing conditions so that 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 prevent the disease from spreading any further.

How Does Damping Off Spread?

Damping off generally spreads by one of these methods:

  • Soil – the pathogens that cause damping off are soil-borne, and so an infection can spread easily through an entire tray of seeds 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 (if you are watering 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.  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?

Luckily, there are lots of ways that you can 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 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.
  • 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).
  • 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.
  • 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.

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

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

Conclusion

By 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.  If you have any questions about damping off of seedlings, please leave a comment below.

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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