What is Damping Off?

Definition: ‘Damping off’ is a condition caused by pathogens that destroy seeds before germination or very young seedlings. The term refers to the outcome – weakened or dead seedlings or seeds. The seedling stem rots and the young plant collapses or seeds fail to germinate.

Is it really that serious? Yes, unfortunately! Damping off can affect up to 80% of seedlings. So, if affected, you could lose a significant number of plants. Research has found that “even a very low population density of soil-borne pathogens can lead to severe epidemic development.” ₍₁₎

What Causes Damping Off?

So, who are the little rotters responsible for this sad end to your plant ‘toddlers’ or seed ‘babies’?

There are over a dozen culprits of soil-borne disease-producing organisms – different species of fungi and fungus-like organisms called ‘oomycetes’. They live in soil and transfer to a seed or seedling when conditions are favourable. Some pathogens are carried inside seeds or on the seed coat. However, only a few are commonly associated with damping off.

Firstly, let’s meet a few pathogens and their tongue-twisting names! They include Pythium species, oomycetes like Rhizoctonia solani, Phytophthora, Fusarium and Aphanomyces cochlioides.

More importantly, where do they hang out? Wet or overwatered soil, particularly in cool temperatures or cloudy conditions, provides favourable conditions for oomycetes called ‘water moulds.’ Why? Clearly, because they require water to multiply and spread. Phytophthora and Pythium species are both parasitic oomycetes.

However, if you have warm, dry soil conditions, Rhizoctonia and Fusarium can thrive and are usually the most likely offenders. Rhizoctonia attacks seedlings causing them to collapse. A dry brown discoloured stem is often a clue.

This fungal pathogen thrives in soils with poor fertility (nutrient-deficient) and insufficient moisture. The brassica family of vegetables including broccoli, rocket, kale and cabbage seem most susceptible to this pathogen. Making your own seed raising mix just before sowing seeds may prevent this fungus from ‘priming’ itself to infect the emerging crop.

What are the Two Types of Damping Off?

Damping off affects both seeds and seedlings. So, what evidence should you look for?

1. Pre-emergence: Seeds rot in the seed raising media before germinating or emerging above the soil level. Your seeds never appear to germinate. So, you may be left wondering what went wrong.
2. Post-emergence: ‘Newborn’ seedlings that have recently germinated wilt, collapse quickly or die from soft rot in the stem. They usually fall over at the soil level. Woody seedlings may start to weaken and wither while still erect, but baby roots may decay soon after. The infected stem looks soft, brown and water-soaked. A bit of a sad story really, isn’t it? It’s devastating for new ‘plant parents’!

What are the Symptoms of Damping Off?

Damping off in Seedlings:

Newly emerged seedling cell walls are still thin. So, they are vulnerable to pathogen attack. Often the first visual clue something is wrong is the seedling suddenly topples over. The stem cell walls collapse at soil level. Sometimes the leaves above the stem won’t have wilted yet. These leaves also tend to decay quickly if the conditions are wet or humid.

Damping off symptoms will vary depending on the pathogen causing the problem. Common signs include:

• The stems often appear brown or grey and mushy.
• Thin, water-soaked infected stems.
• Young leaves are grey or brown.
• Slimy, rotted or grey-brown sunken sections on roots.
• In humid conditions, you may notice furry white cobweb-like growth on the infected plant.

Damping off on seedling stems

Damping off in Seeds:

Germination failure is usually the first sign. You may blame poor quality seed (or yourself!) if your seeds don’t emerge. However, damping off may be the real cause.

What’s happening? The hypocotyl (embryonic stem), radicle (baby root) and plumule (future shoot) die before they reach the soil surface. It all happens out of sight in your seed raising mix! So you’re none the wiser.

How to Exclude Seed Quality as a Cause of Poor Germination

To eliminate this possibility as a factor for germination failure, you could try a seed viability test. Re-sow a small number e.g. 10 seeds in a fresh seed raising medium. It should NOT be the same medium you sowed the other seeds in.

Then count the number of seeds that germinate successfully. If only 1 out of 10 seeds sprout, the viability is only 10%. This may indicate they are old or poor quality and must be used quickly. This may be the real reason you had difficulty with germination. If you get a higher percentage success rate, there should be longer life in your seeds.

Which Plants are Most Vulnerable?

Damping off affects most plants including food crops:

(1) at germination and

(2) in the early stages of seedling development.

As seedlings develop their stem and leaves, the cell walls strengthen. So it’s more difficult for disease-producing organisms to enter the plant. It’s the conditions that make your plants more vulnerable than the actual species.

Collapsing seedlings are an indicator of damping off

What Conditions are Favourable for Damping Off?

Temperature and moisture play a key role in creating a favourable environment. Soil pathogens are most likely to thrive in:

• Overly wet soils due to rain, overwatering or poor drainage properties.
• Seed raising media with poor aeration and structure.
• Over-seeded trays where seeds are competing for air and moisture.
• Alkaline (high pH) levels in the growing media.
• Heavily shaded conditions.
• Soil temperatures between 20-28°C. Unfortunately, many seeds require this temperature range to germinate!
• Indoor or greenhouse growing conditions.

How does Damping Off Spread?

A variety of conditions and materials can cause these pathogens to multiply and disperse including:

• Infected plants.
• Soil, potting mix or seed raising mix.
• Contaminated gardening tools.
• Water – rain, splashing and irrigation systems.
• Wind – transports spores like Fusarium.
• Fungus gnat insects – weaken seedling roots and carry Pythium on their feet.

What to Do if Plants are Infected

What if some seedlings in a tray suffer from damping off, but others look healthy? Avoid the temptation to plant the healthy ones out in your garden! If the soilborne pathogen is in part of the tray, fungal threads or spores will likely be transported too. Don’t risk spreading it.

• First, if the seedlings are particularly precious, you may want to carefully pot up the individual healthy seedlings.
• Second, quarantine them away from seed raising media, potting mix and other plants.
• Third, pay attention to hygiene.
• Lastly, liquid feed with seaweed and microbial inoculants. These help support strong healthy growth to give them the best chance of surviving and thriving.

Liquid fertilising with seaweed adds trace elements and strengthens seedling development

How to Prevent Damping Off

Once damping off has started, you can’t stop it. So the adage ‘prevention is better than cure’ applies!

Considerable research to develop biocontrol solutions for damping off has been undertaken in recent years. Researchers are exploring promising future options. However, managing damping off currently requires an integrated approach combining preventive and curative tactics and strategies. So, what are your options?

Are Fungicides the Answer?

You may be thinking ‘why not just spray a fungicide’? However, according to a review of over 300 articles on managing damping off, this is clearly not the best option. Instead, the study states: “fungicide overuse threatens human health and causes ecological concerns. This practice has led to the emergence of pesticide-resistant microorganisms in the environment.”₍₁₎  

So, rather than fixing the disease, chemical fungicides can create a tidal wave of other problems.

Prevention and Control Strategies for Damping Off

There’s nothing more disheartening than watching plant babies die. Happily, there are plenty of practical actions to avoid, minimise or manage this problem.

Next, are some helpful options to control conditions that:

• Reduce the likelihood of damping off occurring.
• Limit the potential for damage.

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Tips for Seed Raising

• If your seeds don’t germinate within the average number of days, start again. Avoid sowing new seeds into the same mix under identical conditions. Otherwise, you may set yourself up for more disappointment!
• Sow new seeds in a fresh batch of seed raising mix. In particular, make sure it is sterile, well-drained, aerated and lightweight. Ideally, make your own seed raising mix so you have full control over the ingredients. It’s quick, easy and sustainable.
• Be vigilant, observant and act quickly! If you notice symptoms in some of your seedlings, you may be able to save others. Naturally, remove those affected! Next, create unfavourable conditions before it spreads.
• Sow seeds to the correct depth. Roughly twice the seed diameter. e.g. Sow a 1cm (0.4”) long bean seed 2cm (0.8”) deep to avoid rotting.
• Avoid sowing too many seeds close together. Healthy seeds need good air circulation. A build-up of humidity with overcrowded seedlings creates a favourable environment for pathogens. Likewise, correct spacing ensures each seed has access to moisture and light. This provides the best opportunity for healthy robust growth instead of competing for resources, becoming weak and vulnerable.

White fungal growth on overcrowded microgreens seeds

• Promote good air circulation. When raising seeds, ensure adequate airflow and ventilation. Many mini greenhouses have adjustable air holes. Alternatively, lift the cover during the day for a while. You can even use a fan if you have a lot of seedlings. Also, fill the pot or seed raiser to the top with your mix to optimise airflow.
• Avoid direct sowing into garden soil. Control seed raising and transplant later when seedlings are stronger. This minimises plant loss if you suspect the pathogen is soil-borne and a garden bed may be infected.

Tips for Growing Media

• Firstly, did you purchase seedlings and plant them into garden soil or potting mix? If they failed due to damping off, change your mix. Try making your own potting mix or seed raising media free of pathogens. Plant into this instead. I make my own blends with liquid nutrients and microbial inoculants. They encourage fast healthy growth.

• Secondly, avoid recycling seed raising mix from the previous season if you’ve had problems. You could add this to a controlled hot compost where you check the temperature to kill the pathogens.

Tips for Natural Disease Management

• Use hot compost. Do you make your own compost? Do you use it in seed and potting mix blends? If so, ideally ensure your system reaches 55°C (131°F). Exposure of most pathogens to high (thermophilic) temperature kills them. Hot compost is also more likely to contain a diversity of antagonistic microbes that help disease suppression.₍₂₎

“Suppressive compost provides an environment in which plant disease development is reduced, even in the presence of a pathogen and a susceptible host.”₍₂₎ Suppressive soils are “soils in which the pathogen does not establish or persist, establishes but causes little or no damage, or establishes and causes disease for a while but thereafter the disease is less important, although the pathogen may persist in the soil.”₍₃₎

• So, biodiversity or a balance of microorganisms that naturally recolonize compost during the final cooling helps suppress plant diseases like damping off. Surely, another reason to live sustainably by recycling your food waste and making your own compost. Then, use this as a resource for healthier seed raising and potting mixes!
• Introduce biological control agents into your growing medium. Importantly, this strategy increases the diversity and populations of beneficial biologicals that use control mechanisms like outcompeting or parasitizing other fungi in the soil. Cool hey? For example, “Trichoderma are well documented as effective biological control agents of plant diseases caused by both soilborne fungi and leaf- and fruit-infecting plant pathogenic fungi. Trichoderma spp. are often very fast-growing and rapidly colonize substrates, thus excluding pathogens such as Fusarium spp.”₍₄₎
• So, an easy solution is to use microbial inoculant products. For example, incorporate beneficial microorganisms into your growing media, potting mix or soil before you sow seeds or transplant. Consequently, this creates an environment where beneficial antagonistic microbes out-populate and suppress the pathogens.

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Tips for Management Practices

• Minimise watering so the seed raising mix, soil or potting mix is moist but NOT wet. Waterlogged seed raising media is an ideal breeding ground for soil pathogens. However, too dry and seeds won’t germinate. So, it’s a balancing act!
• Strengthen early growth and development. For example, include liquid seaweed and nutrients in your seed raising media or watering routine.
• Clean your garden supplies regularly. Naturally, good hygiene should be a habit! Wash your hands, pots, tools or seed raisers and any other equipment. Importantly, this avoids accidentally spreading contaminated potting or seed raising mix. Wash your hose end after touching the affected soil. Use hot soapy water and a scrubbing brush.
• Maintain good watering practices:
  • Firstly, water your seed raising trays or pots from the bottom up. This avoids potential splashing and controls moisture.
  • Secondly, use a fine nozzle spray bottle to mist small amounts of water rather than a watering can or hose that can waterlog or overwater the area. Obviously, splashing water can move fungal spores around.
  • Lastly, water in the morning. The seed raising or potting mix can warm up and absorb the moisture. Of course, this prevents damp soil at night.
• Crop rotate. Naturally, swap annual crop positions from the same plant family every season. This practice avoids a buildup of soil pathogens in garden beds and pots.

I hope these tips help you learn to recognise and prevent damping off by avoiding the conditions that cause it. Of course, act quickly if you experience this problem! Finally, follow the suggested strategies to minimise damage and save your seedlings.

Related Articles & Resources:

• Seed Starting Guide: Quick Tips for Starting Seeds Successfully
• How to Prevent and Fix Leggy Seedlings
• 5 Mistakes to Avoid When Raising Seeds
• 9 Strategies to Help Combat Common Edible Garden Problems
• Easy DIY Potting Mix Recipe
• How to Make Potting Mix at Home Guide

Want to Dig Deeper? Article References:

1. Integrated Management of Damping-off Diseases: A Review.
2. Suppressive Composts: Microbial Ecology Links Between Abiotic Environments and Healthy Plants.
3. The Nature and Practice of Biological Control of Plant Pathogens. Cook RJ, Baker KF. 1983. St. Paul, MN: APS Press.
4. Biological Control with Trichoderma  With Emphasis on T. harzianum.

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