Damping off – what to do when seedlings die off

Coffee seedlings dying from damping off.

When seedlings die off at a very early stage in life, chances are it’s due to a fungus in the soil.

Damping off” is the term that describes this infection.

Seedlings collapse, lose their leaves, turn yellow and wither leaving a blackish-brown stem behind. The key is to avoid waterlogged soil.

Symptoms of damping

Damping off usually occurs before the first true leaves appear.

  • The stem starts sagging and softening, leading the seedling to collapse and fall.
  • At the same time, the green leaves and stems turn pale and yellow, losing their color.
  • Towards the foot of the stem, a black streak or spot starts appearing. Sometimes the black starts at the leaves.
  • The plant topples over.
  • The foot of the stem is black, constricted and begins to rot. It’s a necrosis, meaning tissue is already breaking down.

There is no hope to save seedlings from damping off once the blackish color appears.

However, there is a slight chance of saving a few seedlings when you catch the damping off at an early phase. If you’ve only reached the point where stems and leaves sag and turn pale, the seedling can still recover.

What is damping?

Damping is the result of fungus attacking and infecting the young seedlings. It’s a fungal disease.

Fungus rot young seedlings alive

Seedling pulled out and showing damping off symptoms.The fungus, almost always dormant in the soil, penetrates the plants through the roots and then works its way up through the young stem.

  • In normal conditions, plant seedlings are able to fend off such attacks.
  • When the soil is too wet, cell barriers weaken and the fungus can pierce through the plant’s “skin”.
  • The fungus then releases enzymes into the plant.
  • These special compounds break plant material down into basic nutrients, which the fungus feeds on.
  • It then spreads through the rest of the seedling.

The infection spreads quickly through the plant, and in a single day a seedling can go from sagging to dead.

Fungus that cause damping

There are four main species of fungus that cause damping off.

  • These are Pythium, Fusarium, Phytophtora and Rhizoctonia.

Usually the one or the other is responsible, because fungus typically operate on a “winner takes all” basis. The first to colonize a patch of soil or plant will fend off competing fungus.

You’ll come across these fungus whenever a plant is weakened. Indeed, they’re almost always present in the soil and on leaf and trunk surfaces.

Fungus always flare up when growing conditions stress the plant. The best way to avoid fungus is to make sure the environment suits the plant’s needs.

Even in grown plants, these fungus might strike. Take a look at this infected Sunpatiens, for instance.

Treatment and cure for damping

Saving seedlings that started damping off is very difficult and often unsuccessful.

  • Stop watering immediately.
  • If your plants are wallowing in water, drain the soil out. For seedlings started indoors, drill extra holes in the plastic of your trays and slant the tray to coax all the water out.
  • Prepare a fresh seedling tray for growing. Especially important is drainage, so focus on that.

Sprinkle ground wood charcoal in a thin layer over the growing bed. A very fine coating is enough, like dust on furniture.

  • Segregate entire portions of your indoor seed tray that still seem healthy, and move them to the fresh tray.
  • Tweak out whichever remaining seedlings are strongest and transplant them carefully to the fresh growing tray.

After that, simply take great care not to overwater. Provide plenty of indirect light. Ventilate to make sure the soil never gets soggy.

Preventing damping off

Prevention is the best course of action, since recovering sick seedlings is rarely successful.

Overwatering, the prime killer

Avoid overwatering. It’s the most important factor.

  • It’s better to mist twice a day instead of drowning the plant in water every couple days.
  • Make sure there’s lots of drainage under the seedlings. Ideally, nursery pots and such shouldn’t rest directly in the water collector. Place gravel or mesh wire to hold them above water level.
  • Increase air moisture around the seedlings, not in the soil. Use moist clay pebbles as an easy solution.
  • Add in a few hydrogel crystals to the soil mix. These will absorb excess water and release it when needed.

Outdoors, if the weather is very wet for days, protect seedlings from rain with a tarpaulin or piece of plastic hung above the growing bed.

Reduce risks of damping

Make sure your seeds come from a good source. They shouldn’t have molds growing on them. It’s possible to disinfect them before planting.

Only use the most mature compost to enrich the soil immediately around your seed holes. It’s fine if further off almost pure manure or compost is used, but in the 4 to 6 inches (10 to 15 cm) closest to your seedlings you’ll only want very ripe compost.

Otherwise, you’ll be attracting fungus with all that fresh material, and it’ll attack your seedlings in stride. Not to mention that having too many nutrients around your seedlings might burn them, too.

Sometimes, when you’re recycling old potting soil, you’ll have to sterilize it completely in an oven (two-three hours at around 400°F or 200°C).

Another technique is to pre-colonize the soil with beneficial fungus. They’ll out-compete the bad rot-inducing fungus and protect your plants.

  • Stores sell such products. Follow prescribed doses.
  • Make your own easily from weeds. Indeed, plants like horsetail and stinging nettle make excellent fermented tea materials that fend off pathogens.

Wood charcoal, a fungus deterrent

Seedlings of radish, healthy, in small sowing cubes.Grated black wood charcoal is excellent against fungus. It keeps the spores from germinating.

  • Simply take a few shards of untreated wood coal and crush it to powder. Cloth pouch and a hammer or stone do the trick.
  • Use pure wood charcoal, not the compacted coal pellets.
  • Avoid any products that have additives such as “pre-doused lighter fluid” or the like.

There are two ways you can use it:

  • mix in two heaping tablespoons for every gallon of potting soil (four liters).
  • sprinkle a dust-like layer on the sowing bed just before placing seeds in.

Either technique is fine but there’s no need to use both.

Plants vulnerable to damping

Young plants are the most vulnerable

These fungus are particularly aggressive against fresh, young sprouts because they don’t yet have a very strong immune system. Almost all plant species in the world can be infected at that stage.

On top of that, the seedling’s structure isn’t yet very marked: root, stem, and cotyledon leaves are more or less a single extension. On the contrary, more mature seedlings have clear “barriers” between root, trunk and leaves. These serve as biological barriers and slow the spread of fungus down in older plants.

Most vulnerable vegetables and flowers

Smart tip about damping seedlings

Prepare several trays when sowing seeds. It’ll avoid loosing all your seeds to damping at once!


Credits for images shared to Nature & Garden (all edits by Gaspard Lorthiois):
Coffee seedlings damping by Scot Nelson ★ under Public Domain
Sick seedling in hand by Scot Nelson ★ under Public Domain
Radish seedlings by Jordan Meeter ☆ under © CC BY 2.0