Damping off – why do seedlings turn brown and die

When seedlings die off at an early stage in life, chances are it’s due to fungus.

“Damping off” is the term to describe this infection.

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

Symptoms of damping

Damping off usually occurs before the true leaves appear.

• The stem sags and softens, leading to collapse and falling over.
• At the same time, green leaves and stems turn pale and yellow.
• 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.

You cannot save seedlings from damping off once the blackish color appears.

However, there is a slight chance of reversing damping off if caught early. If stems and leaves sag and turn pale, the seedling might  still recover.

What is damping off?

Damping results from fungus attacking and infecting young seedlings. It’s a fungal disease.

Fungus rot young seedlings alive

The fungus, already dormant in soil, penetrates into roots and then works its way up through the young stem.

• In normal conditions, seedlings fend off such attacks.
• When 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 five main species of fungus that cause damping off.

• These are Pythium, Botrytis,  Fusarium, Phytophtora and Rhizoctonia.

Usually the one or the other is responsible, because fungus typically operate on a “winner takes all” basis. Indeed, they’re almost always present in the soil and on leaf and trunk surfaces. The first to colonize a patch of soil or plant will fend off competing fungus.

Fungus flare up when the plant is stressed. Avoid fungus by making sure the environment suits the plant’s needs.

Even in older plants, these fungus still strike, like this infected Sunpatiens, for instance.

Treatment and cure for damping

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

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

Sprinkle ground wood charcoal over the growing bed. A fine coat is enough, like dust on furniture. You’ll read below why this works.

• Move healthy seedlings to a fresh tray.

After that, don’t overwater. Provide plenty of indirect light. Ventilate to make sure 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 shouldn’t sit in the water collector. Place gravel or mesh wire to hold them above water level.
• Increase air moisture around seedlings, not soil. Use moist clay pebbles as an easy solution.
• Add in hydrogel crystals to the soil mix. These absorb and release excess water 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 old, mature compost to enrich soil directly in your seed holes. Any pure, fresh manure or compost should be at least 4 to 6 inches (10 to 15 cm) away from your seedlings.

Such fresh material attracts aggressive fungus. Not to mention that having too many nutrients around might burn your seedlings, too.

When recycling old potting soil, sterilize it completely in an oven (two-three hours at around 400°F or 200°C).

Another technique is to populate the soil with beneficial fungus. They’ll out-compete 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 nettle make excellent fermented tea that fend off pathogens.

Wood charcoal, a fungus deterrent

Grated black wood charcoal is excellent against fungus. It keeps spores from germinating.

• Crush a shard of untreated wood charcoal to powder.
• Use pure wood charcoal, not compacted coal pellets.
• Avoid products with additives such as “pre-doused lighter fluid” and the like.

There are two ways you can use it:

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

→ Relevant: 3 natural tips against damping off

Plants vulnerable to damping

Young plants

These fungus are especially aggressive against fresh sprouts that haven’t yet developed a strong immune system. Almost all species can be infected when young.

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

Most vulnerable vegetables and flowers

• Vegetables – eggplant, tomato, lettuce, onion, carrot, cucumber, and all types of cabbage
• Flowers – begonia, dahlia, petunia and nasturtium need maximum attention when sowing from seed.

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