Preventing damping off: How to save your seedlings

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With immense pride, you’ve sown seeds for either a veggie patch or a showy garden. A steady shower of water brings forth the first timid shoots. But –wait– those young sprouts don’t look so good! They’re feeble, soft… probably suffering from damping off!Let’s talk solutions and explanations.

→ Get technical: understand damping off

What is seedling blight?

Seedling blight, or damping, is a fungal disease caused by either of these fungi: Fusarium, Rhizoctonia, Pythium, Botrytis or Phytophthora. These fungi weasel their way into the plant’s core and start wreaking havoc. The poor seedling, too young to fend off such a disease, withers away in hours. Before you know it, all your seedlings are under attack, so speed is of the essence!

More worrying is that these fungi live everywhere: in soil, potting mix, and even on seeds themselves.

• Damping-off pathogens such as Pythium and Rhizoctonia typically attack at the soil line, where seedling stems are still soft and unlignified, and a stem girdled all the way around rarely recovers.
• Pythium and Phytophthora spores actually swim, so a tray left standing in a puddle of water can carry infection from one cell to every neighbour.
• Reused pots and opened compost can harbour damping-off organisms such as Pythium, Phytophthora and Rhizoctonia, so as a precaution scrub containers in hot soapy water and sow into fresh compost.
• Seedlings are most vulnerable to damping-off fungi soon after germination; once true leaves have expanded and the stem base has thickened and lignified, plants become far less susceptible, though not entirely immune.

How to recognize it?

• Plant goes limp, lays down on the ground.
• Stem base turns brown or red and thins out.
• Seedling turns necrotic.
• Sometimes, seedlings rot before they even emerge from the soil.

• Unlike a seedling short of water, one struck by damping off collapses while the compost is still visibly moist.
• Damping-off in seed trays, caused by soil-borne pathogens such as Pythium or Rhizoctonia, can appear as a patch of collapsed seedlings that sometimes expands outwards over following days, though patterns vary.
• A fine white or greyish cottony mould may appear on the compost, but it is often a harmless saprophyte, so rely on collapsed seedlings with thin, darkened stem bases to confirm damping-off.

Which plants are affected?

Every plant can suffer from this disease, be it herbs, flowers, or veggie seeds.

• Tomato, pepper, aubergine and basil seedlings are susceptible to damping-off, so grow them in sterile compost kept warm and evenly moist but never waterlogged, with good air circulation around the trays.
• Cucumbers, courgettes and beans often rot in the compost before a shoot ever shows, which is why they should only be sown once the compost has warmed.

Risk factors

Fungi, like those causing damping-off in seedlings, love moist environments.

If sowing in a greenhouse, remember to air it out often, especially during a rainy spring.

Avoid dense, deep sowings. Sow in loose soil that you’ve worked through beforehand, using a broadfork if it’s in open ground or with well-draining seed compost mixed with sand. Also steer clear of excessively rich substrates.

• Once seedlings emerge, begin ventilating and then remove propagator lids or cling film gradually over the next few days, since prolonged trapped humidity encourages damping-off and weak, drawn growth.
• Water stored in a butt can harbour Pythium and Phytophthora, whose motile zoospores may cause damping-off in seedlings, which is why standard advice is to use mains water on seed trays.
• Seed trays and pots reused from one sowing to the next carry fungal spores in the old compost stuck to their corners, so scrub and rinse them before filling.

Three natural preventives compared

Since nothing will cure a seedling once the fungi are inside it, the only useful comparison is between the preventive preparations described below. Here is how each one is made, measured and applied.

All three work only before symptoms appear, so they belong in the routine from sowing onwards.

Damping off natural prevention and treatments

Today, there are no natural curative treatments, only prevention methods. Disease spreads so fast that, if you spot it, best just toss affected seedlings away.

How to prevent its appearance?

• Use a special seedling compost that’s perfectly balanced, well-drained, and not too rich
• Don’t sow too tightly
• Disinfect your tools before use, this holds for all gardening tasks
• Refer to label for sowing depth
• Air out rooms housing your seedlings regularly
• Water regularly but with a fine mist, substrate should never be waterlogged, only moist
• Keep your seedlings at a temperature between 60°F and 70°F (15-20°C) ideally.

Natural prevention:

Fermented horsetail tea

Fermented horsetail tea is an effective antifungal used in spraying. It’s a concentrate that needs dilution: 1 quart of manure for 9 quarts of water (1L manure for 9L water). Simply spray it on seedlings when watering. You can use a hand sprayer to mist seedlings with your tea. Don’t use it for each watering, only once a week, during germination. The rest of year, a monthly spray is enough for all your plants.

Charcoal

When prepping your substrate, add powdered charcoal to your special seedling compost. It curbs humidity and thus appearance of certain fungi. Count about 3 lbs of charcoal powder per square yard of soil (1.5 kg per square meter). Get horticultural charcoal, not supermarket-sold BBQ charcoal. You can also use your own charcoal if you have a fireplace.

Garlic infusion

Antibacterial and antifungal, garlic is another natural ally against damping-off. Use it like fermented horsetail tea, in spraying. To make this infusion, simply put 1 lb of chopped garlic in 1 gallon of water (500g in 5 liters). Bring to a boil, then let simmer for 30 minutes. Then, turn off the heat, cover the container and let it sit for half a day. All that’s left is to filter and pour this solution into jugs and bottles.

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