It’s always such a shock! Your seedlings germinate, start to grow and everything seems to be going so well, then one morning (it always seems to happen overnight), you awake to discover seedlings that almost seem to have been mown over. They’re lying on their side on the growing mix, with their stem blackened, apparently pinched at the base. But it wasn’t a person with nimble fingers who pinched back your plants, it was a disease called damping off.

This disease is not actually a specific disease, but rather can be caused by any number of soil-borne fungi, including Pythium, Rhizoctonia, Fusarium, Botrytis and Phytophthora, all of which present the same symptoms. Damping off spreads rapidly: entire trays can be killed back in just a few hours. Moreover, once the disease is apparent, it’s too late to save the fallen seedlings.

Less Common That It Used to Be

Damping-off is not as common as it once was. Ever since gardeners adopted so-called artificial soil mixes back in the 1970s (mostly composed of peat, coir, fine bark, vermiculite, perlite, etc., but no longer containing any “real soil” imported from outdoors), damping-off has become much less common. Artificial soils are sterile, or at least, close to being sterile and so are unlikely to harbor disease spores.

Damping off fungi are very common in outdoor soils. In fact, they’re found pretty much everywhere. If damping off doesn’t seem to occur all that often outdoors, it’s only because it tends to strike earlier than indoors, as a pre-emergence disease. Your seeds just don’t germinate at all and you never know that damping off was the cause.

Until artificial soils came on the market, most people simply dug up soil from their gardens to use in starting seedlings. Not only were such soils dense and heavy, allowing little air circulation and thus creating an excellent environment for fungal development, they also came pre-inoculated with a whole range of disease spores. Sowing seed back then was a race against time: the faster the seeds germinated and started to grow strongly, the less susceptible they were to damping off.

That said, fungal diseases are still readily spread indoors, though not as often directly though soil. Contaminated pots and tools, even watering cans, can spread them as can water splashing from one tray to another. Some diseases have wind-borne spores that can be carried by the slightest breeze, even indoors. So even the use of artificial soils doesn’t prevent damping off entirely… but it sure helps!

Once Symptoms Are Visible…

Once you see seedlings mowed down by damping off, it’s too late to save them. The best you can do is to quickly treat the remaining seedlings with a fungicide such as chamomile or clove tea in the hopes of slowing the disease’s progress. In the past, almost every home gardener in North America had a bottle of No-Damp, a fungicide developed expressly to controlling damping off, at hand, but it is no longer available.

Prevention

To help prevent damping-off, use only clean pots and tools. Wash any recycled containers well with water and bleach. Also wash your hands well if you’ve just come in from working in your outdoor garden. Consider opening a fresh bag of potting soil for your seedlings: a bag left open for a few months may have been contaminated by air-borne spores. And never use garden soil on seedlings.

So much for preventing contamination, but seedlings also readily outgrow damping-off, which usually only affects very young seedlings. Applying the best possible growing conditions will help the seedlings get off to a quick start and grow to beyond the stage where they are subject to it. Thus anything that stimulates rapid germination and fast growth also helps prevent the disease. That’s why seedlings should be placed under warm, humid conditions after sowing, in a growing mix that is moist, yet well drained.

Also sow the seeds at the recommended depth (this will vary from one species to another), never too deep, otherwise the seedling will take longer to reach the light… and that gives the fungus more time to get to it.

Freshly sown seeds benefit greatly from being started in a mini-greenhouse, that is, with their tray or pot covered more or less hermetically with some sort of transparent material in order to maintain high humidity. This could be a clear plastic dome, a pane of glass, a plastic bag, a plastic bottle cut in half and placed upside down over the pot, etc.: anything that will let some light in while keeping the humidity high. This, combined with moderate light and warm temperatures around 75°F (24°C), gives ideal conditions for most seedlings to sprout and grow.

As soon as the majority of the seeds have germinated, though, give your seedlings a change in regime. High humidity and high temperatures also encourage fungus growth. So once the seedlings are up and growing, remove their mini-greenhouse and expose them to cooler temperatures, good air circulation, more moderate humidity, say about 50 to 60%, and also stronger light.

Fresh soil, good drainage, cleanliness, decent air circulation, humid air, intense light and moist but not saturated soil, they all add up to good growing conditions that will help keep damping off at bay.

When Damping Off Returns

In spite of the recommendations above, some gardeners have repeated problems with damping off: it just seems to come back year after year. If this is your case, here are some extra tips on preventing the disease:

• When sowing seedlings, instead of covering them with potting soil, use milled sphagnum moss (not peat moss). Sphagnum moss is a natural fungicide. Milled sphagnum moss (one brand is appropriately named No Damp Off) is not necessarily easy to find locally. I prepare my own by running dried sphagnum moss through my wife’s coffee grinder.
• You could also try sprinkling the potting mix with cinnamon powder, another natural fungicide.
• Water or spray your seedlings with cooled chamomile or clove tea. Both have proven antifungal effects.
  

  Treated seed is colored so it can’t be mistaken for untreated seed.

• Sow “treated seed”, that is, seed coated in fungicide. This technique is not, however, considered acceptable in organic gardening circles.

I repeat that the treatments described will not save seedlings that have already been mowed down by damping off. At best, they simply slow down the disease and stop it from spreading.

With damping-off as in so many things, an ounce of prevention really is worth a pound of cure!