It's actually not one, but a set of problems that cause seedlings to die back at the soil line. There are different fungi that cause this to happen. They all have complicated scientific names, which is probably why most of us just identify the problem as damping off.
In the old days, before "Teaming With Microbes," the best of the preventative measures was to use sterilized soil to start seeds. That is still a good way to minimize the problem but not having microbes (as in, "no" fungi) in the soil means you have to feed the plant with chemical fertilizers, because there are no microbes to do the job and these chemical ferts cause a host of other problems.
Today, more and more gardeners are using the best compost they can find to start seeds. We now understand that disease-causing organisms are actually kept in check whenever there is a diversity of microbes. Good compost has a huge and diverse population. When you use compost, some other microbe either eats the fungi or secretes an antibiotic or a substance that changes the pH so that the damping off fungi can't survive in large enough numbers to do their damage.
In addition to using good compost, it pays to take other precautions. Start with good air circulation around seedlings. This is key, as the fungi do not thrive in such an environment.
Next, make sure that the compost you use drains very well. Seedlings should never sit in wet soil. If you need to, add Perlite, vermiculite, diatomaceous earth or sand to increase drainage and, of course, make sure your containers allow excess moisture to drain out. It also helps to water seedlings from the bottom by letting them sit in water for a minute or two. This way the tops of the plant don't get wet.
You should also pay attention to how deep you are burying your seed. Damping off occurs at the crown of plants, the junction between root and stem. Seeds planted too deep create seedlings that are more susceptible to the disease, because more of the vulnerable stem is in the soil. Follow the packet's directions carefully.
As the loyal reader knows, I don't promote chemical fertilizers. If you want to use them, understand that the microbial populations in soil are thrown out of balance when you do. The microbes provide the natural protection from damping off and end up feeding your plants. If you use a chemical fertilizer, be cautious with newly germinated seedlings. Seeds contain all the nutrient the seedling needs until it develops its first leaves, so there is no need to feed them too early. If you do, you have the potential to burn the new roots that are forming. This has the same effect as damping off. If you use chemical fertilizers, dilute your solution by four when you finally feed seedlings so you don't burn root hairs. And don't apply anything until there are four or so leaves.
If you use sterilized starting mix, check to see if it already contains fertilizer. You might be surprised to know that many do and they may not be good for starting seeds precisely because of the fertilizer. It is too strong for seedlings.
Finally, it is always a good practice to plant only a portion of the seed in a packet. That way if seedlings do succumb to damping off, you will have some new seeds to start over. Of course, you may want to pull this article out again, and change things before you do.
Jeff Lowenfels is a member of the Garden Writers Hall of Fame. You can reach him at teamingwithmicrobes.com or by calling 274-5297 during "The Garden Party" radio show from 10 a.m. to noon Saturdays on KBYR AM-700.
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