by Steven Jakobi, Master Gardener Volunteer, Cornell Cooperative Extension of Allegany County
After a winter of planning, preparation and expense, few things are more disappointing for the gardener than the failure of seeds to send shoots up from the soil, or watching seedlings emerge, but then suddenly wilt and die.
Unfortunately, these situations are fairly common in the field or in the greenhouse under certain conditions.
The death of young plants is caused by a group of soil-borne fungi, and the disease is referred to as “damping off.”
Seeds may be attacked in the soil by the fungi right after they germinate. The fungi, attracted by fluids excreted by the plant, enter the tender young cells and dissolve their cell walls.
When the failed seeds are dug up and examined, tissues inside the seed coat are rotted and lack recognizable roots or seed leaves.
Young stems may be attacked just after emerging from the soil because they haven’t yet had a chance to harden off. (Hardening off is the process of moving plants outdoors for part of the day to gradually acclimate them to the direct sunlight, dry air and cold nights.)
Immature stem tissues have a sunken, water-soaked appearance with discolored brown to gray lesions at the soil line. The thread-like, constricted stems are unable to support the tops of the seedlings and they fall over. Seedlings may die within 18-24 hours after colonization by the pathogens.
It’s not a problem with the seeds.
The problem is caused by the spores of fungi that are already present in the potting soil. The spores can also be introduced into the potting medium on the surfaces of tools or pots or trays. The fungi are members of several genera of fungi, including Pythium, Fusarium and Rhizoctonia.
The spores may persist in the soil in a dormant stage for years if plant material is not available as a food source. Spores may be numerous and are typically black and shiny, but they are rarely discernible without the aid of a microscope.
The fungi are indiscriminate in the species or varieties of plants attacked, so you can’t control the problem by trying to find resistant cultivars. Crop rotation doesn’t work either.
Fungicides aren’t effective or desirable, although some companies sell seeds that are coated with fungicides.
So what can be done to ensure that seedlings get a healthy start and are able to ward off these fungi? Take these steps.
- Sterilizing pots, trays and equipment with a 10 percent household bleach (9 parts water, 1 part bleach) for 30 minutes is effective.
- Use soil-less, sterile potting media; don’t reuse potting soil saved from previous years. This reduces the likelihood of contamination.
- Because the fungi like cold and wet soil conditions, avoid those conditions. Make sure the soil temperature is optimum for the given crop. You can place heating mats (with a temperature of about 75 degrees Fahrenheit) under the trays. Avoid water-logged situations by using potting media with good drainage. This can prevent damping-off.
- Adequate duration of light, 12-16 hours per day, ensures rapid and satisfactory seedling growth.
- With fertilizer, maintain the proper ratio of NPK (nitrogen:phosphorus:potassium). The fertilizer should be 1:2:1 (moderate nitrogen levels) to hasten maturation of the stems. While high nitrogen encourages rapid growth, it also retards hardening off of the stem.
- Periodic gentle agitation of the pots or trays of pots is useful in strengthening stems. This can be done by gently running your hand across the top of the plants a few times a day. This works because it triggers the deposition of additional amounts of calcium in the tissues which, like the levels of calcium in bones of animals, responds to movement. (This also may be the reason why playing certain types of music, which cause vibrations of the air, have been shown to be beneficial for plant growth.)
A combination of the methods listed above will most likely result in a strong and healthy crop from seeds we so carefully saved or so happily ordered from our favorite catalogs.