BELMONT, N.Y. — 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 when the pathogens, attracted by plant exudates, 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. Immature stem tissues have a sunken, water-soaked appearance with discolored brown to grey 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.
The fungi may persist 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. Seeds purchased from supply companies are not the source of the fungal contaminants. Rather, the spores are already present in the potting soil or are introduced into the potting medium on the surfaces of tools or pots or trays. Because the fungi are indiscriminate in the species or varieties of plants attacked, neither crop rotation nor resistant cultivars are successful means of control.
So, what can be done to ensure that seedlings get a healthy start and are able to ward off these fungi? Chemical treatment in the form of fungicides is neither effective nor desirable, although some companies sell seeds that are coated with fungicides. Instead, several cultural practices can be employed, including:
- Sterilizing pots, trays and equipment with a 10 percent household bleach (9 parts water, 1 part bleach) for 30 minutes is effective. Bleach stains and dissolves some fabrics, including skin, so it must be used carefully.
- Using soil-less sterile potting media is an option. Alternatively, not re-using potting soil saved from previous years reduces the likelihood of contamination.
- The soil-borne pathogens (mostly as members of several genera, including Pythium, Fusarium, or Rhizoctonia) like cold and wet soil conditions. Thus, potting media with good drainage and the placement of heating mats (temperature of about 75 ⁰F) under the trays, are very useful in greenhouse or home situations. As well, proper heating and humidity control of greenhouses is imperative.
- For direct-sown seeds, soil temperatures optimum for the given crop, and avoidance of water-logged situations can prevent damping-off.
- Adequate duration of light, 12-16 hours per day, ensures rapid and satisfactory seedling growth.
- Keeping the NPK (nitrogen:phosphorus:potassium) fertilizers concentrations to 1:2:1 (moderate N levels) hasten maturation of the stems. While high N 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 is due to 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 catalogues. Happy Spring! Happy planting!
—Steven Jakobi, Master Gardener Volunteer
Cornell Cooperative Extension of Allegany County
For more articles out of New York, click here.