Nematodes

I have written in many of my articles about the problems gardeners encounter from root-knot nematodes. I have often stated that in this area, nematodes are probably the worst problem to face the home grower of tomatoes. Unfortunately, we may not know that we have a problem with nematodes until they are so numerous that prevention, if there really is any, is too late. Nematodes, of all kinds, are so abundant they rank third, after bacteria and protozoans in relative numbers. As many as 10,000 may live in a milliliter of soil. (That is about six one hundredths of a cubic inch less than the size of a pea.)

Nematodes are so small you must use a microscope to see them, and not all varieties are serious plant pests. Because of their numbers, they are important as decomposers of dead plant matter, and feed on bacteria, soil algae and the beneficial nematodes attack crop-damaging insects pests.

Plants do not show symptoms of nematodes until the worms multiply to the point where enough roots are damaged to interfere with water absorption. People ask how they can tell whether or not they actually have a problem. There are several visual signs of nematode activity. Check plants for dwarfed plants with poorly developed leaves, chlortic (pale) foliage and wilting due to the roots’ inability to absorb water. When checking roots for nematodes, do not confuse the galls that they produce, which are a swelling of the root, with the nodules formed by nitrogen-fixing Rhizobium bacteria, which are attached to the roots. Root-knot nematodes attack more than 2,000 kinds of plants, so the problem could be more widespread than just your garden. Check with neighbors for any similar symptoms.

If you are still not sure, there is a simple, unscientific test that you can do. Collect soil samples form several areas in your garden where you suspect worm infestation. Make sure the soil samples come from at least 6 inches deep and from several areas. Place all soil samples together and mix well. Now fill six pots with the soil, which must be kept moist but not wet. Put three of the pots in the freezer (in zip lock bags) and leave there for at least 72 hours, then remove them and allow them to thaw to room temperature. This test is based on the fact that most nematodes are killed by cold temperatures.

Plant all six pots with quick sprouting radish seeds, ten seeds to each pot and cover lightly with sand or potting soil. Keep the soil moist. Place the pots outside if possible or on a windowsill.

Radishes will usually germinate in about 6 day, and if the growth in both frozen and unfrozen soils is the same, you probably do not have a nematode problem.

Whether or not you have an infestation, good gardening practices must be followed to keep any organisms from spreading. Do not transfer dirt from bed to bed, and, above all, keep shovels, wheelbarrows and lawn mower tires clean.

Plant early in the season as the lower soil temperatures will help to restrict nematode movement. Plant nematode resistant varieties (the N on the seed package). Above all, be sure your soil has copious quantities of organic matter. Such as mulch and compost. This will encourage nematode predators. This organic matter will also encourage new root growth, which will also encourage new root growth, which will lessen the effect of nematode damage.

Growing marigolds has been shown to be effective in suppressing nematodes in that they release a nematocidal substance from their roots. This seems to work whether you grow marigolds with other plants or alone and turn them under at year-end to decompose.

If you garden in an area that has hot, dry summers, nematodes can be controlled by letting the infested area lie fallow and withholding water during the hottest months, then planting ray or kY31 as a cover crop to over winter and be turned under in the spring.

Pat Biggerstaff is the gardening columnist for the Claiborne Progress.