Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
12/28/98
Contacts:
Greg Tylka and Paula Flynn, Extension Plant Pathology, (515)
294-0581
Elaine Edwards, Extension Communication Systems, (515)
294-5168
Yard and Garden Column for the Week Beginning Jan. 1
Got worms? Now what?
By Greg Tylka and Paula Flynn
Extension plant pathologists
Iowa State University
Got worms? Not a question that you would ask just anyone, is it? However, if your ornamental or vegetable plants are suffering and you can't figure out why, it's an important question to have answered. As explained in this column a few weeks ago, nematodes are tiny worms that cannot be seen with the naked eye. They live in all natural soils. Some of these nematodes are beneficial, breaking down organic matter or infecting and killing harmful insect larvae, but certain nematodes feed on plants and cause damage.
The key to controlling plant-parasitic nematodes is to recognize the damage they cause and to have the problem accurately diagnosed. The most common plant-parasitic nematodes that home gardeners encounter in Iowa are the pine wood nematode, the foliar nematode and the root-knot nematode. How would one go about controlling a plant disease caused by one of these nematodes? Well, it depends on which nematode is causing the problem.
Resistant plant varieties
A very effective way to manage plant-parasitic nematodes is to grow varieties of plants that have been bred for resistance to nematodes. However, you will be able to find varieties of only a few vegetable crops that are nematode resistant, and they will be resistant only to the root-knot nematode. If you can purchase a plant variety with resistance, this management strategy works great. The resistant plant will not allow the nematode to reproduce and build up in numbers and, consequently, will provide much greater plant growth and yield. Seeds of nematode-resistant plant varieties typically have a "N" designation somewhere on the label of the seed packet to identify that the variety is resistant to root-knot nematodes.
When a truly nematode-resistant plant variety is not available, sometimes you can grow varieties or species of plants that are less vulnerable to attack by a nematode species. The best example of this is with pine wilt, caused by the pine wood nematode. Scots pine trees (sometimes called Scotch pines) are particularly susceptible to pine wilt, but other evergreen species, such as a spruce, are not as susceptible to the disease. Consequently, home gardeners are advised to replace the Scots pine trees that have died from pine wilt and been removed with a pine or spruce species that is not readily damaged by the pine wood nematode.
Cleaning up
Many plant-parasitic nematodes survive over winter in infected plant material. Consequently, these leaf, stem and root fragments serve as a source for new nematode infections each year. Nematode-infected plant material should be removed and destroyed to reduce the chance for future nematode infections. This management strategy works particularly well for the pine wood nematode and the foliar nematode.
Marigolds
Growing a solid planting of French marigolds in the area that is infested with root-knot nematode will reduce the population of this nematode as well as several other plant-parasitic nematodes. The nematodes infect the roots of French marigold, but a toxic compound is released from the marigold roots which kills the worms before they have a chance to feed and multiply. A few marigolds scattered throughout the garden will not control plant-parasitic nematodes. A solid planting should be grown for at least two months and then turned under.
Chemical control
Use of pesticides really is not an option for management of plant-parasitic nematodes in the home garden. Pesticides that kill nematodes, called nematicides, are relatively toxic and not available for use without a pesticide applicator's license. Recall that the pine wood nematode, the cause of pine wilt, is moved from tree to tree by an insect. Many folks inquire whether an insecticide treatment will protect their healthy trees from infection. Unfortunately, such a strategy will not work because nematode-carrying insects can fly in from quite a distance away to introduce the nematode. Extended periods of beetle activity also make timing of insecticide treatment difficult.
Plant-parasitic nematodes are not common plant pathogens, but they do cause problems periodically with ornamental and vegetable plants in Iowa. The key to managing plant-parasitic nematodes is to recognize the damage caused by nematodes and to have the nematode accurately diagnosed. After that, it is just a matter of common sense and persistence to beat these pesky, somewhat mysterious, worms.
ml: isugarden
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