Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
12/7/98
Contacts:
Paula Flynn, Extension Plant Pathology, (515) 294-0581
Greg Tylka, Extension Plant Pathology, (515) 294-3021
Elaine Edwards, Extension Communication Systems, (515)
294-5168
Yard and Garden Column for the Week Beginning Dec. 11.
Nematodes Getting Your Plants Down?
By Paula Flynn and Greg Tylka
Extension plant pathologists
Iowa State University
Everyone knows what a weed or an insect or a fungus is, right? What about a nematode? Ever heard of such a critter? Nematodes play a vital role in plant health. These tiny worms that cannot be seen with the naked eye live in all natural soils &endash; in your lawn, garden, flower bed, in agricultural fields and in forests. Some nematodes are beneficial, involved in the breakdown of organic matter and the cycling of nutrients in the soil. Other nematodes feed on and kill the larvae of insect pests. In fact, beneficial nematodes for control of insect larvae are available for purchase from mail-order organic gardening catalogs.
Although many nematodes are either directly or indirectly beneficial to plants, certain nematodes can damage trees, flowers, shrubs and vegetables. These plant-parasitic nematodes have a specialized structure, called a stylet, that functions like a hypodermic needle and is used by the nematodes to puncture and withdraw food from the plant cells. The key to controlling plant-parasitic nematodes is to recognize the damage caused by nematodes and to have the nematode accurately diagnosed. Each year, the Iowa State University Plant Disease Clinic receives numerous plant samples damaged by nematodes. The most common plant-parasitic nematodes that home owners may encounter in Iowa are the pine wood nematode, the foliar nematode and the root-knot nematode.
Many pine trees in Iowa are infected with the pine wood nematode. The disease caused by the nematode is pine wilt. Scots pine trees (sometimes called Scotch pines) are particularly susceptible to this disease. The nematodes move through the tree's vascular tissue and feed on the living cells that line the resin ducts of the trees. Symptoms of pine wilt are rapid discoloration of needles. Needles will turn brownish-red and often an entire, mature tree can become affected within a matter of a few weeks or a month or two. Most plant-parasitic nematodes are spread by the movement of infested soil or infected plant material, but the pine wood nematode is unique in that it is moved from tree to tree by insects, primarily the pine sawyer beetle. The nematode can be extracted from infected wood, so limb samples are necessary for accurate diagnosis. There are no measures available to cure infected trees and chemical control is not feasible because the vectoring pine sawyer beetle can introduce the nematode from quite a distance away. Once trees are identified as infected with the pine wood nematode, home owners should remove infected trees and chip or burn the wood.
In recent years, a few samples of ornamental plants, namely hostas and chrysanthemums, have been submitted to the ISU Plant Disease Clinic with infections of foliar nematodes. The nematodes migrate through the living cells in the leaf, consuming the contents of the plant cells for food. The characteristic symptom of infection by foliar nematodes is areas of dead leaf tissue. These spots of dead plant cells are rectangular or square in hostas and triangular in chrysanthemums because the nematodes are limited by the veins in the leaves. The foliar nematodes survive and are spread in infected plant tissue, so sanitation is important in managing this nematode. Diseased leaf tissue should be removed from the plant and destroyed as soon as possible, and any dead leaves at the end of the season also should be destroyed. The nematodes require a film of water on the outside surface of the plants in order to move up from the soil and infect leaf tissues, so providing plants with adequate spacing and avoiding overhead watering will help reduce the chance of infection by foliar nematodes.
Root-knot nematodes are found more often in southern environments than in the Midwest, but at least one species of root-knot nematode occurs in Iowa and can infect ornamental or vegetable plants. Root-knot nematodes have been diagnosed on peony and carrots in Iowa in recent years. In contrast to the nematodes discussed above, root-knot nematodes live exclusively in soil, infecting and establishing feeding sites in the vascular tissue of the roots. The nematodes enlarge as they feed, and swellings form on the root around each nematode, creating numerous small knots or galls on infected rots. These galls typically are only a sixteenth to an eighth of an inch in diameter and may have lateral roots emerging from them. As a result of root-knot nematode feeding, plants tend to be stunted and may wilt in the mid-day heat despite having adequate soil moisture because of a disruption of the water-conducting vessels in the roots. Infection of plants by this nematode is easily diagnosed by observation of the root galls and by checking for the presence of swollen nematodes within the gall tissue. Management options include growing nematode-resistant varieties of vegetable plants, replacing the soil from the infested area with noninfested soil and growing marigolds that are toxic to root-knot nematodes.
ml: isugarden
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