NEMATODES Various nematode species   F A C T S H E E T

Introduction

Living in the cold, Northeastern U. S., we are fortunate to have less plant parasitic nematode problems than those living further south. Still, some Northerners who grow plants outdoors do have problems with nematodes. Many annual ornamentals are at risk if planted in soil heavily infested with nematodes. They may develop serious root problems, grow poorly, and fail. There presently are no nematode "cures", but proper planning prior to establishment of plants and the care given to them after planting can have a substantial effect on their performance.

The nematodes discussed here fall into two groups, nematodes that spend there life cycle in the soil and roots of plants and those that spend at least part of their time feeding on the foliage of plants. Of the soil types, the most common is the root knot nematode, Meloidogyne sp. Root knot nematodes have a broad host range of more than 200 reported plants. Other commonly found root feeding nematodes include Stunt nematode, (Tylenchorhynchus sp.), Lance nematode (Hoplolaimus sp.), Spiral nematode (Helicotylenchus sp.), Lesion nematode (Pratylenchus sp.), Cyst nematode (Heterodera sp.), and Ring nematode (Criconema sp.). Most foliar nematodes belong to the genera Aphelenchus or Aphelenchoides. The Pine Wood nematode (Bursaphelenchus xylophilus) is unique as it attacks mainly Japanese Black Pine trees and harms the tree by filling the sieve tubes of the tree's vascular system.

Symptoms

Symptoms may vary based on the plant part(s) attacked, species of nematode(s) present.

Figure 1: Characteristic angled foliar lesions. ( provided by the Plant Disease Diagnostic Clinic, Cornell University) Figure 2: The appearance of damage on strap-like leaves. (provided by the Karen L. Snover-Clift, Cornell University)

Above Ground Symptoms
Nematodes that feed on the roots cause above ground symptoms that are similar to those resulting from many kinds of root injury. Foliage loses its luster and wilts. Prolonged root stress caused by nematodes may result in yellowing and eventual loss of foliage. New flushes of growth are stunted and weak, with fewer and smaller leaves than healthy plants. Plants tend to wilt more readily during low water or drought conditions than uninfected plants. The damage is usually distributed irregularly, since nematodes are rarely distributed evenly in the soil.

Nematodes that feed on the foliage produce characteristic angled lesions on broad leaved plants (Fig.1). These legions are very characteristic of a nematode infection. Infections on narrowed leaved plants can be misleading due to the small, strap-like leaves lacking those characteristic angled lesion. Instead the random leaves appear to turn completed brown and die (Fig. 2).

Figure 3: Root knot nematodes cause symptoms of galling on the roots. (provided by the Plant Disease Diagnostic Clinic, Cornell University) Figure 4: Vermifom plant parasitic nematodes feeding within root tissue. (provided by Dr. Eric B. Nelson, Cornell University)

Root Symptoms

Root symptoms vary widely. Some kinds of nematodes cause tissues on which they feed to grow strangely (root-knot and some foliar nematodes, for instance); some stop the growth of the roots; others kill the cells on which they feed as they move through the roots (Fig. 4), leaving patches of dead tissue as they move on. Depending on the kinds of nematodes involved, damage may include galls, stunting, and decay of roots; roots are often darker in color than healthy roots. Fungi and bacteria which cause root rots, wilt, and other plant diseases often infect nematode-damaged roots earlier and more severely than uninjured roots. Some viruses can also be transmitted by nematodes.

Among the dozens of nematode species associated with landscape ornamentals, only a couple seem to cause most of the serious problems. The root-knot nematodes (Meloidogyne spp.) are by far the most important. Their easily-recognized galls on the roots make their presence obvious (Fig. 3). Galls result from growth of plant tissues around juvenile nematodes which feed near the center of the root. Root-knot gall tissue is firm without a hollow center, and is an integral part of the root; removing a root-knot gall from a root tears root tissue. Nodules formed on roots of many legumes because of beneficial Rhizobium spp. (nitrogen-fixing bacteria) and most other natural nodules or bumps are loosely attached to the root and have hollow centers. Active Rhizobium nodules have a milky fluid in their centers.

Laboratory Soil Sample Analysis

Laboratory soil sample analysis or foliar analysis is the only way to detect the kinds of nematodes associated with a problem and may be necessary to identify the most effective control measures. Contact your local county extension agent for more information on nematode assays.

Disease Cycle

Nematodes are microscopic, worm-like, animals with life cycles that include one egg stage, four juvenile stages and one adult stage. Nematodes overwinter using a number of strategies in egg phase, within the dead female body, or in plant material. As the weather warms in the spring, the nematodes become active and begin feeding. Some female species become enlarged when mature. They either produce and then deposit a mass of eggs outside of their bodies--as do female Root Knot nematodes, or they house the eggs within their bodies--as do female Cyst nematodes.

Management Strategies

In most cases, there are many attractive plants which could be planted into a site without serious damage or immediate losses. With proper care, an attractive landscape can be established and maintained, despite nematodes. The keys to minimizing the effects of nematodes on landscape ornamentals are good horticultural care and preventive maintenance as outlined below.

1. Prepare new planting sites properly.
Give plants the best chance to become rapidly established. Native soil into which annuals are to be planted should be prepared well, including removal of any old roots, debris, etc., from the site. Water and nutrient holding capacity of the soil and activity of natural enemies of nematodes are improved by incorporating organic soil amendments into the soil before planting.

2. Replace infested (contaminated) soil.
It may be simpler to remove all soil or planting mix from an annual bed and replace it with new nematode-free planting medium. Nematodes eventually will invade the new medium, and bedding plant roots can grow out of the new medium into the infested native soil, but infection will be delayed and damage less severe.

3. Use nematode-free stock.
No matter how perfect and pest-free the planting site, a nematode infection already started in roots of transplants will do the most damage. Buy only top quality plants. Reject any that have clear evidence of nematodes or other problems.

4. Select plants that are well adapted.
Plant suitability to the location is important at all levels: region (climate), soil type, shade, drainage, etc. Plants that are "out of place" are more likely than well-adapted ones to suffer environmental stress. Moreover, a plant species that is "well adapted" to an area probably has some degree of tolerance or resistance to locally common pests, such as nematodes.

5. Avoid nematode-susceptible plants.
Do not use plants that are very susceptible to nematodes known to occur in a planting site. Plants vary widely in their susceptibility to different nematodes, just as they vary in form, flower color, and season of bloom. Most references to "nematode susceptibility" in popular literature refer to one or more root-knot nematode species, unless they specify others. Such lists are often incomplete, but it is quite likely that any plant identified as "nematode susceptible" should not be planted where root-knot nematodes are known to be serious. See Table I for information on herbaceous plants and their responses to specific nematodes.

6. Keep other pests under practical control.
Stress from pests can set plants up for nematode infection just as readily as over- or under-supply of nutrients or water. Overuse of pesticides can also injure plants, so use judgment in their application and always follow the instructions on the pesticide label --- it's the law.

7. Maintenance.
Give the plants optimum care from the start and for as long as you want them to perform well. "Optimum" does not mean "maximum". Fertilize as needed to maintain healthy growth, not to produce excessive, succulent growth that invites attack by nematodes and other pests. Water deeply, to encourage development of a deep root system which can exploit a large volume of soil for water and nutrients. Frequent shallow watering causes plants to develop a shallow root system. A large root system can better withstand a small amount of nematode damage without major compromise to the overall health of the plant than can a shallow, already minimal root system.

8. Do not allow maintenance to lapse.
Sudden dry periods or pest outbreaks can weaken plants in an incredibly short time. Even under normal conditions, erratic or inadequate watering can weaken a plant so that it can no longer tolerate a modest nematode population that had existed for years.

9. Mulches.
Keep the plant root zone mulched to keep roots cool in hot weather and minimize evaporation of water from the soil surface. Organic mulches also contribute organic matter to the soil, thus enhancing the capacity of the soil to retain water and nutrients. Mulches reduce stress on the plant as a whole and on the root system specifically. Greater soil organic matter content also stimulates activity of natural enemies such as certain fungi, predatory nematodes, etc. which can help suppress nematode populations.

10. Chemical controls.
Most chemicals that have historically been used for nematode control or suppression are highly toxic and have very limited uses usually involving soil sterilization before planting. In addition, these chemicals have often been recommended only for crop production or other commercial uses, and had few if any applications in landscape plantings. Currently only one product has limited lawn & landscape uses in New York State. Basamid Granular will kill nematodes and plants and can be used to fumigate soil when renovating lawns or prior to planting landscape beds. However, this is a toxic material that can be harmful to beneficial soil organisms, and alternatives should be carefully considered before use. Basamid granular should be applied by a trained applicator.

Table I was compiled from various fact sheets and scientific articles. Bedding plants as well as vegetables and grasses are included. Plants listed were either not infested, lightly infested, resistant to, or actually suppressive of the various nematodes listed.

Sources:

Hagan, Austin et al. Nematode Suppressive Crops. 1994. Circular ANR-865. Alabama Extension Service.

Jones, R. K. Nematode Management in Bedding Plants in the Landscape. 1995. Plant Pathology Information Note. Ornamental No. 31. North Carolina Cooperative Extension Service.

Lehman, P. Factors Influencing Nematode Control With Marigolds. 1974. Nematology Circular No. 50. Florida Department of Agriculture and Consumer Services, Division of Plant Industry.

McSorley, R. and Frederick, J. Response of Some Common Annual Bedding Plants to Three Species of Meloidogyne. 1994. Journal of Nematology. 26 (4S):773-777.

Updated, SLJ, 1/07 ____________________________________________________________________________________
This publication contains pesticide recommendations. Changes in pesticide regulations occur constantly, some materials mentioned may no longer be available, and some uses may no longer be legal. All pesticides distributed, sold, and/or applied in New York State must be registered with the New York State Department of Environmental Conservation (DEC). Questions concerning the legality and/or registration status for pesticide use in New York State should be directed to the appropriate Cornell Cooperative Extension Specialist or your regional DEC office. READ THE LABEL BEFORE APPLYING ANY PESTICIDE. __________________________________________________________________________________
The Plant Disease Diagnostic Clinic at Cornell University is located at 334 Plant Science Building, Ithaca, NY, 14853. Phone: 607-255-7850, Fax: 607-255-4471, Email: kls13@cornell.edu or slj2@cornell.edu