By: Teo Spengler
You may never have heard of stunt nematodes, but thatdoesn’t mean that these microscopic worms aren’t affecting you. What are stuntnematodes? These destructive pests are among the plant parasites that cause themost damage to field and vegetable crops in the country. Once you understandthe damage done by these pests, you’ll want to know how to prevent stuntnematodes from destroying your crops. But control is not easy. Read on for adescription of stunt nematode symptoms, plus a few tips on stunt nematodecontrol.
Stunt nematodes are not big bugs that you can readily spoton your veggie vegetation. They are tiny worms, microscopic, termed Tylenchorhynchus spp. by scientists.Stunt nematodes are parasites that damage the roots of vegetablesin your garden, exposing the plants to various destructive pathogens in thesoil. They are not limited to backyard gardens. In this country, these pestscause almost $10 billion in economic loss.
It’s not easy to pin down the financial loss caused by stuntnematodes. That’s because scientists don’t know enough about theircharacteristics and how they operate.
There are a variety of plant parasitic nematodes, including rootknot nematodes, spiral nematodes and needle nematodes. Like these otherplant parasitic nematodes, stunt nematodes feed on plant roots. They can liveboth in the soil and on plant tissues and are able to infest a wide variety ofdifferent crops.
Stunt nematode symptoms also vary from one crop to another.They often involve non-specific issues like wilting, yellowing and stunting.
Every gardener wants to stop these worms from damaging hisor her crops. So, if you are wondering how to prevent stunt nematodes fromeating your veggie plant roots, you aren’t alone. But stunt nematode control isnot easy. And the geographical spread of the worms depends on temperatures,soil types and crop history.
It is more appropriate to think of stunt nematode managementthan stunt nematode control. First, put into practice the cultural practicesthat do not involve toxics, like proper sanitation and keeping your plantshealthy. Only if these fail should you turn to chemicals.
Sanitation is essential if you find stunt nematodes in yourplants. You need to plow under the infected plant and be sure to give healthyplants everything they need to thrive, include sufficient water and nutrients.Wash down your garden tools and equipment to prevent the spread of theinfection.
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There are several species of plant parasitic nematodes associated with turf in Indiana. These microscopic soil organisms are not visible and must be isolated from soil around the root area by special laboratory methods. The symptoms associated with these pests are yellow stunted and sometimes wilted grass. These symptoms are often visible in random patches. The symptoms are more apparent when turf is under stress. Plants weakened by nematodes are more vulnerable to other pests as well.
Major nematode species known to cause substantial damage to golf courses in Indiana are: Ring Nematode, Stunt Nematode, Lance Nematode and Root Knot Nematode.
Soil samples need to be taken to a root-depth of 4 inches with soil probes. The accuracy increases as more sub-samples are taken. A pint of soil is required for nematode analysis. We recommend a sample from a "good" area and a comparable sample from a "problem" area to better diagnose the problem, especially if the turf has not been analyzed for nematodes recently. Soil samples should not be exposed to high temperature and should not be allowed to dry, as nematodes will not survive these conditions. The most common turf parasitic nematode has been ring nematode followed by stunt nematode.
A soil sampling program on a regular basis, at least once a year, must be established. The best management practice starts with eliminating stress and providing proper fertilization. A restricted use pesticide, like Nemacur, is effective in helping turf for a short period of time. The efficacy of the nematicide needs to be determined by taking soil samples before and after application.
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New information about the anatomy of microscopic worms called stunt nematodes could help scientists identify one or more of the 111 known species in this class of destructive pests.
Stunt nematodes (Tylenchorhynchus spp.) are among the most costly plant parasites. Overall, plant parasitic nematodes in the United States cause annual economic losses estimated at nearly $10 billion from decreased food, fiber and ornamental production.
Stunt nematodes damage the roots of field and vegetable crops. Once damaged, plants become exposed to many destructive soilborne microorganisms and pathogens. A major problem with determining the damage these nematodes cause is inadequate knowledge of their distinguishing characteristics, numbers, relationships and geographic distribution.
After several years of studying specimens and pertinent literature of all described stunt nematode species, Agricultural Research Service microbiologist Zafar A. Handoo--an expert on identifying nematodes at the ARS Nematology Laboratory, Beltsville, Md.-- recently completed an identification key. This is the first accurate, all-inclusive guide to diagnose and identify all known stunt nematode species. The research has been published in the Journal of Nematology.
Handoo examined and evaluated all the information on these species contained in the USDA Nematode Collection, one of the world's largest and most valuable archives of these worms. It contains information on thousands of nematode species important to agriculture. His compendium details the most important diagnostic characteristics of each stunt nematode species.
Handoos key is based on the overall morphology--the external features--of females, since males are not known in several species. In some cases, he used the differences in male reproductive organs. In his key, he identified the main characteristics useful in distinguishing species, such as the shape of the lip region and shape of the tail. Further studies are needed of the worms morphology, including scanning electron microscopy to magnify male and female nematodes from a broader range of habitats.
To make management decisions, it is important to know the nematode species present and their population estimates. If a previous crop had problems caused by nematodes that are also listed as pests of potato, their numbers may be high enough to cause damage to subsequent potato crops. Soil samples should be sent to a diagnostic laboratory for enumeration and identification.
Take soil samples in fall from within the root zone of the previous crop after harvest or, preferably, just before harvest. Divide the field into sampling blocks of not more than five acres. Each block should be representative of the field's cropping history, crop injury, or soil texture. Take several subsamples randomly from a block, mix them thoroughly and make a composite sample of about 1 quart (1 liter) for each block. (See UC ANR Publication 3316, Integrated Pest Management for Potatoes, for more details.) Place the samples in separate plastic bags, seal them, and place a label on the outside with your name, address, location, and the current/previous crop and the crop you intend to grow. Keep samples cool (do not freeze), and transport them as soon as possible to a diagnostic laboratory. Contact your farm advisor to help you find a laboratory for extracting and identifying nematodes, and for help in interpreting sample results.
Needle nematode damage to corn seedlings. (Tom Hillyer)
Many species of plant-parasitic nematodes feed on corn throughout the Midwest. Most are commonly found anywhere that corn is grown. Nearly all of these corn nematode species likely are native to Iowa and probably fed upon native grasses long before corn was cultivated in the state. The common and scientific names of the most common genera of corn nematodes are listed in Table 1.
Table 1. Plant-parasitic nematodes that commonly feed upon corn in the Midwest.
|Common Name||Scientific Name|
Most corn nematode species can maintain their populations when soybeans or alfalfa are grown, but repeated cropping of corn may cause nematode populations to flare up. Also, use of transgenic, insect-resistant corn hybrids for corn rootworm control may reduce the amount of soil-applied insecticide used in the state. Some have speculated that these insecticides may have provided some suppression of plant-parasitic nematode populations, and reduction in use of soil insecticides also may lead to increases in corn nematode population densities.
Aboveground symptoms of nematode damage to corn include thin stands, uneven plant height, stunted plants, uneven tasseling, leaf yellowing, and small ears and kernels. Swollen roots, lack of fine roots and root branching, and necrotic lesions (black or dark brown dead spots) are common symptoms of nematode feeding on roots. These symptoms are not unique to nematode feeding and, thus, cannot be used to definitively diagnose nematode damage. The only way to accurately document the occurrence of damage from plant-parasitic nematodes on corn is through collection and analysis of a soil and root sample.
Several points should be considered when collecting a sample for diagnosis of a corn nematode problem.
Once the nematodes are extracted from the soil and roots, identified, and counted, various pieces of information will be considered in deciding whether or not the nematodes present in the sample are partially or primarily responsible for the damage observed in the corn crop. Collecting a good sample and providing pertinent and complete background information about the circumstances in the field are critical steps in making an accurate assessment of the potential for damage. Information about the field history, soil type, and rainfall can be useful in making an accurate judgment as to whether the numbers of nematodes recovered from the sample are sufficient to cause damage to corn.
Corn seedlings stunted by nematode feeding
If it is determined that the corn crop is being damaged by corn nematodes, two management strategies are available: nonhost crops and soil-applied nematicides. Neither of these management options can be used to minimize damage or "rescue" the current corn crop.
If the corn crop is being damaged by needle nematode, certain species of lesion nematode, or a combination of these nematodes, growing nonhost crops such as alfalfa and soybean will reduce nematode population densities and, thus, the potential for damage to future corn crops. One or two years of growing nonhost crops may be sufficient to lower the numbers of needle and lesion nematode to below damage thresholds for corn.
Stunt nematode on corn root.
There are only a few nematicides that currently are labeled for use in controlling plant-parasitic nematodes on corn. Damaging population densities of plant-parasitic nematodes in corn fields may occur in discrete patches or "hot spots." Consequently, fieldwide application of these pesticides might not be necessary or economical. Also, the benefits of nematicide use in controlling nematodes usually does not carry over to subsequent cropping seasons. Thus, nematicide use will be a management option that is necessary with each corn crop.
Lesion nematodes, stained red, inside infected corn root. (Don Norton)
Soil and root samples for analysis of corn nematodes can be sent to the Iowa State University Plant Disease Clinic, 327 Bessey Hall, Iowa State University, Ames, IA 50011. The test for corn nematodes is called a complete nematode count. Samples should be accompanied by a completed Plant Nematode Sample Submission Form and a check for the $30 per sample processing fee.
This article originally appeared on pages 10-11 of the IC-498 (1) -- February 12, 2007 issue.