Part II: Warm-season turf, Diagnosing and managing nematodes
Nematodes are among the most difficult pests to diagnose and manage in warm-season turf. No one can simply look at poor turf and be sure that its condition is due to nematodes. How can a turf manager tell for certain if nematodes are at fault? And what can you do about it?
Plant-parasitic nematodes are microscopic worms that feed in and on the roots of most kinds of plants, including the warm-season turfgrasses. Nearly all of their effects on plants are due to their feeding. Because all of the nematodes that are serious turf pests feed on roots, their direct effects are hidden from view. Some cause specific symptoms on roots while others seem simply to weaken the turf because of general injury to roots. Therefore, you must learn to recognize the kinds of above-ground symptoms that suggest nematodes, know what root conditions to look for that confirm your suspicions and how to take samples to finish your diagnosis.
Diagnosis Above-ground symptoms are the same as those that result from almost any kind of root damage: * Areas that wilt more easily than most and recover from wilt more slowly after rain or irrigation. * Chlorosis (yellowing) or other "off" colors indicating that nutrients are not getting to the foliage despite soil levels. * Thinning or "melting out" of turf. Nematode-stunted turf has low vigor, so it cannot easily replace foliage lost to wear or natural attrition. Turfgrass damaged by nematodes wilts more readily than other areas. Turfgrass responds to this additional stress by decreasing or ceasing growth. * Weeds often invade easily where nematodes have reduced the competitive vigor of turf. * Erratic or irregular shape and distribution of affected areas-nematode populations can vary a great deal within a small area, so the severity of their effects on turf also varies across an affected area. Symptoms often appear in a mosaic pattern-weak (severely affected) areas may be scattered within areas of much better growth, with a fairly smooth transition between them. Nematode effects on turf almost never follow straight lines or appear abruptly (unless human activity such as nematicide application can explain the pattern).
Below-ground symptoms of some nematodes may be fairly distinctive, but others may have serious effects on turf growth and vigor without causing any easily recognized symptoms on the roots. Among the root conditions that indicate that nematodes may be a problem are: * Roots are unusually short-bermudagrass turf seriously affected by some kinds of nematodes often has roots less than 1 inch long, compared to healthy roots, which may reach 6 to 8 inches. * Roots are sparse-less numerous than in healthy turfgrass. * Roots are simple-they have fewer branch roots, fine feeder roots and root hairs than healthy grass. * Unusual growth may be present, such as swollen root-tips or excessive production of short lateral roots from a small area. * Roots usually are darker than normal. Many nematodes kill individual cells as they feed, and the cells turn brown as they die. You sometimes can see small areas of these dead cells as tiny dark lesions, which may become quite large depending on the kinds and numbers of nematodes feeding on the roots.
Not all nematodes are alike Many different genera (kinds) of nematodes can affect turf in different parts of the country. Some are limited by conditions (for instance, sting nematodes are serious only in sandy soils, usually those with more than 85-percent sand content). Others affect relatively few kinds of turfgrasses-or at least are much more serious pests to some turfgrasses than others (for example, ring nematodes affect centipedegrass much worse than St. Augustinegrass or bermudagrass). The table below lists nematodes commonly associated with decline of warm-season turfgrasses in the United States.
Laboratory assays of soil samples are an important diagnostic tool, but you should use them carefully. If you do not collect, handle and submit samples correctly, they can yield false results. Even accurate lab results can lead to wrong conclusions if their interpretation does not take into account all circumstances surrounding the problem. Most states provide nematode diagnosis at laboratories in their land-grant university or state Department of Agriculture. Private laboratories in many areas also offer nematode analysis. Each laboratory has its own specific instructions, services and fees, but most require that you prepare nematode samples similar to the following "generic" instructions:
1. Each sample should consist of 15 to 20 cores of soil from the suspect area to provide an "average" nematode population estimate. A single sample is risky because of the erratic distribution of nematodes within any area-it is likely to provide an unrealistically high or low estimate of one or more of the kinds of nematodes present in the area than a sample made up from many cores. Take cores to normal rooting depth, about 4 inches. Some labs will ask that you discard the top growth from each core. Most labs want a sample of about 1 to 2 pints. If you have more soil than you wish to ship, mix it thoroughly in a bucket or large plastic bag before pulling out the proper volume to submit.
2. Submit the sample in a tough plastic bag that you can seal well. Zipper-seal bags sometimes do not seal perfectly when soil gets into the zipper and can pop open during rough handling. To avoid these problems, put adhesive tape over the zipper to keep it closed.
3. Label the bag for each sample with a permanent black marker. Never put the sample identification information on a paper label inside the bag. A few days in moist soil will turn most paper labels to compost, and the information will not be legible at the laboratory. Fill out laboratory forms as completely as possible, clearly linking the information for each sample with the label on the bag.
4. Do not allow any nematode sample to overheat-keep it between normal air temperature and refrigerator temperature. Avoid direct sunlight and hot spots in a vehicle, such as on the floor directly over the exhaust pipe or on the dashboard in the sun. A few minutes of overheating to as little as 120oF can kill some of the nematodes in a soil sample, skewing the lab results. If you are collecting several samples in the field, place each sample in the shade (or better yet, in an insulated cooler). Take the sample(s) to the laboratory as quickly and freshly as possible: Express shipping companies can get samples almost anywhere in 1 to 2 days. Be sure to enclose the laboratory's completed forms and your payment when you ship the sample(s).
5. The laboratory should return information to you about the number of each kind of nematode in the sample and whether those levels are likely to cause damage to your turf. If they return only nematode counts, contact a local expert (for example, at your cooperative-extension office) to help interpret the results. You won't find any magic number to use as an absolute threshold-soil and other environmental conditions, cultural practices and quality standards differ among locations. You must consider them all when deciding what action to take, if any.
Managing nematodes * Control measures before planting. You can avoid the effects of nematodes and other soil-borne pests temporarily by either treating the soil in place or removing it and replacing it with a pest-free soil or sand mix. Both are, of course, only temporary measures because nematodes and other pests will return from deeper in the soil profile or come in with sod or sprigs.
Soil fumigation can help reduce nematode populations that built up in a previous planting and also may help control soil-borne fungal diseases and weed problems, depending on the product and application method. Several soil-fumigation products are available for many areas but may require special equipment and skills to apply. State regulations and label restrictions are quite variable, so check with local authorities before planning to use the following: * Dazomet (BASF's Basamid Granular) * Metam sodium (a restricted-use pesticide for "small-area" applications) * Methyl bromide (a restricted-use pesticide that will be phased out in the United States by 2001)
Other options may exist in your state, so check with a local extension specialist.
Soil excavation and replacement may be an alternative to fumigation for small areas. It is laborious but requires no waiting period and, obviously, is not subject to pesticide regulations. However, you must take care that the replacement soil does not bring its own problems that might be equal to the nematodes, diseases and weeds removed with the original soil.
* Control measures for established turf. Management must emphasize cultural practices to improve the ability of the turf to withstand some nematode pressure, because in some states, no traditional nematicide is registered to treat living turf. Even where nematicides are available, their effects are temporary and partial. Treatment will be far more effective if you take appropriate cultural measures to increase turf tolerance and vigor.
Cultural practices that most affect the ability of turf to deal with nematodes include: * Cutting height. Too little foliage (low cutting height) means that turf cannot manufacture food fast enough to replace tissue lost to nematode damage. * Fertilizer. Don't let a lack of potassium, calcium, phosphorus or other nutrients important for sound root growth limit turf's ability to replace lost roots. Over-use of nitrogen can rob turf of carbohydrates that it would use for root growth by forcing more foliage growth (which you then mow off, so that the plant loses these resources entirely). * Watering practices. Water deeply enough to encourage deep rooting. Shallow watering may discourage roots from growing more than 2 or 3 inches deep, making turf much more sensitive to the loss of roots to nematodes. * Managing soil problems. Poor drainage, hard pans and other impervious layers can hinder root growth.
Nematicides for established turfgrasses. A limited number of products are available for this purpose (see table, page G 29). The traditional nematicides include: * Fenamiphos-Bayer's Nemacur 10% (golf courses, sod farms, cemeteries and industrial grounds). * Ethoprop-Rhone Poulenc's Chipco Mocap (golf courses only). Consult local experts-state and local restrictions may determine what you can use in your area. Several biorational products are now in development, but none has yet proven to be as effective against nematodes in a wide range of conditions as the traditional nematicides (fenamiphos and ethoprop). For now, turf managers must maximize turf vigor and rely on the limited number of available chemicals when treatment is necessary.
Dr. Robert A. Dunn is professor of nematology at the University of Florida-Gainesville.
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