Melissa B. Riley and Paula Agudelo
Department of Entomology, Soils, and Plant Sciences
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ANSWERS TO QUESTIONS
What are nematodes? Describe their general characteristics. Why can they be problems on many different plants including grass on golf courses? Nematodes are multi-cellular animals; they are non-segmented roundworms belonging to the phylum Nematoda. It is estimated that there are over 1 million species of nematodes, only 20,000 of which have been described. They are ubiquitous in nature and occur in any environment on earth from the deep oceans to mountain tops and from the polar ice caps to the equator. They commonly occur in soil and water, and may be free-living or parasitic to animals or plants. They range in size from a fraction of a millimeter to several meters in length but most commonly are microscopic. Nematodes are typically elongate, described as threadworms, but some stages exist in globose or sac-like shapes. The general morphological features are shown in
Figure 5. The body has bilateral symmetry and is protected by an external cuticle which is replaced as the nematode grows in a process called molting. Nematodes are detrimental to turf and other plants due to their feeding upon roots and/or their penetration and movement within roots. This penetration and movement results in portals through which fungi and bacteria may enter roots causing further damage. The type of feeding is dependent on the nematode type.
What are the general symptoms of nematode damage that can be observed on golf courses? Why are these symptoms present? When are the symptoms most commonly observed and why? The most common symptoms in turf where nematode are problems are overall reduced growth, chlorosis and spotty appearance of the grass as well as a thinning of grass. The grass is more sensitive to drought and other pathogens due to reduced root mass and nematode damage. The grass in general does not respond to watering and fertilization as it would if the problems were due to under-fertilization and/or lack of water. The symptoms occur due to a loss of the root system which is necessary for uptake of water and nutrients from the soil. Symptoms most often occur when conditions are not optimum for growth, such as during hot summer conditions when plants demand more water.
How should Bennett take samples of the greens to make sure to get a representative sample? Why is how the sample is taken so important? It is suggested that Bennett take multiple soil samples at the edge of the chlorotic area to a depth of 8-10 cm (3-4 inches, the depth where most of the grass roots occur) using a T-bar soil probe or small trowel. Multiple samples should be taken because nematodes are not present in a uniform pattern and multiple samples ensure an accurate determination of the nematodes present in the green. If only one core is taken, you could get an over- or under-representation of what is present in the soil.
How can you differentiate a plant-parasitic nematode from a free-living nematode in a soil sample extract? Why is this important? A plant-parasitic nematode is differentiated from the free-living nematodes based on the presence of a stylet. Not all nematodes with a stylet are plant-parasitic but all plant-parasitic nematodes have a stylet. The stylet is a needle-like structure that the nematode uses for feeding and to penetrate plant tissue. Free-living nematodes are common in soil, and being able to distinguish the plant-parasitic from the free-living nematodes is an important step in determining if nematodes are causing problems in turfgrass. The shape and type of stylet are also important in determining the nematode genus.
What types of nematodes are known to be problems on golf courses? List them and describe how they can be distinguished from each other. Table 1 lists the different nematodes known to cause problems with turf.
Morphological Characteristics (See Figures 6-20)
Long stylet, spiked tail (female), weak to moderate striations (horizontal grooves in cuticle)
Adult female is lemon shaped cyst observed on root surface, juveniles have strong stylets with larger knobs
Stylet with basal flanges
Robust body, bluntly rounded tail, tulip-shaped knobs at base of stylet
Stylet with no flanges, long, thin nematode
Small in size, long stylet, hook-shaped
Heavy striation, long stylet in relation to body length, short and stubby, sluggish in movement
Females in roots cause a galling of the root, juveniles found in soil, weak stylet, thin and short, tapered tail with rounded tip
Strong stylet with rounded knobs, flat labial region, straight shape, stout
Long stylet, loose sheath
Long stylet and more heavily striated than
Hemicycliophora (sheath), sheath adheres closer to body, tail comes to point abruptly
Moderate stylet, no distinct striation, commonly observed in a spiral shape
Long stylet, bluntly rounded tail (female), weakly striated
Trichodorus cuticle does not appear loose as is observed in
Moderate to short stylet, distinct striations
Look at the sample which you obtained from the Centrifugal-Flotation method using the dissecting and compound microscopes and identify and draw at least three different plant-parasitic nematodes present. Could these nematodes be causing the problems with the grass? The answer to this question depends on the nematodes extracted and observed in the soil samples. Students will have to identify the nematodes present in their soil sample and determine if these nematodes are problems on turf. To determine if the nematodes are a problem the number of nematodes/100 cm3 should be determined and compared with the values in Table 1.
What are possible measures that Bennett can implement to manage nematodes on his golf course if they are present? What are the advantages and disadvantages of the different measures? How would you decide what methods to use in this situation (remember people are playing golf on this course every day)?
Comments on usage
Curfew® soil fumigant (1,3-dichloropropene) is a liquid soil fumigant for the management of nematodes but it is only available for use the following states: Alabama, Florida, Georgia, Louisiana, Mississippi, North Carolina, South Carolina and Texas on golf course putting greens, fairways and tees, and also on athletic fields. Curfew soil fumigant dissipates rapidly and must be custom applied by authorized operators. These regulations are subject to change and proper usage requires checking current availability and usage. Until May 2008, fenamiphos (Nemacur) was another nematicide available for use on turf. Stocks on hand can still be used up but it can no longer be purchased.
Use deep, infrequent watering to encourage deep, healthy roots, light midday watering may be necessary to minimize wilting when nematode populations are high
Avoid over and under-fertilization, aerate soils to encourage deeper root development, reduce the thatch layer
Some varieties are better adapted to certain sites and using a variety that is adapted to a site can decrease nematode problems. There is no complete resistance to all nematodes. Different grass varieties have different levels of susceptibility.
The major factor that should be used in considering what management options should be implemented is the numbers and types of nematodes that are present in the greens. The nematode thresholds are important in consideration of the management measures that should be implemented. Nematicides are commonly used as the last resort and some of the cultural measures are needed to improve the effectiveness of the nematicide – reduction in thatch, aeration/cutting and core-cultivation. Regulations on nematicide usage are constantly changing and it is best to check the current state regulations and label recommendations for those products that are registered for use in your state.
What would you recommend to Bennett based on the analysis of your soil sample? Justify your answer based on the information that you have obtained from your sample in the laboratory. The answer to this question should be based on the number and types of nematodes that are present in the extracted soil sample. Damage thresholds (Table 1) can be used as a justification for the answer to the question. In many cases nematicide applications will not be recommended, but instead, the alteration of cultural measures to encourage good growth of grass and prevention of nematode infestations such as making sure that equipment used on the golf course is clean. Nematodes should not be moved into areas where they do not currently exist if it can be prevented.