The intent of this fact sheet is to outline, in general terms, the distribution of nematodes in soil, how to collect soil samples, and how to assess the role of nematodes in the decline of turf on golf course putting greens. The results of the a nematode assay are the result of a systematic extraction procedure followed by careful identification and enumeration. Nevertheless these results are only an estimation of the actual population. Additional samples taken in the same general area would result in similar but not exactly the same data. Some of the factors that contribute to this variation are as follows:
- Distribution - Nematodes are unequally distributed in the soil both vertically and horizontally. Thus, two different samples from the same putting green will generally yield different results. In the horizontal plane, nematodes have a clumped distribution. For example, if assays were done in a transect across a putting green, a series of high and low counts would result. Populations also vary according to the depth at which the sample was taken. Some tend to be concentrated in the upper portion of the soil profile. For example, a sample taken at a two-inch depth may have twice the concentration of stunt nematodes as the same sample taken at four inches. Other nematodes may be more evenly distributed through the four-inch depth.
- Time of year - During the winter months nematode populations decline. As the grass begins to resume growth in the spring, nematodes begin feeding on the roots. The reproductive potential of the nematodes increases as the soil temperature rises. In the southern New England area, populations peak anytime from mid-June through mid- to late August. An additional peak of activity may occur later in the season.
- Assay procedure - It is not possible to extract all of the nematodes from the soil sample. Also, extraction techniques vary from one laboratory to another and some techniques are more efficient than others in recovering certain species.
Assessing Damage Caused by Nematodes
High populations of certain species of nematodes may indicate that the root system has become debilitated to a significant extent. However, this information should be evaluated in context with past and prevailing circumstances. For example, all of the following factors are important in the development of a diagnosis and recommendations:
- species and numbers of nematodes present
- depth at which sample was taken
- species composition of grasses present
- depth of root system
- soil texture
- symptoms and extent of damage
- presence of other stresses to the turf
- potential contamination of the environment by a nematicide
The threshold levels (the nematode population levels which justify the implementation of control measures), are dependent on the variables listed above. At this time there is very little experimental evidence to establish threshold levels for turfgrasses in New England. The numbers listed in the following table are based on research from other states as well as from case-history information and survey results compiled in our region. They are to serve as a guide only.
|Threshold Levels for Nematodes that Parasitize Turfgrasses|
|Common Name||New England*||Other**|
|* Developed by Robert Wick, PhD, University of Massachusetts. These threshold levels are based on field observations and laboratory assays from the New England region. Extenuating circumstance such as geographic location, sample collection, assay methods, host and prevailing environmental conditions can affect the interpretation of threshold levels.|
|** From: Turfgrass Trends, Oct 1995. Eric Nelson, Ph.D.|
|*** Sting nematodes do not occur in the Northeastern U.S.|
The decision to use nematicides must be carefully evaluated. The simple presence of plant parasitic nematodes does not warrant application. Furthermore, when high populations of nematodes occur in the absence of appreciable damage to turf, it is difficult to justify applications. Nematicides should not be used where contamination of water may occur. Use only in accordance with the label and local regulations.
Acknowledgements: Information regarding nematode populations and "threshold levels" in the New England region was the result of research of Drs. Robert L. Wick, Patricia J. Vittum (University of Massachusetts), and Stan Swier (University of New Hampshire).
Written by: Dr. Robert Wick