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Verdure: The Sting (nematode, that is)

Sting nematodes are known to have the lowest threshold levels of any nematode on turfgrass. But just how low?

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“The Sting” is a famous movie from 1973. The plot centers on two con artists, portrayed by Robert Redford and Paul Newman, who hatch a hilarious scheme against a big-city crime boss. It won seven Academy Awards, including Best Picture. For a look at a classic movie, stream “The Sting,” and you’ll still be surprised by the ending. Because of its cultural significance, “The Sting” is listed in the U.S. National Film Registry of the Library of Congress.

The Sting movie
Paul Newman and Robert Redford in “The Sting.” Photo courtesy of Universal Pictures


But the sting nematode is a con artist of another sort.

Nematodes are considered microscopic roundworms or eelworms from the animal kingdom, and most species are free-living in the soil. Some species are plant-parasitic and cause devastating losses to food crops in many regions of the world. The sting nematode (Belonolaimus longicaudatus) is an aggressive parasite of turfgrass roots, especially bermudagrass (Cynodon spp.). Sting nematodes feeding on bermudagrass roots can cause turf chlorosis, a weak and thinning stand vulnerable to invasion from weeds and algae, and, of course, root damage and loss of root function.

The sting nematode can be devastating to bermudagrass at low populations in the root zone, as it is considered to have the lowest threshold level of any nematode on turfgrass. But how low? Researchers at Clemson University wanted to find out.

Bermudagrass root and soil samples were collected from three golf courses in South Carolina during 2012 and 2013. Sampling test plots were set up on Tifway and Tifgreen bermudagrass (C. dactylon × C. transvaalensis) fairways. Nematode population density was determined by taking six soil cores of 0.5-inch (1.3-centimeter) diameter × 6-inch (15-centimeter) depth across each plot at each location. For each plot, the six cores were mixed, and nematodes were extracted from a standard 100-cubic-centimeter volume subsample using the sugar centrifugal flotation method. A sample of the liquid extract was viewed under the dissecting microscope, and nematode species present were identified and counted.

At each location for both years, soil core sampling and nematode extraction and counts were conducted at the start of the field study, once in late spring/early summer and once again in late summer/early fall. For the two-year study, 678 soil cores were collected and analyzed for nematodes.

To measure bermudagrass roots, a similar number of soil cores also were collected during both years. The top 1-inch (2.5-centimeter) layer of turf and thatch was removed from each soil core, and the remaining soil was washed from the roots. Roots were oven-dried at 176 F (80 C) for five days, weighed and then placed in a muffle furnace at 977 F (525 C) for 2.5 hours. Final root weights were recorded as the difference between oven-dry root weight and ash weight.

The researchers used a complicated statistical regression analysis method to make sense of all that data. Regression is a powerful statistical tool that allows researchers to determine the strength of the relationship between two things being measured — in this case, sting nematode counts and bermudagrass root weights.

Not surprisingly, they found that a higher sting nematode population meant a lower amount of bermudagrass roots, and of course that also meant poor bermudagrass visual quality. Their mathematical modeling of that data revealed a threshold of 17 sting nematodes per 100 cubic centimeters of extracted soil, meaning bermudagrass root damage and lower bermudagrass turf quality is associated with sting counts of ≥17 per 100 cc of soil.

Editor’s note: Nematologist William T. Crow, Ph.D., offers insights on the two other kinds of nematodes that make up the “big three” in Managing root-knot nematodes in warm-season turfgrass and Managing lance nematodes in warm-season turfgrass.

Ideally, turfgrasses should tolerate some reduction in roots from nematode infection before aboveground damage symptoms become visible.

Sting nematodes prefer to feed on newly formed lateral roots, thus decreasing the root system’s ability for optimum water and mineral uptake, which then indirectly affects aboveground turf quality and playability.

The results from this Clemson University study also indicated that a reduction in turf quality can certainly occur before any measurable reduction in root weight is detected. So, when it comes to the sting nematode and bermudagrass, you don’t want more than 17 per 100 cubic centimeters of soil to show up at your “The Sting” watch party.

Source: Shaver, B.R., P. Agudelo and S.B. Martin. 2017. Damage functions for sting nematode (Belonolaimus longicaudatus) on bermudagrass turf. International Turfgrass Society Research Journal 13(1):517-523 (https://doi.org/10.2134/itsrj2016.06.0478).


Mike Fidanza is a professor of plant and soil science in the Division of Science at the Penn State University Berks Campus in Reading, Pa. He is a 20-year member of GCSAA.