Nematodes are one of the few turf pathogens that can be managed using an integrated pest management strategy. Because plant-parasitic nematodes are ubiquitous in turfgrass soils, it would be impractical and impossible to eliminate them. Thus, turf managers in the Northeast must live with a certain number of plant-parasitic nematodes in any turfgrass stand. When the population of nematodes reaches a high level, it may cross a disease threshold, and symptoms might be observed. When this occurs, treatment is warranted to knock the nematode numbers below the disease threshold.

Quick Tip

Dr. Robert Wick at the University of Massachusetts has been actively working on the population dynamics of plant parasitic nematodes on putting greens for at least 20 years. His research has helped clarify the damage thresholds for numerous plant-pathogenic nematodes in the Northeast and has established the seasonal population growth curves for these nematodes (Wick, 1989). Using this information, diagnosticians can tell which nematodes are likely to cause damage, the number of nematodes associated with damage and the likely time of year that the damage will be most noticeable. One of the major pieces of missing information, however, is being able to predict which courses will have nematode problems and in what particular years. Not every golf course has a nematode problem, and nematode populations do not reach dramatically high levels every year, even on those courses that frequently have high nematode populations.

During the past three years, we have been conducting a project to examine the factors that can contribute to high nematode populations on golf course putting greens. During 2003 and 2004, soil samples were taken from 38 golf courses in southern New England (three greens each) during May, July and September. Nematode numbers were counted, and a wide array of cultural and environmental factors were recorded. Once all the data was obtained, it was statistically analyzed to determine what factors, if any, might lead to increased nematode populations.

Figure 1. Changes in nematode population in 2003 and 2004 expressed as nematodes per 100 cubic centimeters (cc) of soil.

Initially, we examined data to determine how populations of different nematode species changed over the course of a year and between two different years (Jordan and Mitkowski, 2006). Surprisingly, there was a major statistical and biological difference between total nematode numbers between 2003 and 2004, with 2004 having much higher total average nematode numbers, regardless of the species of turf parasitic nematode. By examining the graphs in Figure 1 it is apparent that most nematode populations increased into September, with the exception of theTylenchorynchus(stunt) nematodes. These findings agree with what is generally observed on golf course putting greens: a steady increase in nematode numbers that often peaks at some point in August or September.

Unfortunately, it is difficult to determine exactly why nematode populations were so much greater in 2004 than in 2003, an entirely unexpected result. Two plausible explanations are rainfall amount and temperature. Spring 2003 was extremely wet. Soil-borne nematodes require oxygen and excessive precipitation is likely to keep soil oxygen low, thereby reducing nematode viability and fecundity.

However, temperature may have played an even more significant role in nematode survival. While nematodes in temperate climates can survive the freezing temperatures of Northern soils, the fluctuation in temperatures plays an important role in their survival. Although the average temperatures for winter 2003 and winter 2004 were similar, temperatures in 2003 varied widely throughout the winter. The fluctuation in temperature was dramatic, and little ground froze. In 2004, however, temperatures changed gradually, and some areas of southern New England had frozen ground to a depth of 3 feet. We expected to see fewer nematodes in 2004, as it seemed to be a harder winter. The opposite was true. Because nematodes acclimate to their environment over weeks or months, it is possible that a hard (or consistently cold) winter allows them to maintain a regular level of dormancy.