Consider Nematode Thresholds Before Treatment

Plant parasitic nematodes on golf courses present special problems for both disease diagnosticians and superintendents. In contrast to fungal diseases, which are unacceptable at any level, low to moderate levels of plant parasitic nematodes should be expected in any stand of turf. The difficult part of managing a potential nematode problem is determining when the nematode population is actually a cause for concern. The point at which nematode populations become a problem is called the population, disease or damage threshold. The threshold concept is used widely in plant pathology and is often the underlying mechanism of an integrated pest management strategy. Many factors contribute to nematode thresholds. While some of these factors are obvious, others are not. Additionally, interactions between these factors have the potential to dramatically influence a threshold. In short, thresholds are not as absolute as they may seem. Nematodes are aquatic organisms — even terrestrial nematodes live in films of water. Pathogens: A nematode populations species is commonly recognized as the most important consideration in determining a nematode threshold. However, it is extremely difficult to identify the majority of individual nematodes to species in any soil sample. As a result, identifications are usually made to the genus level even though management recommendations are made using numbers published for an explicit species. While this would seem to be a major problem, it is generally considered an acceptable practice. The biological differences between species within most genera of turf pathogenic nematodes are considered to be slight. For instance, few morphological differences exist between most turf pathogenic species of the lance nematode, Hoplolaimus, and the assumption is usually made that pathological differences are also few. Whether this assumption is true is debatable, and it points to a potential source of significant uncertainty. Is a published threshold for another species valid for the species of nematode found on your putting green? If experimental data doesn't exist, a guess may be in order. Such a situation rarely occurs in most other agricultural systems because the diversity of serious plant parasitic nematodes is much lower than that in turf stands. A cyst nematode on soybean is most likely to be Heterodera gylcines, and a positive identification is a relatively straightforward procedure. A root-knot nematode on lettuce in New York is almost certainly Meloidogyne hapla. In both of these cases, the thresholds are well-established for the identified species, and that species is the only one likely to be encountered. A stunt nematode on bentgrass, on the other hand, could easily be one of half-a-dozen different Tylenchorkynchus species, but it is unclear whether this fact is important in using established thresholds. Heterodera and Meloidogyne are unique examples on turf because, at least in the Northern states, no generally recognized thresholds are available for these two genera. While Heterodera is encountered sporadically, its damage potenContinued on page 52 IN THIS ISSUE