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NC1035: Practical Management of SCN and Other Nematodes of Regional Importance: With Special Reference to Invasive Biology

Statement of Issues and Justification

The North Central Region (NCR) includes the major production areas of corn, soybeans, and small grains in the US. All of these crops are susceptible to highly virulent and invasive plant-parasitic nematodes. Fortunately, the soybean cyst nematode (SCN), Heterodera glycines, is the only one widely distributed through the region. SCN is the major yield-limiting pathogen of soybean, a crop that contributes $18 billion annually to the economy of the nation. Management tactics currently are limited to the use of nonhost crops, resistant soybean cultivars, and one or two soil-applied nematicides. Unfortunately, each of these management tactics has serious shortcomings. There is an urgent need to improve and integrate management tactics for control of this widespread, persistent, yield-decreasing soybean pest. It is our aim to conduct region-wide coordinated research directed toward reducing losses due to SCN.

In addition, SCN is widely considered to be a classic example of an invasive species and therefore a good model for studying the fundamental dynamics of invasive species to avoid major detrimental economic effects from other nematode pests not yet of major significance in our region. Examples of such pests include nematodes such as the corn cyst nematode, cereal cyst nematode, potato cyst nematode, potato rot nematode, and Columbia root-knot nematode, which are not established throughout the NCR, but which have the potential to do so: they are established in surrounding regions. It is imperative, therefore, for us to understand the fundamentals of invasion biology as exemplified by SCN.

In order to effectively address the current and potential economic consequences of plant-pathogenic nematodes in the NCR, we have developed three near-term objectives for coordinated regional research:

1) To develop, evaluate, improve and integrate management techniques for soybean cyst nematode in the NCR to increase grower profitability;

2) To better understand and apply the concepts of invasion biology as revealed by SCN epidemiology;

3) To develop a decision-support database for management of SCN and other regionally important nematodes.

The specific approaches identified to achieve these objectives fit very well within the following four NCRA Priority Research Objectives for Integrated Pest Management:

Soybean cyst nematode management

Although a number of different management tactics have been investigated (Niblack and Chen, 2004), the only ones that consistently increase yields in infested fields or reduce SCN population densities, or both, are rotation to nonhost crops (such as corn) and the use of resistant soybean cultivars. The recommendation to grow two or more consecutive years of nonhost crops for management of the nematode is of limited utility because many of the fields in the NCR have corn and soybean grown in alternating years for economic and agronomic reasons; therefore, planting multiple years of nonhost crops for nematode management purposes often may not be an option. In the NCR, in contrast to the southeastern US, cultivation of a nonhost reduces SCN population densities only about 35 to 50% and overwinter survival is typically 100% (Jackson et al., 2006).

There are hundreds of soybean cultivars available with resistance to SCN, but more than 90% of the resistant cultivars contain resistance derived from the soybean plant introduction (PI) 88788 (Diers et al., 1998; Shier, 2005; Tylka, VIPS). Only a few soybean cultivars possess resistance from the soybean lines PI 548402 (Peking) and PI 437654 (the source of resistance in the CystX® germplasm). Soybean cultivars with resistance derived from PI 88788 and PI 548402 allow some level of reproduction by most SCN populations. Consequently, selection for populations of the nematode that can readily reproduce on the resistant soybean cultivars can occur when resistant cultivars are used repeatedly. For example, in a 1990 Illinois survey, researchers found that only about 34% of SCN populations in the state could attack PI 88788 and cultivars derived from it (and marketed as resistant), and none of these nematode populations were considered strong, i.e., able to cause yield loss (Sikora and Noel, 1991). By 2005, based on a more extensive survey, more than 85% of the SCN populations were able to attack PI 88788 and derived cultivars (Niblack, unpublished). Growers with high SCN population densities in their fields are advised to rotate sources of SCN resistance, if possible, in order to reduce selection pressure (Niblack, 2005); however, little if any information is available on the long-term efficacy of this approach.

Nematicides also are an option for SCN management, and one or more are labeled for this purpose in most NCR states. But use of these pesticides costs $25 to $35 per acre, and there is little evidence that they consistently increase soybean yields sufficiently to pay for their use (Smith et al., 1991). Additionally, increases in SCN population densities can be measured in the fall following nematicide use at planting, which makes use of these compounds a recurring proposition and militates against their use unless other options have been exhausted. For the immediate future, use of nonhost crops and resistant soybean varieties will probably remain the most viable options for management of SCN.

Although there are some management options available, much work is needed to broaden and stabilize our management of this persistent, widespread, devastating soybean pathogen, particularly in light of its ready adaptation to resistant cultivars.

SCN as a model for invasion biology

Severe nematode pathogens of corn and small grains, and soybean pathogens other than SCN, occur elsewhere in the United States and have yet to become well established and of major significance in the NCR states. These nematodes may affect important crops directly, and also indirectly through interactions with other pathogens, as does SCN. Their appearance may also have economic consequences due to phytosanitary regulations and the trade restrictions placed on localities known to sustain populations of regulated nematode species. A current example is the detection of infestations of a potato cyst nematode (Globodera pallida) in Idaho; the consequences of this invasion by a quarantined pathogen are yet to be determined as of this writing (2006). It is imperative, therefore, for us to understand the fundamentals of invasion biology in relation to SCN and other nematodes that have major detrimental economic potential as invasive species.

SCN is a classic example of an invasive species. In the United States, SCN is an exotic. Its origin is suspected to be northern China, where it had been observed for 50 years prior to its first known appearance in the US. SCN was first reported in North Carolina in 1954. During the following three years, it was reported from Tennessee, Arkansas, Kentucky, Mississippi and Missouri (Riggs, 2004) and was the object of a federal quarantine until the early 1970s. Being easily transported by any means capable of moving soil, this major plant pathogen is now well established in 39 states, including every state of the NCR. Rapid spread of SCN can be attributed to a number of invasive characteristics: relatively short age to first reproduction, high fecundity, dispersal ability, and capacity to survive drying in the protective cyst stage. Like many invasive species, SCN most likely escaped specialized predators and parasites during its initial establishment in North America. The rapid expansion of soybean production in the US was accompanied by management practices that facilitated the spread of this pathogen by moving soil and seed across broad geographic regions.

In this regional research project, SCN will be used as a model system for development of the principles of invasion biology of soil-borne pathogens and soil invertebrates in general (Shea and Chesson, 2002). This knowledge will be used to develop new and innovative practices to both minimize losses caused by SCN and decrease risk to other invasive species that are not currently of significance in the region.

Decision-support database

Useful nematode management information for farmers in the NCR is available from a wide variety of sources, both public and private. Unfortunately, that information is also fragmentary and may be difficult to locate. For example, due to large screening programs in Iowa and Illinois, and smaller programs in other states, we have access to specific data about the actual levels of resistance in hundreds of soybean cultivars labeled as resistant by the companies who produce them. An effort to coordinate these programs and to simplify access to the data would be of tremendous value to soybean farmers. In addition, such coordinated efforts are of great value to researches because they highlight areas that are in need of concentrated, collaborative research. Our aim for this objective is to identify and coordinate databases that will be of value to nematode management strategies in the NCR states.

Conclusions

The current and future threats to the economy of the NCR states represented by plant-pathogenic nematodes such as SCN require a collective approach - collective in the sense of comprising research and extension nematologists from each state, and in the sense of taking both fundamental and applied approaches to the research questions outlined in this proposal and giving our outreach responsibility the status of a separate objective. As a group, we have the expertise, collaborative relationships, and commitment required to address and achieve the objectives we have proposed.

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