S1046: Improved management of plant-parasitic nematodes through modern diagnostic tools and increased use of host resistance
Statement of Issues and Justification
Plant-parasitic nematodes have been referred to as the 'Hidden Enemy' of crops because the losses resulting from these plant pests are often over-looked and under estimated. Yet nematodes are among the most wide-spread pests affecting crop production, especially in the southern United States where the warm climate and abundance of sandy soils provides an ideal environment for this group of pests. As a group, nematodes are estimated to cause 10-14% yield losses annually (Sasser & Freckman, 1987). They are especially important on cotton (Koenning et al., 2004), peanut (Dickson, 1998), soybean (Kinlock, 1998), turf (Crow, 2005), and numerous vegetable crops (Johnson, 1998). Although several species of nematodes are recognized as important pests of individual crops, the root-knot nematodes (Meloidogyne spp.) (Perry et al., 2009) are widely considered to be the most important nematode pests because of their extensive host range, with one or more species able to parasitize nearly every crop grown in the United States. The soybean cyst nematode (Heterodera glycines) is also a major pest because of its aggressive behavior and widespread distribution in soybean production areas of the South and the Midwest. Recently the reniform nematode (Rotylenchulus reniformis) has received increasing recognition and attention as a pest of several crops, especially cotton (Koenning et al., 2004).The importance of plant-parasitic nematodes and the losses they cause is substantiated by the level of financial support given by stakeholders to the research efforts of many of the scientists that will be involved in this project. These stakeholders include Cotton Incorporated, the National Soybean Board, the National Peanut Board, and state level growers associations for each of these major crops. Much of the support for the proposed research will come directly from these stakeholder groups. Historically, nematodes were controlled (when growers were cognizant of their effects on crop yields) by the application of either a fumigant nematicide or one of a few granular insecticides that also had nematicidal activity. All of these products have been shown to present significant hazards to both environmental and human health. The recognition of these risks have resulted in either a loss of registration or greatly increased restrictions on the use of these pesticides. No new nematicide or nematicidal chemistry that has proven effectiveness has been developed in nearly 30 years to fill this void. A few new products have been developed, such as the use of abamections as a seed treatment but with limited success (Faske & Starr, 2007). Crop rotation has the potential to suppress nematode populations but is not widely practiced by growers. The wide host range of several important nematode pests limits the number of crops that a grower can use in rotation with their primarly crop of interest to suppress nematode populations because effective rotation crops are frequently of lesser economic value (ie, velvetbean for management of root-knot nematodes on soybean; Weaver et al., 1998). Because of the intensive crop production systems utilized by many growers, if a rotation crop is to be adopted it must have substantial benefit beyond just suppression of nematode pests.
Biological control has received much attention as an ecologically sound approach to the management of nematode pests. The limited mobility of nematodes and the relative stability of the soil environment suggest that biological control has significant potential. Unfortunately, this potential has been difficult to achieve. It has proved difficult to stimulate activity of fungal species that parasitize nematodes or to successfully introduce sufficient numbers of propagules of these agents into a large area to effect suppression of the desired nematode population. The bacteria Pasteuria spp. are obligate parasites of several nematode species and have effectively suppressed nematode populations in several tests (Dickson, 2004). However, large scale application is hampered by the inability to culture the bacteria in quantities needed for commercial purposes.
Several scientists from across the Southern and Midwestern United States are currently researching the use of host resistance as a corner stone for the sustainable management of plant parasitic nematodes. Several notable successes have been achieved as a result of the collaborative efforts of scientists participating in the S-1015 multistate project. These include introgression of resistance to M. arenaria and M. javanica into peanut cultivars adapted to both the southeast (Georgia) and western (Texas) production regions. These new peanut cultivars also have the highly desirable and new traits of improved resistance to other pathogens and a high ratio of oleic to linoleic fatty acids. Finally, scientists in Georgia have utilized the initial efforts from scientists in Texas to map the nematode resistance gene in peanut (research completed in Texas) to develop a high through put marker-assisted selection system. Similarly, groups of scientists from Alabama, Georgia, Mississippi and Texas have collaborated on efforts to identify, characterize, and develop sources of resistance to both the reniform and root-knot nematodes in cotton. Genetic relationships among several genes for resistance to the root-knot nematode are being characterized and several genes are being mapped. In other studies, new sources of resistance to root-knot nematodes are being identified. Similarly, several scientists have collaborated to screen different collections of Gossypium germplasm for resistance to reniform nematodes, have identified several sources of resistance, and are working to introgress these different sources of resistance into upland cotton. Because plant breeding efforts such as those being pursued by current project members are, by their very nature, long term projects it is important that our project be renewed to enable us to build on our past successes. The multistate effort has been successful because it has allowed scientists to share resources, especially germplasm, and avoided duplication of efforts.
Additionally, we seek to expand our efforts to include a renewed emphasis on the identification of nematode species, with special emphasis on Meloidogyne spp. As little as 25 yr ago there were approximately 30 recognized Meloidogyne spp. All but four species were considered to be of minor importance in the southern United States. It has long been accepted that M. arenaria, M. javanica, and especially M. incognita were the most wide spread and economically important root-knot nematodes. Meloidogne hapla was also important in the cooler climates, at higher elevations, and in the northern edge of the region. At present, however, there are nearly 100 valid species, many of which are becoming recognized as widely distributed in the southern United States (Hunt and Handoo, 2009). Some of these species are virulent on crops with resistance to the four species which have heretofore received the most attention from researchers. Because host resistance is typically a species, and sometimes a race-specific trait, it is increasing important that we continue to develop diagnostic tools for rapid identification of many of the newly described species known to exist in our region and considered to be of economic importance. Future successes in the development and deployment of resistant crop cultivars will be dependent on knowledge of the distribution of many more Meloidogyne spp. than were previously recognized.
The proposed research addresses important agricultural crop management issues as identified by the REE Advisory Board. In the Report on Agriculture Research Needs of Commodity Crops , the REE Advisory Board states that sustainable, efficient production of these staples (including cotton, soybean, and peanut) is critical to the US food supply and economy. They further list as top priorities, nematode resistance in cotton and improved varieties and disease resistance in peanut. (http://www.ree.usda.gov/nareeeab/reports/070609commoditycrops_rpt.pdf).
If this research is not conducted, growers will continue to see a shrinking profit margin due to greater production costs and yield losses due to nematodes. The deployment of nematode-resistant crops will lower production costs by reducing the need for nematicide applications and less profitable crop rotations while at the same time improving yield by reducing losses due to nematodes. Additionally, the reduction in use of nematicides such as the carbamate aldicarb and the fumigant 1,3-dichloropropene with reduce the potential for contamination of the environment and adverse effects to public health. An economic analysis of the implications of nematode-resistant crops will be evaluated through budgeting analysis. Crop enterprise budgets will be developed for traditional crop production and for nematode-resistant crops, comparing the gross returns and net returns (returns to land).
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