Location: Crop Protection and Management Research2009 Annual Report
1a. Objectives (from AD-416)
New (additional) objectives per PDRAM memo #HQ01d dated July 5, 2007: 1) Determine whether increased aflatoxin production in nematode-infected peanuts is due to a greater percentage of immature kernels, and the role of nematode infection of roots vs. pods. 2) Determine whether nematode-resistant peanut genotypes reduced the risk of preharvest aflatoxin contamination in soil infected with root-knot nematodes. Identify, characterize, and move genes for resistance to Meloidogyne spp. into cotton and peanut germplasm and cultivars. Utilization, mechanisms, and interactions of classical and contemporary methods in integrated nematode management. Enhance native and introduced antagonists of nematodes in cotton and peanut cropping systems.
1b. Approach (from AD-416)
Field and greenhouse experiments will be conducted to improve management of plant-parasitic nematodes in cotton and peanut. The approach will be multi-tactic including host-plant resistance, antagonistic crops, and biological control. Host-plant resistance to root-knot nematodes is the foundation of our nematode management strategy. Cooperative research will be conducted with plant breeders to develop cultivars and germplasm of peanut and cotton with desirable agronomic traits and a high level of nematode resistance. Plant material will be selected for resistance using traditional and marker-assisted selection. Durability of resistance genes is an important consideration in our research. Towards this end, we will search for new nematode resistance genes to deploy with previously identified resistance genes and determine the frequency and distribution of a species (the northern root-knot nematode) capable of reproducing on resistant peanut. We will also investigate ecologically-based control strategies that can be integrated with resistant cultivars to increase the durability of resistance and control a broader spectrum of nematode pathogens. Specifically, we will determine the potential of Bt toxins, antagonistic cover crops, and antagonistic microorganisms to reduce root-knot and reniform nematode populations. Central to an effective management strategy is a thorough understanding of how nematodes interact with biotic and abiotic factors to reduce crop yield and quality; therefore, we will examine the interaction between root-knot nematodes and water stress, weeds, and fungi that produce aflatoxins. These studies will result in ecologically-based, cost-effective management options to reduce nematode populations, reduce damage from nematodes, and foster natural biological control.
3. Progress Report
We completed the final year of testing for the release of a cotton germplasm line that has a high degree of resistance to the cotton root-knot nematode. We are currently finishing the final greenhouse evaluations of resistance prior to a germplasm release. This highly resistant germplasm has yield and fiber qualities that are superior to other resistant germplasm and are similar to many modern cultivars. We completed a second year of evaluating the yield and fiber quality of cotton isolines, which are genetically the same except for nematode resistance, to maximize the yield and fiber quality within each isoline. We created the isolines so we could evaluate whether incorporation of resistance introduces any undesirable traits such as reduced yield or fiber quality. We made significant progress in identifying new sources of resistance to root-knot nematodes in cotton germplasm by screening resistant plants with DNA markers for the known resistance genes. So far we have identified one germplasm line with previously unidentified resistance genes. A second line is still being analyzed. One trial of a greenhouse study to evaluate the effect on root-knot nematodes of the Bt toxin produced by transgenic cotton plants was completed, and the toxin had no effect. That project will not be continued because the lack of effect is already widely accepted as true by scientists and farmers, and therefore this project is no longer a productive use of our resources. We completed a field study to determine whether incorporation of winter rye reduces root-knot nematodes and weeds in the succeeding cotton crop. Nematode populations were not suppressed in any of the rye treatments. However, densities of pigweed in the strip tillage plots were lower in treatments with rye than in winter fallow. In a greenhouse experiment, the fungicide azoxystrobin increased nematode populations in both a suppressive soil and the same soil heated to remove fungal antagonists of the nematode. These results suggest that the fungicide may have a direct effect on the nematode or the host plant. In a greenhouse study, an antibiotic-producing bacterium (Wood1R), applied as a seed treatment, reduced numbers of root-knot nematodes on corn, cotton, and soybean in sterilized soil, but was not effective in reducing nematode numbers in natural soil. In an ongoing field study, populations of predatory nematodes were dramatically reduced throughout the year in plots treated with the fumigant nematicide Telone; however, the reduction in predators did not lead to greater survival or reproduction of root-knot nematodes. Boll mapping data (also known as box picking data), which shows how cotton bolls are distributed on a plant, was collected from a field experiment to study how root-knot nematode stress and drought stress interact to affect cotton yield and fiber quality. The data from multiple years has been compiled, but it has not yet been analyzed. In a greenhouse experiment to determine the mechanism by which root-knot nematodes increase aflatoxin contamination in peanut, root galling by the nematode was very low and did not influence aflatoxin levels.
1. Crop choice affects an antagonist of root-knot nematodes. The peanut root-knot nematode (PRKN) can cause severe yield losses in peanut. The bacterium Pasteuria penetrans is an obligate parasite of these nematodes and is commonly found at low densities in peanut fields throughout the southeastern U.S. Continuous planting of peanut can greatly increase abundance of Pasteuria spores and suppression of PRKN; however, peanut yield is diminished by diseases under this cultivation practice. ARS research in Tifton, GA showed that in a three-year rotation, densities of Pasteuria spores increased to a higher level when peanut was rotated with another host (eggplant) for PRKN than when peanut was rotated with a nonhost (cotton), but peanut diseases were not increased. Peanut growers can use this rotation strategy (rotating with host rather than nonhost crops for PRKN) to promote nematode-suppressive soils by native or introduced Pasteuria spores.
2. New cotton germplasm has high yield, quality, and root-knot nematode resistance. Root-knot nematodes cause more damage to the U.S. cotton crop than any other pathogen, and host plant resistance is the most consistently effective means of minimizing the losses. Germplasm with a high level of resistance to root-knot nematodes has been available to cotton breeders for more than 30 years, but the yield and fiber quality of those lines is much less than that of contemporary germplasm. After 8 years of breeding and selection, we successfully created a germplasm line with a high level of resistance but which has yield and fiber quality similar to modern cotton varieties. This germplasm is an improvement over previous resistant germplasm lines and will be a valuable resource for cotton breeders in developing cultivars with resistance to root-knot nematodes.
Olson, D.M., Davis, R.F., Wackers, F.L., Rains, G.C., Potter, T.L. 2008. Plant-herbivore-carnivore interactions in cotton, Gossypium hirsutum: Linking belowground and aboveground. Journal of Chemical Ecology. 34:1341-1348. DOI:10.1007/s10886-008-9532-1.