Location: Crop Protection and Management Research2008 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
Progress relates to National Program 303 Plant Disease, Component 2 Biology, Ecology, Epidemiology, and Spread of Plant Pathogens and Their Relationships with Hosts and Vectors, Component 3 Plant Disease Resistance and Component 4 Biological and Cultural Strategies for Sustainable Disease Management. For peanut, we completed a study to evaluate breeding lines and interspecific hybrids for resistance to the three main root-knot nematodes attacking peanut. We identified possible new sources of resistance to these nematodes; a manuscript is in press. For cotton, lines with resistance to root-knot nematodes were developed previously in this project and were evaluated in 2008 for yield, fiber quality, and nematode resistance to support future public release as germplasm lines for use by breeders. We planted resistant and susceptible lines (developed earlier in our project) that are genetically identical except for the presence of resistance genes; these will be used as a tool to determine whether the resistance genes (or DNA linked to the resistance genes) have any unintended effects on plant growth, yield, or fiber quality. To identify potential new sources of resistance to root-knot nematodes in cotton, we crossed two resistant germplasms with a susceptible germplasm and evaluated segregation of resistance in the offspring. DNA from all the offspring was also collected, and we are in the process of using markers for previously identified resistance genes to determine if resistance genes in these plants are located in a different place on the chromosomes and, therefore, likely to be different genes. We completed the first trial of an experiment to determine whether incorporation of winter rye reduces root-knot nematodes and weeds in the succeeding cotton crop. Nematodes populations were not suppressed in any of the rye treatments (4A). Densities of pigweed, however, were lower in treatments with rye (both strip-tilled and incorporated). In a greenhouse experiment to determine whether fungicides used in peanut reduce natural biological control of nematodes, we showed that only azoxystrobin increased nematode populations in a suppressive soil (4B). In a greenhouse experiment to determine the mechanism by which root-knot nematodes increase aflatoxin contamination in peanut, we found that nematode infection of the roots led to an increase in aflatoxin in the kernel suggesting a physiological effect on the plant (also NP108 - Component 2.1.2). A field experiment studying how drought stress and nematode stress interact to affect cotton yield and fiber quality was concluded. Preliminary analysis indicates that the combined yield loss from drought stress and root-knot nematode stress was equal to the sum of damage from the individual stresses (an additive effect). An experiment to determine the effects of a nematode susceptible weed infesting a nematode-resistant cotton crop was concluded. The weed reduced the nematode-suppressive effect of the resistant crop in proportion to the number of weeds present.
1. Root-knot nematode resistance in peanut: Three root-knot nematode species parasitize peanut in the USA--Meloidogyne arenaria (Ma), M. hapla (Mh), and M. javanica (Mj). Some commercial cultivars have resistance to Ma and Mj, but no peanut cultivar or released germplasm has resistance to Mh. Sources of resistance to all three nematodes are needed for broad and durable resistance to root-knot nematodes. ARS and University of Georgia scientists in Tifton, Georgia, tested cultivars and breeding lines for resistance to the three nematode species and for a molecular marker (197/909) linked to a known resistance gene. Three breeding lines were identified with high levels of resistance to Mh, with two lines also having resistance to Ma and Mj; none of these lines contained the 197/909 marker indicating that they might be different resistance genes. These breeding lines will be valuable for developing peanut cultivars with resistance to multiple species of root-knot nematodes, which will improve the durability of resistant cultivars. National Program 303, Component 3 Plant Disease Resistance, Problem Area 3B Disease resistance in new germplasm and varieties.
5. Significant Activities that Support Special Target Populations
Dong, W., Holbrook Jr, C.C., Timper, P., Brenneman, T.B., Mullinix, B.G. 2007. Comparison of methods for assessing resistance to Meloidogyne arenaria. Peanut. J. of Nematology 39(2):169-175.