Location: Crop Genetics and Breeding Research
2018 Annual Report
Objectives
1. Identify and characterize nematode resistance genes and work with breeders to combine them with commercially valuable agronomic traits in cotton and peanut.
1.A. Determine the phenotypic expression of Meloidogyne arenaria resistance in peanut isolines 48 (moderate resistance) and 46 (high resistance).
1.B. Evaluate the phenotypic expression of the Meloidogyne incognita resistance QTLs qMi-C11 and qMi-C14 in cotton isolines.
1.C. Identify sources of resistance to Meloidogyne incognita in cotton that differ from Auburn 623 RNR.
1.D. Identify specific Meloidogyne-resistance genes within quantitative trait loci (QTL) regions and determine their functions in cotton and peanut.
2. Evaluate antagonist-nematode interactions, and develop novel integrated strategies, including biological control methods for management of nematodes in cotton, peanuts, and biofuel crops.
2.A. Evaluate environmental factors that influence Pasteuria penetrans endospore movement in soil and attachment to nematodes.
2.B. Monitor changes in adhesion phenotypes of Pasteuria penetrans to determine the drivers of phenotypic/genetic changes occurring in a population of the bacterium and its host, Meloidogyne arenaria.
2.C. Evaluate factors that affect the general suppression of Meloidogyne spp. in field soil.
2.D. Evaluate integrated management options including resistance, suppressive cover crops, and an improved decision model for managing Meloidogyne incognita.
Approach
Field and greenhouse experiments will be conducted to develop management options for root-knot nematodes in cotton and peanut. We plan a multi-tactic approach utilizing host-plant resistance (Objective 1), crop rotation, antagonistic crops, seed treatments, and biological control (Objective 2). Host-plant resistance to nematodes is the cornerstone of our strategy. We will determine mechanisms of resistance in cotton and peanut, determine effects of nematode resistance genes on the Fusarium wilt disease complex in cotton, and try to identify new resistance QTLs in cotton. However, we cannot rely exclusively on host-plant resistance for managing nematodes. 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 nematodes. Specifically, we will evaluate factors that influence the ability of the nematode-parasitic bacterium Pasteuria penetrans to suppress nematodes; determine whether frequency-dependent selection occurs between the bacterium and its host; and determine whether considering P. penetrans abundance improves nematode management decisions. We will evaluate the effects of winter cover crops on the natural suppressiveness of soils to nematodes; evaluate integrated management options including combining high residue rye with resistant cotton cultivars and nematicidal seed treatments; and evaluate nematode suppression and crop damage in the novel crop rotation of cotton with double cropped sweet sorghum (summer crop) and sugar beet (winter crop).
Progress Report
We are finishing two manuscripts documenting differences in the effects of two root-knot nematode resistance quantitative trait loci (QTLs) in cotton on nematode development and reproduction, as well as on the relative levels of nematode penetration into the roots and subsequent egression if they fail to establish a feeding site. The two QTLs have different effects, which is why combining the QTLs leads to more effective nematode suppression. Understanding the modes of action of the two QTLs allows us to determine if resistance genes identified in the future have unique modes of action.
Evaluations of the effects of the QTLs imparting resistance to root-knot nematodes in cotton on the interaction of the nematodes with the Fusarium wilt fungus have been completed. Although additional study is warranted, we found that the QTLs did not affect the interaction and did not reduce the incidence or severity of Fusarium wilt. Our results suggest that host plant resistance to the nematode is not an effective control of Fusarium wilt.
The second field trial evaluating whether the nematode resistance QTLs in cotton impart any deleterious effects (linkage drag) on yield or fiber quality have been established. A companion study evaluating the individual contribution of each QTL to reducing damage from nematode infection has been established.
We collected 20 cotton germplasm lines previously identified as having some level of root-knot nematode resistance for evaluation to identify new QTLs for resistance to the nematode. With the assistance of a Brazilian collaborator, we are attempting to locate about 20 additional genotypes in Brazil. We are currently attempting to increase the amount of seed we have of each genotype to allow replicated evaluations.
We completed the first cycle of a multi-year crop rotation sequence. The first summer crop (sweet sorghum in 2016), winter crop (sugar beet in 2016-17), and second summer crop (cotton in 2017) were completed. A second rotation cycle beginning with sweet sorghum in 2018 has been started.
We have made numerous thin sections of stained roots to look at the expression of resistance in peanut. We have observed signs of a hypersensitive response (HR) in isoline 46 (high level of resistance) and Tifguard peanut. We have not observed an HR in isoline 48 (moderate resistance) or the susceptible peanut; however, we are continuing to make thin sections because finding the nematode and its feeding site in these sections is very difficult.
We had hypothesized that winter cover crops would serve as a source of carbon and nitrogen to stimulate the soil community leading to greater suppression of root-knot nematodes compared to winter fallow. Although the soil community suppressed survival of root-knot nematode juveniles (70% suppression compared to heated soil), we did not observe any difference in survival between treatments containing either clover or rye and fallow soil.
The effect of root exudates on attachment of Pasteuria spores to root-knot nematodes was recently published.
Using Pasteuria incidence to predict nematode damage in cotton: In the fall of 2016, our plots differed in the incidence of Pasteuria from 59% to 9%. In the summer of 2017, we had severe mortality of cotton due to Fusarium wilt predominantly in the plots containing low levels of Pasteuria. In the fall, we did not observe a relationship between Pasteuria incidence or root-knot nematode densities in the fall (2016) and cotton yield (2017). This lack of a relationship was likely due to the confounding effects of Fusarium wilt.
We have established the 3rd year of a field study to evaluate different combinations of treatments for management of root-knot nematodes in cotton. The treatments are winter cover crop for nematode suppression (fallow, rye, and high residue rye), cotton cultivars with and without resistance to root-knot nematodes, and seed treatments with Avicta on the susceptible cotton.
Accomplishments
1. Two resistance genes in cotton have different modes of action. Two genes that impart resistant to the cotton root-knot nematode (Meloidogyne incognita) have been identified. ARS researchers at Tifton, Georgia and researchers at the University of Georgia, demonstrated that the two genes have different modes of action with one gene inhibiting an early stage of nematode development and the other gene inhibiting a later stage. Combining genes with different modes of action increase the durability and level of resistance in plants, and understanding the modes of action of the two genes will allow researchers to search for additional resistance genes with a third mode of action to be included in cotton breeding programs.
2. Crop pest and biocontrol agent locked in arms race. The most powerful and unpredictable aspect of biological control strategies is that both the target pest and it's control agent have the potential to evolve and even coevolve. An ARS researcher at Tifton, Georgia, led a team of researchers from Emory University and the University of Georgia that found rapid changes in host specificity of populations of the biocontrol agent Pasteuria penetrans, a bacterium that parasitizes root-knot nematodes, which are major agricultural pests. These observations indicate that as the nematode becomes resistant to Pasteuria attachment, the bacterium changes so that it can attach to the nematode in a process that is akin to an arms race. The study demonstrates rapid, local evolution of a biological control agent allowing them to overcome resistance in the host and more effectively control root-knot nematodes.
3. Sting nematode in cotton is often involved in Fusarium wilt disease. Sting nematodes are a damaging but infrequent pathogen of cotton. Fusarium wilt is a serious disease of cotton that was believed to result from the interaction of the fungal pathogen with the cotton root-knot nematode, Meloidogyne incognita, and recommendations for controlling the disease required controlling the root-knot nematode. In a multi-county survey over two years of individual plants showing Fusarium wilt symptoms, ARS researachers at Tifton, Georgia and researchers at the University of Georgia, demonstrated that sting nematodes were frequently associated with Fusarium wilt often in the absence of the root-knot nematode. These results prove that the sting nematode frequently interacts with the Fusarium wilt fungus to cause disease and must be controlled to minimize Fusarium wilt in cotton.
Review Publications
Liu, C., Timper, P., Ji, P., Mekete, T., Joseph, S. 2017. Influence of root exudates and soil on attachment of Pasteuria penetrans to Meloidogyne arenaria. Journal of Nematology. 49:304-310.
Timper, P. 2017. Rye residue levels affect suppression of the southern root-knot nematode in cotton. Journal of Cotton Science 2017. 21:242-246.
Morris, K.A., Langston, D.B., Davis, R.F., Timper, P., Li, X., Grey, T.L. 2017. Fluensulfone sorption and mobility as affected by soil type. Pest Management Science. 74:430-437. https://doi.org/10.1002/ps.4724.
