Project Summaries

04-518TX  Project Manager: D. C. Jones

IMPROVING WEST TEXAS COTTON - DISEASE

Robert J. Wright and Jane Dever, Texas AgriLife Research; Thea Wilkins, Texas Tech University; Terry A. Wheeler, Texas A&M University

Our first objective was to assess resistant and susceptible germplasm against Verticillium dahlia. Six producer fields that have significant levels of Verticillium wilt were used to evaluate commercial cultivars. Each trial contained 32 lines (60 to 75 will be examined in 3 to 6 different locations) grown in a randomized complete block design (RCBD) with 4 replications of 2-row plots 35 ft in length. An assessment of wilt incidence, defoliation, yield, HVI fiber characteristics and loan value for each cultivar was done. Sites for 2012 were in Floydada (Ron Graves), Plainview (Glen Shur), Littlefield (Blake Davis), Wilson (Bruce Lehman), Seminole (Chuck Rowland), and Garden City (Mitchell Janis). Each of these sites had a history of moderate to high densities of Verticillium dahliae. All sites were successfully planted (or replanted) and stand counts were collected. Wilt incidence was measured at all sites, and in three of the sites, Floydada, Plainview, and Garden City, significant levels of wilt developed. Wilt was < 10% for the Seminole site, which had somewhat lower densities of V. dahliae, and very low for the Wilson and Littlefield sites. The Littlefield site appeared to have developed irrigation problems. The Wilson site was chosen to demonstrate the value of drip irrigation in managing Verticillium wilt. This site had high pathogen pressure, but developed almost no wilt. Defoliation ratings were made for the sites with good wilt incidence. All sites were harvested successfully. Results for the three sites with significant wilt and three sites without wilt were reported. F1 seed of eight new crosses for Verticillium wilt resistant populations were sent to Mexico winter nursery for increase to F2 generation. The 2012 nursery included 68 F5, 8 F4, 37 F3 and 48 F2 progeny rows; and 10 new sources of resistance. The Verticillium wilt nursery was rated for incidence of disease symptoms and defoliation of the plants. Individual plant selections (280) were harvested as well as five whole rows for 2013 multi-location performance testing. Disease ratings were fairly low overall, due to the high temperatures in 2012. We have therefore setup greenhouse studies this winter to assess resistant and susceptible responses when challenged to pathogen infection. Initially, all of the new lines in 2012 preliminary strains testing will be screened for Verticillium wilt resistance (~30 entries). Selections made in the nursery were also screened in the greenhouse. Twenty advanced strains, 30 intermediate strains and 30 preliminary strains were analyzed in multi-location performance testing in 2012. Strains tests were not ginned as yet by January, 2013, so data are not available. Seven lines in 2012 preliminary testing were from the Verticillium wilt nursery, but new lines tested from all nurseries have been screened for resistance to Verticillium dahliae. CA 4002 (08-10-706) was officially released by the Texas A&M PRC, and has been accepted for publication in the Journal of Plant Registrations.

Our second objective was characterization of root-knot nematode (RKN) resistance in parental lines and SNP maps. In recent years a number of laboratories have investigated the genetics of resistance to RKN. Both the genetic and historical pedigree information suggests that only a few sources of resistance have contributed to breeding improvements. Most cultivars with resistance were developed for other cotton growing regions and thus were poorly adaptation to the Texas High Plains when tested. In recent years, we have been developing germplasm with improved resistance, adaptation, and storm-tolerance for the Texas High Plains. The resistance sources we've used can be traced to the historical sources and a few that yet have been characterized. Since a number of genetic markers diagnostic to the few resistant loci have been published, we tested the hypothesis that our resistant germplasm would have a higher than expect frequency of the diagnostic resistant allele(s). We tested 12 marker loci that mapped to two well characterized genes/QTLs that confer resistances to RKN across a panel of 94 germplasm lines. Only one published SSR marker has a weak association with resistant material. It is clear that the utility of this marker or any of the other published markers as the sole selection criteria (i.e. marker assisted selection) for resistant genotypes is low and that additional genetic tools are needed. The lack of this association could be to a number of factors but is likely due to a significant number of recombination events between the gene and maker loci. This would occur normally during the breeding process and suggest that markers more tightly linked to the gene are needed to ensure a higher level of confidence with the marker assisted selection process.

Progeny rows planted in the presence of RKN, including 85 advanced generation, 24 F3 and 10 new F2 populations were evaluated and all individual plant selections (248) examined in the greenhouse for root galling and reproduction. The RKN nursery had some very promising combinations of boll type, lint percent and fiber quality; 15 new lines were selected for multi-location performance testing in 2013 to add to two lines tested in 2012. Initial greenhouse assays have been completed, and while there are definitely tolerant lines among the nursery selections, some have less than desirable lateral root formation. Combining resistance with healthy root formation will be a breeding goal going forward. To examine the relationship of a DNA marker genotype with resistant phenotypes, published simple sequence repeats (SSRs) diagnostic to two characterized RKN resistant genes will be studied. Any association between marker genotype and resistant phenotype will be verified in 2013 using a segregating population. The expected result would be an enabling technology for marker assisted breeding of RKN resistance.

Our third objective was identification of recombinant individuals targeting the BRR5.1, BRR9.1, and BRR13.1 quantitative trait loci (QTLs). This phase of the study will increase the number of informative recombinants in each of the target QTLs. Progeny lines will be identified and selected that contain recombination events in at least one of the three target QTLs. Lines that contain a recombination event in a single target but fixed (homozygous) at the other two targets (i.e. for the susceptible allele) is a highly desirable outcome. These lines will enable us to more accurately estimate the phenotypic effect of each individual QTL. We have cataloged our seed and have identified several recombinant progeny to begin the fine mapping of each targeted region. A new PhD student Mr. Anh Bui began his studies in January 2013 and will conduct the research activities of this project. Mr. Bui is a student from Vietnam who received his M.S. from Wageningen University in the Netherlands and is supported by a VIED fellowship. To discover the desired density of recombination events (i.e. one per 100 Kb/one per 0.1 cM) for fine-mapping, a population 1000 F2 progeny will be genotyped using markers that flank each QTL region. This may allow discovery of 20 recombination events per cM (1 cM/0.05% recombination rate (1/2000 meioses). Putative recombinant plants will be transplanted to pots to produce F3 progeny of phenotypic assessment in 2012. If necessary, this step will be repeated until a desired number of recombinants are identified. Additional recombinants will be identified by expanding upon the population already developed. Sets of 200 F2 plants will be genotyped until the desire number of recombinant individuals is achieved.

Our fourth objective was to develop enabling technology to facilitate fiber quality improvement in breeding line development. From 2003 to 2009 a study was done to select divergent fiber quality traits in mutant populations of TAM 94 L-25 and Acala 1517-99 Upland cotton cultivars known to have good fiber quality. High and low selections were made for micronaire, strength, length, and AFIS maturity. The divergent phenotypically stable lines identified by this process should be ideal for both breeding studies and the development of molecular markers specific for these fiber quality characteristics. The selected mutant lines ranged from a mean low fiber length of 1.05 in. to a high of 1.28 in.; low fiber strength mean of 26.6 g/tex to a high of 35.2 g/tex; a low micronaire value mean of 3.5 to a high of 5.1; and low maturity ratio mean of 0.85 to a high maturity ratio of 0.97 across both populations. These highly divergent fiber quality lines will help to create high quality fiber upland cotton cultivars while maintaining high fiber yields and wide adaptation across the cotton growing areas of the U.S. This experiment will also hopefully result in the development and release of germplasm lines specific fiber properties.

In 2012, 29 M5 lines selected from TAM 94-L25 and 33 lines selected from Acala 1517-99 were compared to four check cultivars in replicated trials at Las Cruses, NM, Lubbock, TX, College Station, TX, and Stoneville, MS. By December of 2012, agronomic data was being analyzed by location and fiber quality samples were being prepared for submission to the FBRI Laboratory at Texas Tech University for HVI and AFIS fiber quality evaluation. This process will ensure that marking lines that are selected that produce stable fiber quality across a broad range of production environments.

 

Project Year: 2012
 

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