Project Summaries

06-908GA  Project Manager: D. C. Jones


Peng W. Chee and Edward Lubbers, University of Georgia

State surveys on nematodes reveal that major cotton-producing counties in Georgia have damaging levels. From 1991 to 1998 almost 100 thousand bales per year valued at a total of $300 million were lost (National Cotton Council, 1998).  Crop rotation, while a recommended cultural practice to lessen soil populations of RKN, is not an option for most Georgia growers because of the lack of suitable non-host crops with which to rotate their cotton. Therefore, inherent genetic resistance provides an attractive alternative to pesticides and crop rotation.

Poor profit potential of cotton production from yield stagnation and pest management costs call for creation of cultivars with genetic resistance to enhance economic returns for cotton producers. Insect, nematode, and weed pest management costs are among the highest expenditures growers face in cotton production, thus their reduction would enhance profitability of cotton production. Since Georgia is the second ranked producing state, cotton cultivars adapted for our rainfall patterns, soils types, and pests must be developed to give the best available genetics to producers.

Despite the widespread occurrence of RKN in Georgia and genetic resistance to RKN since 1974, private cultivar developers have previously exhibited little interest in fulfilling this need. Commonly cited reasons for the slow progress in developing RKN resistant cultivars is the current screening process is costly, tedious, time consuming and destructive for identifying resistant genotypes. Further, most breeding stations have neither the facilities nor personnel with expertise in nematology to carry out the screening process to identify resistant material.

Our objective is to develop Georgia-adapted, value-added cotton germplasm with RKN resistance. This will benefit the state's producers by providing increased yield and decreased production costs. In a previous project, Drs. Chee, May, and Davis developed advanced RKN parents from a backcross breeding populations using M120-RNR and M155-RNR root-knot nematode resistant donor parents with the elite breeding line PD 94042 (May, 1999). The best resistant BC3F3 lines were crossed with adapted  lines from our UGA Cotton Breeding program. A ten plant sample of the RKN resistant parental material was challenged twice with a very high rate of RKN in a pot-based greenhouse test by Shen et al. (2006). Further samples were then grown at the Gibbs Farm, University of Georgia-Tifton campus in an RKN infested field following the procedure of Davis and May (2005). The resistant lines were verified in an additional pot-based greenhouse test. Resistant lines 103-7, 201-A, 506-5, and 506-11 were selected as parents to introgress the RKN resistance into the Georgia-adapted germplasm GA 98028 and GA 2001078. One objective is to assist in further testing of these selections and use any improved material as parents to ultimately develop more elite germplasm.

DNA markers, developed in a companion project with the preliminary work described by Shen et al. (2006 and 2010), were intended to be used to select the resistant offspring using marker-assisted selection (MAS). The chromosomal region bearing the RKN resistance by these molecular markers on chromosome 11 was verified independently (Ynturi et al., 2006). The presence of this QTL results in decreased galling compared to susceptible plants when roots are challenged with RKNs. Another RKN resistance QTL was verified by He et al. (2011) on chromosome 14. It results in decreased numbers of nematode eggs as compared to susceptible plants when roots are challenged with RKNs. These markers are polymorphic between the parental line and both original parents of the RKN resistance donors that led to the BC3F3 population. Any additional markers for RKN resistance will be utilized as they become available.  Three rounds of crossing/backcrossing to the agronomic elite parents while ensuring the presence of the markers and the RKN resistance should give Georgia-adapted, value-added cotton germplasm with RKN resistance.

The following is our approach after using MAS to maintain the resistance and the marker up to the 2nd backcross. Coupling single plant selections in the BC2F1 population of the backcross with fiber quality testing, an unreplicated modified augmented design yield test (with every 5th row in the trial assigned to a conventional check cultivar) was to planted to select for yield and to test/verify the homozygosity of the RKN resistance marker(s). This trial was machine harvested and the seed-cotton yield of each F4 progeny row compared with seed-cotton yield of the nearest check row. For the rows that significantly out-yield the nearest check plot, boll samples were picked for lint %, fiber quality, and for seed in a parallel increase field. Next, the preliminary trial (PT) was conducted near Tifton or Plains. Advanced generation germplasm lines promoted from the PT were tested in an advanced yield trial (AT) in both Plains and Tifton. Elite germplasm lines from a successful performance in the ATs were tested in locations throughout the state in both dryland and irrigated fields in the University of Georgia Official Variety Trials.

After the preliminary crossing, the molecular markers (Shen et al., 2006) were used in a three-cycle backcrossing program in the greenhouse to insert the RKN resistance gene during 2007. However, since our crossing schedule was disrupted by inviable seed from the second backcross during 2007, F1 seed was sent to the winter nursery in Mexico to obtain seed for the 2008 growing season to test samples of the F3 population with the molecular markers for RKN resistance. Afterwards we were to continue backcrossing with the plants that tested positive with the DNA markers for the RKN resistance. The F3 populations of RKN resistance donors crossed to GA adapted lines were returned from the winter nursery at Tecoman and were determined to not be polymorphic in any of the populations with any of the RKN-resistant markers in our possession. The parents and related RKN donor lines were also sampled and found to have no polymorphisms between the immediate parents of these populations. The linkage between the markers and the RKN-resistance gene must have been broken in the development of the PD 94042 RKN-resistant population. Immediate phenotypic testing was not possible due to resource constraints. To continue our aggressive approach, new crosses were made in 2008/2009 utilizing the more advanced RKN-resistant lines (155-R2-B1, 120-R1-B3, and 120-R1-B1) with newer GA-adapted lines to renew the effort to develop better GA-adapted, RKN-resistant cultivars via backcrossing utilizing phenotypic testing if needed.

In 2009, after a loss of a greenhouse planting from drifting herbicide, crosses from available remnant seed along with the backcross parents were planted into the 2009 field crossing block for further crossing/backcrossing. Further crosses of 155-R2-B1, 120-R1-B3, and 120-R1-B1 with newly released cultivars GA 2004303 and GA 2004230 along with additional elite GA 2007 material were made to develop lines with enhanced GA adaptability along with this superior root-knot resistance. Crosses of M120-RNR with GA 303 and GA 230 were later made to utilize the first markers developed in the companion research project described earlier.

The first of the three sets of crosses/backcrosses that we have made since 2007 used RKN-resistant parents derived from a M120-RNR by PD 94042 population was crossed with Cotton States lines derived from GA-adapted lines GA 98028 and GA 2001078. This set was challenged by RKN during the summer of 2010. Out of 633 F4 plants of various crosses, 21 plants (3.3%) were selected as being very strongly tolerant with less than 10% root galling. These were grown for seed increase in 2011 and then grown in the selection nursery for single plant selections in 2012. Further tests will subsequently be done to verify the RKN resistance of these plants. In 2011, another group of plants were challenged by RKN. They included 155-R2-B1, 120-R1-B3, and 120-R1-B1 as RKN-resistant parents as well as 3 of the original donors, 201-A, 506-5, and 506-11. In this group of 546 plants, 55 plants (10.1%) were selected as being very strongly tolerant with less than 10% root galling.

In 2012, the successful populations derived from the first two sets of RKN donor parents were grown for generational advance and seed increase for use in a replicate test in 2013 to determine yield and fiber quality suitability. This is to determine if we bring forward an elite RKN resistant line more quickly while we continue with the complete backcrossing effort to BC2. The third set (GA 230 and GA 2004303) using M120-RNR for the RKN-resistant parent are now at BC1F1 generation and will be tested using the DNA markers for the chromosome 11 and 14 RKN-resistant QTLs at the BC1F2 generation. These lines will also be used as the building blocks to stack the RKN genes with fiber quality genes and commercial transgenic genes utilizing MAS.

GA 120R1B3, one of our RKN resistance donors in set 2, performed very well against the checks (GA 230, DP 147RF, FM 966, and ST 4664RF along with the recurrent parent PD 94042) and has been officially released as a germplasm line (Davis et al., 2011). It was tested in 2008 (Lubbers and Chee, 2009) and retested in 2009 (Lubbers et al. 2010) to verify its agronomic performance. It is presently being compared with five relatively current commercial cultivars, four current University cultivar releases from the University of Arkansas, the University of Georgia, and Louisiana State University, and an outdated commercial cultivar which is considered very resistant to RKNs. We are expecting our approach to provide a solid performing release of GA-adapted, RKN resistant germplasms/cultivars. Even though MAS is generally considered a reliable procedure, it is a relatively recent innovation and has not been extensively utilized. Phenotypic analyses of the new populations are likely to present resource allocation difficulties that will decrease the size of the populations that can be tested.


Project Year: 2012

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