Esdras Manuel Carbajal Melgar
ABSTRACT
St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] is a warm-season grass widely grown across the southern US. Characteristics such as rapid stolon production, superior shade tolerance, and moderately low input requirements make this grass well suited for home lawns, sod production, and commercial landscapes. The genus Stenotaphrum includes seven species with different ploidy levels and has a base chromosome number of x=9. Because of fertility barriers across ploidy levels, diploid genotypes have been used as the primary source of genetic material for breeding efforts. While many genotypes are cross-fertile, all St. Augustinegrass cultivars are propagated vegetatively in sod production. In other warm-season grasses, the development of sterile triploid hybrids by crossing tetraploid and diploid genotypes has been successfully used as a means of ensuring varietal purity. Applying this model in St. Augustinegrass would be beneficial to sod producers and turf managers who require purity for certification and uniformity for performance, respectively. Therefore, the present study was conducted to develop colchicine-induced tetraploid lines of St. Augustinegrass. Seeds of cultivar ‘Raleigh’ were treated with four colchicine concentrations at four exposure times. Seedlings recovered after treatment and were screened for genome size changes using flow cytometry. A set of putative tetraploid lines was initially identified and evaluated for stomatal density, stomatal length and pollen stainability. Further analysis of putative tetraploids indicated all lines were found to be diploid with the exception of one, DSA 13005. Pollen stainability of this line was high and it produced 33 progenies through selfing, two of which were identified as putative tetraploids via flow cytometry. Average stomata lengths for DSA 13005 and its two progeny lines, DSA 16001 and DSA 16016, were found to be significantly (P<0.05) larger than that of diploid control Raleigh. All three lines were corroborated as tetraploids (2n = 4x = 36) through chromosome counts. These lines will be used in future breeding efforts in an attempt to develop sterile triploid cultivars.
One of the most common diseases in St. Augustinegrass is gray leaf spot (GLS), caused by the fungal pathogen Magnaporthe oryzae. While previous studies have reported polyploid lines with resistance to GLS, no comprehensive evaluations of sources of resistance have been performed in the genus. Screening diploid and polyploid St. Augustinegrass germplasm for response to GLS will enable breeders to identify resistant parents for cultivar development. In this study, a collection of 62 genotypes of Stenotaphrum spp. were screened for resistance to three different M. oryzae sources of inoculum under controlled environmental conditions. The traits evaluated were incubation period, number of leaves with lesions, mean lesion length and derived parameters disease incidence, area under the disease progress curve (AUDPC), and area under the lesion expansion curve (AULEC). Significant differences among genotypes were identified with polyploid genotypes PI 365031, PI 290888, PI 300130, FX-10 and PI 300129 and diploid genotypes PI 410353 and PI 647924 consistently showing high levels of resistance across trials, inoculum sources and traits. The identification and utilization of resistance genes in diploid genotypes is of great importance for future St. Augustinegrass breeding efforts, as this germplasm pool can be more readily exploited because of the lack of reproductive barriers with most commercial cultivars and plant introductions.
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