Pedigree selection under field conditions within Acala 1517-08 and its glandless derivatives for development of cotton resistant to Fusarium wilt caused by Fusarium oxysporum f. sp. vasinfectum race 4

Abstract

The soil- and seed-borne fungus, Fusarium oxysporum f. sp. vasinfectum race 4 (FOV4), causes Fusarium wilt in cotton (Gossypium spp.) and represents a significant threat to cotton production in the US. The objectives of this study were to enhance FOV4 resistance in heterogeneous Acala 1517-08 and its derivatives through three cycles of pedigree selection in 2018, 2019 and 2020 under FOV4-infested field conditions. In 2017, in a field not infested with FOV4, 46, 12, and 23 single plants were selected for productivity from a base population of 1100–1400 plants of Acala 1517-08 and its derived glandless lines, Acala 1517-18 GLS and NM 13P1125, respectively. In 2018, the single plant selections, controls and other lines were tested in progeny-rows in an FOV4 infested field for disease responses including mortality rate (MR), root vascular staining (RVS), and disease incidence (DI), resulting in 41, 15, and 26 single plants selected from progenies of the three lines, respectively, based on RVS. These single plant selections were tested in 82 progeny-rows in the same field in 2019, and the second selection cycle based on RVS advanced 40, 16, and 37 plants selected from the progenies of the three lines, respectively, for further evaluation and the third selection cycle in a progeny test in 2020. In 2020, an average of 81–84% plants died in highly susceptible controls, Pima S-7 and Pima DP 744, due to FOV4 infections, while the resistant controls, Pima S-6, Pima PHY 841 RF and NuMex COT 17 GLS, incurred up to 19–24% mortality. As a result of the three cycles of single plant selection, a total of 12 progenies were identified to be as resistant as the resistant controls. On the progeny basis, MR between years was significantly correlated, and MR at different evaluation dates was also significantly correlated with one another and with mean RVS and DI within year. However, due to possible escapes of plants from FOV4 infections, the original RVS scores of individual plant selections were not correlated with their respective progenies; and no significant differences were detected among progeny means grouped based on RVS ratings for single plant selections. The results demonstrate that early plant mortality is a superior means of selecting against FOV4 susceptibility, while using RVS rating late in the season if excluding seedling MR may inflate and distort the resistance level of genotypes, rendering it less reliable in assessing FOV4 resistance in cotton. In addition, many single plants selected for FOV4 resistance with no RVS may be escapes, necessitating repeated progeny tests and selection. The importance of pedigree selection within germplasm, choice of resistant and susceptible checks, FOV4 inoculum density, and the use of RVS, and resistance versus tolerance in response to FOV4 infections under field conditions are discussed in detail.