Abstract
Sclerotinia sclerotiorum, the causal agent of Sclerotinia stem rot, is a devastating fungal pathogen of soybean that can cause significant yield losses to growers when environmental conditions are favorable for the disease. The development of resistant varieties has proven difficult. However, poor resistance in commercial cultivars can be improved through additional breeding efforts and understanding the genetic basis of resistance. The objective of this project was to develop soybean germplasm lines that have a high level of Sclerotinia stem rot resistance to be used directly as cultivars or in breeding programs as a source of improved Sclerotinia stem rot resistance. Sclerotinia stem rot-resistant soybean germplasm was developed by crossing two sources of resistance, W04-1002 and AxN-1-55, with lines exhibiting resistance to Heterodera glycines and Cadophora gregata in addition to favorable agronomic traits. Following greenhouse evaluations of 1,076 inbred lines derived from these crosses, 31 lines were evaluated for resistance in field tests during the 2014 field season. Subsequently, 11 Sclerotinia stem rot resistant breeding lines were moved forward for field evaluation in 2015, and seven elite breeding lines were selected and evaluated in the 2016 field season. To better understand resistance mechanisms, a marker analysis was conducted to identify quantitative trait loci linked to resistance. Thirteen markers associated with Sclerotinia stem rot resistance were identified on chromosomes 15, 16, 17, 18, and 19. Our markers confirm previously reported chromosomal regions associated with Sclerotinia stem rot resistance as well as a novel region of chromosome 16. The seven elite germplasm lines were also re-evaluated within a greenhouse setting using a cut petiole technique with multiple S. sclerotiorum isolates to test the durability of physiological resistance of the lines in a controlled environment. This work presents a novel and comprehensive classical breeding method for selecting lines with physiological resistance to Sclerotinia stem rot and a range of agronomic traits. In these studies, we identify four germplasm lines; 91–38, 51–23, SSR51–70, and 52–82B exhibiting a high level of Sclerotinia stem rot resistance combined with desirable agronomic traits, including high protein and oil contents. The germplasm identified in this study will serve as a valuable source of physiological resistance to Sclerotinia stem rot that could be improved through further breeding to generate high-yielding commercial soybean cultivars.
Highlights
Soybean [Glycine max (L.) Merr.] is an important, globally grown source of protein, and it is the largest source of edible oil
Continuous evaluations of desirable traits are necessary for the development of elite soybean breeding lines. The objectives of this project were to: (1) develop soybean germplasm lines that have a high level of Sclerotinia stem rot resistance, that yield competitively with commercial cultivars, while maintaining acceptable protein and oil profiles which would allow them to be used directly as cultivars or in breeding programs as a source of Sclerotinia stem rot resistance; (2) conduct a search for genetic markers associated with Sclerotinia stem rot resistance in the newly developed germplasm lines which can be used to select for resistance and to improve progress in breeding for Sclerotinia stem rot resistance; and (3) compare the response of the generated germplasm lines in a controlled greenhouse environment to multiple isolates of S. sclerotiorum and in field environments
quantitative trait loci (QTL) associated with reduced S. sclerotiorum infection were identified in the four populations of germplasm lines generated from W04–1002 as a source of Sclerotinia stem rot resistance
Summary
Soybean [Glycine max (L.) Merr.] is an important, globally grown source of protein, and it is the largest source of edible oil. Cultural practices include crop rotation, tillage, weed control, irrigation management, and modification of seeding rates and row spacing (Peltier et al, 2012). Fungicides such as picoxystrobin (Aproach R ) and boscalid (Endura R ) have resulted in suppression of Sclerotinia stem rot in field trials and are most effective when applied at the R1 (first flower) to R3 (beginning pod development) growth stages (Smith et al, 2014). The most commonly available and well-studied biological control agent for Sclerotinia stem rot is Coniothyrium minitans (Contans R ) (Peltier et al, 2012) This beneficial fungus is known to degrade sclerotia, the resting structure of S. sclerotiorum
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