Soybean breeding strategies for resistance to Sclerotinia sclerotiorum

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. Although there is variability in Brazilian germplasm, there are still many issues to be elucidated in the S. sclerotiorum - Glycine max (L.) Merrill. The objectives of this study were to evaluate the variability of different isolates of S. sclerotiorum, as well as to study the genetic resistance of soybean cultivars to the pathogen; to determine the most appropriate phenological stage in the soybean crop to inoculate S. sclerotiorum, as well as to compare different inoculation methods for the identification of soybean cultivars resistant to the pathogen; to verify the existence of pathogenic variability among monoascosporic strains of a single isolate of S. sclerotiorum using the straw test method in soybean cultivars and to evaluate the potential of some segregant populations to obtain soybean progenies resistant to sclerotinia stem rot. To achieve these objectives, four studies were conducted: a) To clarify the type of genetic resistance and virulence of the isolates, 20 soybean cultivars were inoculated with four isolates: Mauá da Serra, Ingaí, Nazareno e UFLA 24 using the detached-leaf method. The plants were kept in a greenhouse until the V 2 stage, characterized by the first fully expanded trefoil. The isolates were grown in culture medium for five days. Each leaflet of the trefoil received an agar disk containing the mycelium. The experiment was conducted in a completely randomized design, and each leaflet was considered a replicate. After 72 hours, the leaflets were evaluated using a scoring scale ranging from 0 (no symptoms) to 5 (susceptible). Statistical analyses were performed using the diallel method (Griffing IV model), which provided information on the vertical and horizontal resistance of the cultivars, as well as the aggressiveness of the isolates. The cultivars BRS Baliza RR, M-SOY 8001, Emgopa 316 and M-SOY 8329 showed horizontal resistance; BRS Favorita RR, Emgopa 315, MG/BR 46 (Conquista), 7166RSF IPRO, BRS Silvânia RR and BRS Milena presented specific resistance to most isolates. The UFLA 24 and Ingaí isolates were the most effective at causing symptoms in the soybean cultivars. b) To compare different inoculation methods, 20 cultivars were inoculated with the UFLA 24 isolate in four different experiments. There were three experiments using the straw test method: V3 stage inoculation, evaluation by lesion length three days after inoculation; R1 stage inoculation, evaluation by lesion length seven days after inoculation; R1 stage inoculation, evaluation by scale of notes seven days after inoculation. For these three experiments we have used the completely randomized design with five replicates. The fourth experiment was used the detached leaf method, inoculation at the V2 stage, evaluation by scale of notes, 72 hours after inoculation. The design used was completely randomized with three replicates. From the obtained data were performed the analysis of variance and the means of the four experiments 14 were compared. A coincidence index and the Spearman correlation were used for the comparison of the classification of the cultivars against the four experiments. It was verified that in relation to the means for the four experiments, the cultivar EMPOPA 315 was classified as more resistant and the cultivar TGM123RR was the most susceptible. There was a mean and significant correlation between the detached leaf test method and the straw test method at R1, regardless of the evaluation strategy (lesion length or note scale) and also between the experiments inoculated at R1. The coincidence index indicated that when the length of the lesion is adopted as an evaluation strategy, both inoculation in stage V3 and in R1 gives the same reaction to the pathogen, in the case of cultivars considered resistant. c) - To verify the existence of pathogenic variability among S. sclerotiorum monoascosporic isolates, 20 soybean cultivars and five monoascosporic isolates of S. sclerotiorum were used. A greenhouse experiment was conducted to evaluate the variability of aggressiveness between monoascosporic isolates. Plants in the R1 stage were inoculated by the straw test method. The experiment was conducted in a completely randomized design, with three replicates in the factorial scheme. Evaluations were performed at 7, 14 and 21 days after inoculation with the aid of a graduated ruler, by means of the proportion of the injured area. The area under a disease progression curve (AUDPC) was estimated. The monoascosporic isolates 7.3, 7.4 were more aggressive and the cultivars BRSMG 790A and BRSMG 850GRR presented more stable resistance to the different monoascosporic isolates. The GGE biplot analysis characterized the cultivars in relation to the level of resistance and the monoascosporic isolates regarding the aggressiveness. The cultivars BRSMG 790A and BRSMG 850GRR showed a more stable resistance to the different monoascosporic isolates. In addition, these two cultivars were grouped into distinct clusters, that is, they have different groups of favorable alleles related to resistance to white stem rot. There was variability among the monoascosporic isolates and these should be preferred for inoculation in breeding programs that aim to obtain soybean cultivars resistant to S. sclerotiorum. d) - To evaluate the potential of segregating populations to obtain soybean progenies resistant to sclerotinia stem rot, 149 F 2:3 progenies from six populations: population 1 (BRSMG Garantia x MG / BR 46 (Conquista)); population 2 (MG / BR 46 (Conquista) x Emgopa 316); population 3 (BRSMG 68 Vencedora x Emgopa 316); population 4 (BRSMG 68 Vencedora x MG / BR 46 (Conquista)); population 5 (Emgopa 316 x BRSMG Guarantia) and population 6 (BRSMG 68 Vencedora x BRSMG Guarantia), and 20 controls were evaluated in the field, in the 2017/2018 crop season in a simple latice 13 x 13 and a plot of 1 line of 2 meters. The progenies were evaluated for resistance to S. sclerotiorum using the detached leaf method, when the plants reached the V2 stage (first fully expanded trifolium). The trifolium was collected and taken to the laboratory to install the experiment. Each leaflet of the trifolium received a mycelial disc of the UFLA 24 isolate, which was maintained under controlled conditions until the time of evaluation, 72 hours after inoculation. The evaluations 15 were by scale of scores ranging from 0 (no symptoms) to 5 (susceptible). The experiment in the laboratory was conducted in a completely randomized design (DIC), with each leaflet of the trefoil constituting a repetition. The data were evaluated through analysis of variance, individual and joint. Progenies show variability in relation to resistance to S. sclerotiorum. Progenies from population six (BRSMG 68 - Vencedora x BRSMG Garantia) obtained the greatest potential to obtain soybean strains with a certain level of resistance to sclerotinia stem rot.