Diallel analysis for soybean resistance to the fungal pathogen Sclerotinia sclerotiorum
Sclerotinia stem rot is a common soybean disease caused by the fungus Sclerotinia sclerotiorum, resulting in economic losses in Brazil and worldwide. The development of resistant cultivars is a good option for the management of this disease; however, it has been difficult, largely due to the variability found in the fungus. We assayed for the genetic resistance (vertical and horizontal) of soybean cultivars inoculated with various isolates of S. sclerotiorum. Twenty soybean cultivars were selected and tested; 10 were relatively resistant and 10 relatively susceptible to the pathogen. The cultivars were inoculated with mycelium from four fungal isolates: Mauá da Serra, Ingaí, and Nazareno, collected from soybean production areas and UFLA 24, an isolate normally used by the Lavras University Laboratory of Plant Resistance to Diseases team, for assays with the detached-leaf method. The experiment was conducted in a completely randomized design. Detached-leaves at V2 trefoil were placed on an agar disk containing the mycelium 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 soybean 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 fungal isolates were the most aggressive, indicating that these isolates should be preferred for evaluating the level of resistance of soybean genotypes.