Development of a new field inoculation technique to assess partial resistance in soybean to Sclerotinia sclerotiorum

Abstract

The use of appropriate techniques to assess disease resistance is crucial to understanding the genetic control of host resistance to pathogens to develop resistant plant varieties. The objective of this study was to develop and evaluate a novel field-based inoculation technique for sclerotinia stem rot in soybean [Glycine max (L.) Merr.], caused by Sclerotinia sclerotiorum (Lib.) de Bary. The barley kernel inoculation technique involves stem inoculation using an infected barley kernel inserted into a wound made in the soybean stem. The damage on soybean plants is measured by the length of the lesion on inoculated stem and expressed as inoculation severity index (ISI). Soybean varieties were evaluated for 2 yr at different sites using the barley kernel inoculation technique, and compared with other controlled environment and natural infection-based techniques. The ISIs obtained at Elora, ON, were significantly correlated (r = 0.21, P = 0.021) between the 2 test years (1998 and 1999), and in 1998 with natural infection [as measured by the disease severity index, DSI] at the same location (r = 0.31; P = 0.0005). Using a small set of lines that were specifically selected over several years to respond to DSI, the correlation between the DSI from some locations and years and the ISI in 1999 was negative. The significant positive correlations obtained using soybean lines that were not previously selected for DSI suggest the potential of this technique for pre-screening the breeding material for partial resistance to sclerotinia stem rot prior to confirmation with natural infection levels as measured by DSI. This may be of value to soybean breeders trying to develop soybean lines partially resistant to sclerotinia stem rot because of the difficulty in obtaining consistent and reliable DSI scores across locations and over years. Key words: Sclerotinia stem rot, white mould, inoculation, resistance, soybean