Evaluation of Detached Shoot and Leaflet Inoculation Techniques to Screen Peanut Genotypes for Resistance to Rhizoctonia Limb Rot

Abstract

Abstract Identifying peanut genotypes with resistance to Rhizoctonia limb rot in the field is slow and costly. The objectives of this study were to compare detached leaflet and shoot inoculations as methods of screening for resistance to Rhizoctonia limb rot and to assess the relationship between genotype reactions in vitro and in the field. Eleven peanut germplasm selections and four cultivars, which exhibited a range of susceptibility to limb rot in previous field experiments in 1996 and 1997, were selected for evaluation. Detached lower reproductive limbs were suspended in Hoagland's solution and inoculated using a mixture of soil and oat seeds infested with Rhizoctonia solani. Detached leaflets on moistened filter paper in petri dishes were inoculated with hyphal plugs. Georgia Green and selections 512 and 283 were the most susceptible of all the genotypes to leaflet infections although Georgia Green is partially resistant to limb rot in the field. Disease levels for all genotypes were higher in the detached shoot inoculation study than in previous field experiments and symptoms observed were typical of those seen in the field. However, there was no correlation in relative susceptibility of the genotypes between the detached leaflet, shoot, or previous field experiments. Susceptibility of genotypes in the field may be determined by the combined resistance of the different plant parts or by factors not evaluated in this study. The detached shoot inoculation technique did not reproduce the same range of disease reactions on diverse peanut genotypes that were observed in the field experiments, but it did prove to be a good method of generating high levels of disease in vitro. The technique could probably be adapted to evaluate other factors such as relative fungicide performance or effects of environment on disease.