Evaluation of early-screening methods and peanut germplasm for resistance to Rhizoctonia limb rot

Diseases caused by Rhizoctonia solani lead to significant reduction in peanut yields and quality throughout the world. This study was conducted to evaluate a subset of accessions from the core collection of peanut germplasm plus the commercial cultivars Florunner, Southern Runner, Georgia Browne, and Georgia Green for resistance to limb and seedling hypocotyl infections caused by R. solani. Also, several early-screening methods using detached shoots and leaflets in growth chambers were evaluated to determine their effectiveness in identifying potential sources of limb rot resistance. The results were then correlated with the results from the field screenings in 1996 and 1997. Seedlings were also evaluated for resistance to hypocotyl infections and correlated with the results from the field experiments. The field screening project identified six core accessions and the commercial cultivar Georgia Green as having a level of resistance to limb infections equal to that of the partially resistant cultivar Georgia Browne. The seedling screen identified two core accessions and the commercial cultivar Georgia Green that had significant levels of resistance to seedling hypocotyl infections. The detached shoot and leaflet inoculation techniques worked well. However, there was no significant relationship between the results of these techniques and the results from the field experiments. The detached shoot technique did identify three of the four most resistant genotypes in the field experiment indicating that it could have potential as an early-screening method. A temperature study was also conducted to provide a better understanding of the environmental conditions responsible for limb rot and seedling hypocotyl infections. Cooler temperatures increased seedling hypocotyl infections. However, there was no optima temperature consistently for limb rot infections. Disease levels were low for all temperatures indicating that a key component of the infection process may have been missing in our study. After several preliminary experiments, the detached shoot experiment, and the pod rot experiment, we have concluded that sustained periods of high relative humidity seems to be more important than cool temperatures in the development of severe R. solani epidemics. The last component of the research was to evaluate four peg and pod developmental stages for susceptibility to Rhizoctonia pod rot. The data showed that the younger peg tissue was more susceptible to infections than mature pods. This indicates that there could be a greater yield loss than first expected based on the occurrence of pod rot in mature pods.