Crop Protection 18 (1999) 127 — 130 Stem rot disease evaluation of mass-selected peanut populations W.D. Branch *, T.B. Brenneman  Department of Crop and Soil Sciences, University of Georgia, Coastal Plain Experiment Station, Tifton, GA 31793-0748, USA  Department of Plant Pathology, University of Georgia, Coastal Plain Experiment Station, Tifton, GA 31793-0748, USA Received 13 October 1998; received in revised form and accepted 20 November 1998 Abstract Stem rot is a widespread, devastating disease of the cultivated peanut (Arachis hypogaea L.). This disease is caused by the soilborne pathogen Sclerotium rolfsii Sacc. Resistant cultivars are needed to reduce cost of peanut production. ‘Toalson’ and ‘Southern Runner’ are two genetically diverse cultivars with partial stem rot resistance; whereas, ‘Florunner’ and ‘Sunbelt Runner’ are two stem rot susceptible cultivars. The objective of this study was to evaluate mass-selected populations derived from different combinations of crosses among these two resistant and two susceptible cultivars. Four mass-selected populations, two unselected bulks, and four parental cultivars were evaluated in the F  —F  generations for pod yield and stem rot resistance in replicated field tests during 1993 —1996, respectively. The resistant cultivar Toalson produced 40% higher pod yields and had 50% fewer stem rot disease loci than the other partially resistant cultivar Southern Runner. Mass-selected populations derived from the Sunbelt RunnerToalson cross combination also had significantly higher yield and better stem rot resistance than mass-selected populations derived from the FlorunnerSouthern Runner cross combination.  1999 Elsevier Science Ltd. All rights reserved. Keywords : Arachis hypogaea L.; Sclerotium rolfsii Sacc.; Groundnut; Cross combination; Cultivars; Disease resistance 1. Introduction Stem rot, also known as white mold or southern blight, is a devastating disease on peanut (Arachis hypogaea L.) caused by the soilborne fungus Sclerotium rolfsii Sacc. Stem rot occurs throughout the world in temperate, subtropical, and tropical regions, and causes significant peanut yield loss when disease incidence is high (Kolte, 1984; Melouk and Backman, 1995; Porter et al., 1982). Fungicides currently used to control stem rot are very effective but expensive. Consequently, development of disease resistant cultivars is needed to reduce peanut production costs. ‘Florunner’, a medium maturing runner-type, was the predominant cultivar in the southeastern US for the past two decades (Norden et al., 1969), but it has been re- placed by other cultivars in recent years. It is highly * Corresponding author. Fax: 001 912 386 7293; E-mail: wdbranch@tifton.cpes.peachnet.edu susceptible to major diseases, including stem rot (Branch and Brenneman, 1993, 1996; Branch and Csinos, 1987). ‘Sunbelt Runner’ is another runner market type cul- tivar that has slightly earlier maturity and larger pods and seed than Florunner (Mixon, 1982). Sunbelt Runner is also susceptible to stem rot (Branch and Csinos, 1987; Brenneman, et al., 1990), but two high yielding and dis- ease resistant cultivars, ‘Georgia Browne’ and ‘Georgia Green’ were derived from crosses involving Sunbelt Run- ner as one parent. ‘Toalson’ and ‘Southern Runner’ represent genetically diverse sources of peanut stem rot resistance (Gorbet et al., 1987; Simpson et al., 1979). Toalson is an early maturing, spanish-type cultivar that has partial stem rot resistance in Georgia (Branch and Csinos, 1987). Toalson also has resistance to pod rot caused by Pythium myr- iotylum Dreschler and Rhizoctonia solani Kuhn. Southern Runner is a late maturing runner-type cul- tivar that has partial stem rot resistance (Gorbet et al., 1987). Southern Runner was released primarily because of its moderate resistance to late leaf spot caused by 0261-2194/99/$ — see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 1 - 2 1 9 4 ( 9 8 ) 0 0 1 0 3 - 3