3.4.6
BREEDING FOR RESISTANCE TO S. ROLFSII IN GROUNDNUT

D GORBET and F SHOKES

University of Florida, Marianna, FL, USA

Background and objectives
Sclerotium rolfsii is an economically important pathogen in warm, moist climates worldwide, causing disease on more than 500 species of plants. This fungus causes stem and pod rot on groundnut (A. hypogaea) with potential plant death and estimated field yield losses of 10% or more in the south-eastern USA. Cultivars of peanut with resistance or tolerance to S. rolfsii are needed, but field screening has been complicated by the non-uniform spatial distribution of inoculum. Recently developed field and greenhouse techniques have shown promise in identifying resistance in groundnut to S. rolfsii. The objectives of these studies were to identify useful resistance to S. rolfsii in groundnut breeding material from the University of Florida program and evaluate their utility as potential cultivars.

Materials and methods
Irrigated and replicated field screening studies were conducted during 1994-97 at the NFREC, Marianna, Florida. Single row plots (6.1 m) of 111-136 genotypes (breeding lines/cultivars) were inoculated at 50 dap with virulent isolates of S. rolfsii grown on sterilized oat seed. Plants were rated for disease symptoms, hits and severity, at harvest. Selected genotypes showing the best resistance were grown in irrigated replicated yield tests (2-row plots, 3-4 replications), in 1995-97. These tests were inoculated and rated for disease incidence in the same manner as for the screening studies. Pod yield and grading data were collected.

Results and conclusions
Significant differences for disease resistance were obtained among the 300 genotypes evaluated in the field screening studies. Values ranged from less than 10% to over 70% plants diseased in these studies. Seventeen genotypes, including Florunner (susceptible) and Southern Runner (partially resistant), were selected for yield tests during 1995-97. All entries had less disease and produced pod yields exceeding 200% of Florunner. One resistant line (F84x47-10-1) produced pod yields that were 353% of that for Florunner (5556 vs 1575 kg/ha). All resistant breeding lines produced pod yields greater than Southern Runner (3781 kg/ha), which was significantly better than the susceptible Florunner. Most of the breeding lines produced acceptable grades, based on US market standards. Pod yields were significantly (P<0.05) and negatively correlated with disease incidence. The r values for pod yields and disease severity for 1995, 1996, and 1997 were -0.826, -0.828, and -0.522 (all P<0.001). The r values for pod yields and white mould hits were also negative and very highly significant. These studies indicate that several Florida breeding lines have significantly better resistance to S. rolfsii than the resistant cultivar Southern Runner, with excellent yield potential and acceptable grades.

One breeding line from these studies has recently been released by the Florida Agricultural Experiment Station as a new multiple disease resistant cultivar. Tested as UF91108, Florida MDR 98 has resistance to S. rolfsii, Cercosporidium personatum and tomato spotted wilt virus.

References
1. Melouk HA, Backman PA, 1995. Peanut Health Management, pp. 75-82.
2. Shokes FM, Rozalski K, Gorbet DW, Brenneman TB, Berger DA, 1996. Peanut Science 23, 124-28.