Characterization of isolates of Sclerotium rolfsii and evaluation of peanut for reaction to southern blight

Scope and method of study. This study has four objectives: (1) To investigate the effect of Sclerotium rolfsii on peanut at three developmental stages (50, 75, 100 days after planting). (2) To evaluate the effect of calcium nutrition on peanut pod breakdown caused by S. rolfsii. (3) To determine production of oxalic acid in vitro and its relation to pathogenicity of isolates of S. rolfsii. (4) To evaluate genetic variability among isolates of S. rolfsii using mycelial compatibility groups (MCG) and RAPD-PCR.; Findings and conclusions. This study has the following findings: (1) S. rolfsii induced southern blight in peanut genotypes Okrun, Tamrun 96 and TX 961678. Disease severity was greatest for all three genotypes when inoculation of peanut was performed 50 days after planting (DAP). Thus, 50 DAP appears to be the crucial period to select a management strategy for southern blight. The study confirmed the moderate resistance of Tamrun 96 to southern blight and its usefulness in managing S. rolfsii in peanut fields with a known history of southern blight. (2) Treatment of peanut inoculated with S. rolfsii with 227 kg/ha and 454 kg/ha of calcium chloride or 2,272 kg/ha and 3,409 kg/ha of gypsum resulted in a reduction of peanut pod breakdown, particularly with both rates of gypsum. (3) Production of oxalic acid (OA) in vitro by seventeen isolates of S. rolfsii was correlated to an increase in mycelial biomass produced by the isolates. The study determined that OA was not the sole factor determining pathogenicity in S. rolfsii. (4) Mycelial compatibility groups (MCG) and RAPD-PCR analysis are valuable methods for differentiating seventeen isolates of S. rolfsii from Oklahoma and other locations. Isolates were placed in fourteen different MCGs. Three isolates from Oklahoma where placed in MCG 2, and others in MCG 1 and MCG 13. The remaining isolates were placed in individual MCGs. RAPD-PCR analyses revealed similarities in banding patterns of isolates belonging to the same MCG when primer 335 was used. DNA polymorphisms were detected among isolates using 21 oligonucleotide primers, and four distinct RAPD groups of isolates were identified. Isolates were clustered together according to the crop from which they were isolated.