VIEW ARTICLE

Determination of Sclerotial Populations of Sclerotium rolfsii in Soil by a Rapid Flotation-Sieving Technique. R. Rodriguez- Kabana, Alumni Associate Professor, Department of Botany and Microbiology, Agricultural Experiment Station, Auburn University, Auburn, Alabama 36830; P. A. Backman(2), and Elizabeth A. Wiggins(3). (2)(3)Assistant Professor, and Graduate Assistant, Department of Botany and Microbiology, Agricultural Experiment Station, Auburn University, Auburn, Alabama 36830. Phytopathology 64:610-615. Accepted for publication 1 November 1973. DOI: 10.1094/Phyto-64-610.

A flotation technique using an aqueous solution of blackstrap molasses, was developed for rapid extraction of sclerotia of Sclerotium rolfsii from soil. A 50-ml (70 g) subsample of air-dried, screened (4-mm mesh) soil was placed in a 600-ml beaker and enough extracting solution added to provide a final volume of 350 ml. The mixture was stirred with an electric stirrer (1,600 rpm) for 30 s and allowed to settle for 30 s. The liquid was decanted through a 250 µm (60-mesh) screen of 7.5 cm diam, and the sclerotia were counted and plated on selective oxalate-gallic acid medium to determine viability. The extracting solution consisted of 200 ml blackstrap molasses (sp gr 1.300, 25 C), 800 ml tap water, and 12.5 µg/ml Separan NP 10 (a flocculating agent). This extracting solution had a sp gr value of 1.073 (25 C), and was equivalent in extracting efficiency to a 0.75 M sucrose solution. The recovery of sclerotia using this technique was greater than 80% of those present in a soil sample and was sensitive to a concn as low as one sclerotium per 50 ml soil. When used to study a 2-yr peanut-corn rotation, the technique revealed no sclerotia in soil from fields under corn which had been under peanuts the previous year. An average of 3.33 sclerotia/250 ml soil was found in fields under peanuts which had been under corn the previous year. Results of studies on the distribution of sclerotia in two fields under peanut monoculture indicated a heterogenous distribution of sclerotia and a pronounced “row effect” where plots were contiguous.