Techniques for Inoculation of Peanut with Sclerotium rolfsii in the Greenhouse and Field1

Abstract

Four greenhouse experiments and a field trial were conducted in 1994 to determine the most effective technique for inoculation of peanut (Arachis hypogaea L.) with Sclerotium rolfsii Sacc. The cultivar Florunner was grown in the greenhouse in 20-cm-diameter pots and inoculated 48 d after planting (DAP). Plants in the field experiment were grown in plots 2.7 m long on 0.9 m centers, thinned to 11 plants per plot, and alternate plants were flagged for inoculation at 48 DAP. Inoculation techniques for all experiments were (a) a germinating sclerotium on a 1-cm-diameter agar disk [potato dextrose agar (PDA)] appressed to the base of each central stem; (b) mycelia of a composite of six isolates growing on sterilized oat seed placed on the soil near the base of each central stem (20 g/pot) in the greenhouse or in the center of rows (a full 150 mL beaker of inoculum per row) in the field; (c) 2-3 mL of a PDA slurry with actively growing mycelia applied to the base of each central stem; (d) mycelia on toothpicks impregnated with potato dextrose broth (PDB) inserted into the base of each central stem; (e) a toothpick with mycelia [as in (d)] inserted into the soil near the base of each central stem; and (f) mycelia on PDB-impregnated clothespins clamped around the base of each central stem. The most effective methods in all experiments were the agar disk technique (a) and the clothespin technique (f). The oat inoculum technique (b) was only slightly less effective than techniques a and f. Techniques a and f have the advantage of allowing distinct single-plant inoculation. The oat inoculum technique allows the use of a composite of multiple isolates and inoculation of entire rows of plants or large areas of a field. The other techniques (c, d, and e) produced significantly (P ≤ 0.05) less disease. Inoculation with mycelia cultured on toothpicks inserted into stems (d) worked well sometimes but lacked consistency. This technique wounds plants and may bypass some possible natural defense mechanisms. We prefer the clothespin technique for evaluation of individual plant resistance because it is rapid and reliable. This is the first report for use of this technique for inoculation of peanut with S. rolfsii.