Increased activity of soil microorganisms near sclerotia of Sclerotium rolfsii in soil

Abstract

Microbial activity of soil near sclerotia—"mycosphere" (MS)—increased markedly within 24 h after sclerotia of Sclerotium rolfsii were pressed into the soil, but no changes occurred in the microbial activity of soil not near the sclerotia—non-mycosphere (NMS). Increased microbial activity in the MS, called the "mycosphere effect," was attributed to increased bacterial activity, since numbers of fungi and actinomycetes remained relatively unchanged. The qualitative selection of specific bacteria in the MS by sclerotia was suggested by the increase in bacteria that were tolerant to streptomycin and oxgall bile salts in the plating medium. Exposure of the soil to volatile compounds from alfalfa hay before adding the sclerotia enhanced the rate of the microbial increase in the MS. The energy substrate for the increase in MS organisms was of sclerotial origin and not from culture nutrients adhering to the sclerotia. Addition of concentrated sclerotial leachate to soil, and exposure of soil to alfalfa volatiles also induced an increase in the MS organisms.Bacteria from the MS affected S. rolfsii in vitro more than did bacteria from the NMS. Most of the MS bacteria were inhibitory to S. rolfsii on Difco plate-count agar, had no effect on potato-dextrose agar, and were stimulatory on Czapek-Dox agar. These results suggest that the nutritional status in the MS, as determined by sclerotial exudation or external nutrients, may be of primary importance in determining whether MS organisms will inhibit, will stimulate, or will have no effect on the sclerotia of S. rolfsii.