Effect of Microsphaeropsis sp strain P130A on germination and production of sclerotia of Rhizoctonia solani and interaction between the antagonist and the pathogen

Microsphaeropsis sp. strain P130A was evaluated for the control of tuber-bo rne inoculum of Rhizoctonia solani based on the viability of sclerotia prod uced in vitro and on both the viability and production of tuber-borne scler otia. The interactions between the antagonist and the pathogen, as well as the effect of the toxins produced by the antagonist on mycelial growth of R . solani were studied using transmission electron microscopy. On sclerotia produced in vitro, for all incubation periods (1 to 42 days), Microsphaerop sis sp. significantly reduced germination. Percent germination of sclerotia treated with Microsphaeropsis sp. decreased with increasing incubation per iod from an average of 82.0% after 1 day to stabilize at an average of 5.8% after 35 days. Similarly, percent germination of tuber-borne sclerotia was significantly lower when tubers were treated with Microsphaeropsis sp. Bot h 2% formaldehyde and Microsphaeropsis sp. treatments significantly reduced sclerotia germination to approximately 10% after 42 days of incubation at 4 degreesC. Furthermore, on tubers treated with the antagonist, the number of sclerotia per square centimeter decreased from 1.6 to 0.5 during the 8 m onths of storage at 4 degreesC, whereas an increase from 1.2 to 7.8 sclerot ia per square centimeter was observed on untreated tubers. Microsphaeropsis sp. (strain P130A) colonized hyphae of R, solani within 4 days after conta ct on culture media. Transmission electron microscopic observations showed that the antagonist induced a rupture of the pathogen plasma membrane and t hat a chitin-enriched matrix was deposited at sites of potential antagonist penetration. Host penetration was not associated with pathogen cell wall a lterations, which occurred at the time of progress of the antagonist in the pathogen cytoplasm. In the presence of a crude extract of Microsphaeropsis sp., cells of R, solani showed cytoplasm disorganization and breakdown of plasma membranes. Antibiosis and mycoparasitism were involved in the antago nism of R. solani by Microsphaeropsis sp., but the sequence by which these events occur, as well as the significance of wall appositions produced by R . solani, is yet to be established.