USING MARKED STRAINS TO ASSESS THE EFFECT OF HAIRY VETCH AMENDMENT ONTHE INOCULUM DENSITIES OF THIELAVIOPSIS-BASICOLA, PYTHIUM-ULTIMUM ANDRHIZOCTONIA-SOLANI

Controlled environmental studies were performed to assess the survival and activity of Thielaviopsis basicola in hairy vetch-amended and non -amended soils by using benomyl tolerance as a biochemical marker. Ben omyl-tolerant T. basicola populations in infested soil were assayed by dilution plating on benomyl-amended T. basicola-carrot-etridiazol-nys tatin (TB-CEN), a selective medium for T. basicola. Both soil populati ons and isolation frequency of the denomyl-tolerant T. basicola strain from cotton seedlings were reduced in soils amended with hairy vetch. Disease severity was also lower in seedlings grown in hairy vetch-ame nded than in non-amended soils. Results were consistent with results o btained from the direct observation of T. basicola chlamydospores, ind icating a loss in chlamydospore viability in hairy vetch-amended soils . The effects of hairy vetch amendment on Pythium ultimum and Rhizocto nia solani populations were assessed by using a kanamycin-tolerant P. ultimum strain and an AG-11 isolate of R. solani. Populations of the k anamycin-tolerant P. ultimum strain and R. solani AG-11 did not change significantly with hairy vetch amendments, but there was a trend towa rd larger populations; indicating that these fungi were not suppressed in amended soils. The results agree with field studies where populati ons of T. basicola, its isolation frequency from diseased cotton seedl ings and the seedling disease severity caused by T. basicola were redu ced in hairy vetch-amended soils, but not those of Pythium spp and R. solani. The use of marked strains of T. basicola may be useful in iden tifying agricultural practices that are potentially suppressive to thi s pathogen. (C) 1998 Elsevier Science Ltd. All rights reserved.