Using marked strains to assess the effect of hairy vetch amendment on the inoculum densities of Thielaviopsis basicola, Pythium ultimum and Rhizoctonia 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. Benomyl-tolerant T. basicola populations in infested soil were assayed by dilution plating on benomyl-amended T. basicola–carrot–etridiazol–nystatin (TB–CEN), a selective medium for T. basicola. Both soil populations and isolation frequency of the benomyl-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-amended than in non-amended soils. Results were consistent with results obtained from the direct observation of T. basicola chlamydospores, indicating a loss in chlamydospore viability in hairy vetch-amended soils. The effects of hairy vetch amendment on Pythium ultimum and Rhizoctonia solani populations were assessed by using a kanamycin-tolerant P. ultimum strain and an AG-11 isolate of R. solani. Populations of the kanamycin-tolerant P. ultimum strain and R. solani AG-11 did not change significantly with hairy vetch amendments, but there was a trend toward larger populations, indicating that these fungi were not suppressed in amended soils. The results agree with field studies where populations of T. basicola, its isolation frequency from diseased cotton seedlings and the seedling disease severity caused by T. basicola were reduced 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 identifying agricultural practices that are potentially suppressive to this pathogen.