CHARACTERIZATION OF THE SUPPRESSIVENESS OF HAIRY VETCH-AMENDED SOILS TO THIELAVIOPSIS-BASICOLA

Factor(s) involved in soil suppressiveness to Thielaviopsis basicola w hen hairy vetch was used as a green manure were studied in a cotton pr oduction system. Soil suppressiveness was assessed in vitro at hairy v etch amendment levels of 0, 0.25, and 0.75% (wt/wt) by observing chlam ydospores, using a nylon fabric technique. Chlamydospore germination i n all soils was below 5%, and microscopic examination showed no germ t ube lysis or visible propagule destruction. Viability (chlamydospore g ermination on T. basicola-carrot-etridiazol-nystatin [TB-CEN] medium) was reduced by 29% within 48 h after hairy vetch amendment. Viability also was reduced in atmospheres of amended soils, suggesting that the suppressiveness was due to a volatile factor. In a field study, chlamy dospore viability in amended soils was reduced by 16%. T. basicola hyp hal growth was more sensitive to ammonia than Rhizoctonia solani or Py thium ultimum, and chlamydospore mortality of T. basicola was 100% in petri dish atmospheres with 0.4 ppm of ammonia (50% lethal dose = 0.15 ppm). Soil atmospheric ammonia was 0.08 and 0.10 ppm for 0.25 and 0.7 5% amendment levels, respectively, both at 3 and 7 days after incorpor ation. In the field, 0.11 and 0.14 ppm of ammonia were detected in soi l atmospheres 3 and 7 days after incorporation, respectively. The leve ls of ammonia detected were sufficient to account for the loss in T. b asicola chlamydospore viability, indicating that ammonia is responsibl e for the suppressiveness observed.