Initial and residual toxicity of soil-applied thiram on the vesicular-arbuscular mycorrhizal symbiosis

The effect of the non-systemic fungicide thiram on the vesicular-arbuscular mycorrhizal (VAM) symbiosis and on Leucaena leucocephala was evaluated in a greenhouse experiment. In the uninoculated soil treated with P at a level optimal for mycorrhizal activity, mycorrhizal colonization of roots was low, and did not change as the concentration of thiram in the soil increased with the from 0 to 1000 mg/kg. When this soil was inoculated VAM fungus Glomus aggregatum, with VAM colonization was enhanced significantly, but decreased increase in thiram concentration until it coincided with the level observed in the uninoculated soil. Similarly, symbiotic effectiveness was reduced, its expression delayed or completely eliminated with increase in the concentration of thiram. Amending soil to a P level sufficient for non-mycorrhizal host growth fully compensated for thiram-induced loss of VAM activity if the thiram levels did not exceed 125 mg/kg. In soil treated with 50 mg thiram/kg, the toxicity of the fungicide dissipated within 66 days of application. At higher concentrations, the toxicity of the chemical on the mycorrhizal symbiosis appeared to be enhanced.Contribution from the Hawaii Institute of Tropical Agriculture and Human Resources Journal Series No. 3716