Inhibition of mycorrhizal symbiosis in Leucaena leucocephala by chlorothalonil

The effect of the fungicide, chlorothalonil, on vesicular-arbuscular mycorrhizal (VAM) symbiosis was studied in a greenhouse using Leucaena leucocephala as test plant. Chlorothalonil was applied to soil at 0, 50, 100 and 200 μg g⁻¹. The initial soil solution P levels were 0.003 μg mL⁻¹ (sub-optimal) and 0.026 μg mL⁻¹ (optimal). After 4 weeks, the sub-optimal P level was raised to 0.6 μg mL⁻¹ (high). The soil was either uninoculated or inoculated with the VAM fungus, Glomus aggregatum. The fungicide reduced mycorrhizal colonization of roots, development of mycorrhizal effectiveness, shoot P concentration and uptake and dry matter yields at all concentrations tested, although the highest inhibitory effect was noted as the concentration of the fungicide was increased from 50 to 100 μg g⁻¹. Phosphorus applied after four weeks tended to partially offset the deleterious effects of chlorothalonil in plants grown in the inoculated and uninoculated soil which suggests that the fungicide was interfering with plant P uptake. The results suggest that the use of chlorothalonil should be restricted to levels below 50 μg g⁻¹ if the benefits of mycorrhizal symbiosis are to be expected. Contribution from Hawaii Institute of Tropical Agriculture and Human Resources Journal Series No. 3464. Contribution from Hawaii Institute of Tropical Agriculture and Human Resources Journal Series No. 3464.