Biostatic effect of fumigation and pesticide drenches on an endomycorrhizal-Rhizobium-legume tripartite association under field conditions

The effects of formaldehyde fumigation and pesticide drenching with Bavistin, Cuman, Copperthom, Sulfex, Furadon, and Termix at recommended rates on vesicular-arbuscular mycorrhizal (VAM) colonisation and Rhizobium sp. nodules were assessed regularly for a period of 90 days in the legumes Cajanus cajan, Dolichos biflorus, Vigna mungo, and V. unguiculata under field conditions. The fumigant and the pesticides initially reduced VAM fungal colonisation and the number of spores in all plants. Following the initial decrease there was a slow recovery, but by 90 days after emergence, root colonisation was either parallel to or still lower than the control, and the number of spores was still well below control levels for all species except C. cajan, which had more VAM spores than the control in all treatments except fumigation and Furadon. Although the number of nodules did not differ from control levels at 30 days after emergence, differences were evident during the later stages of plant growth for all species except V. unguiculata. The effect of pesticides on VAM fungi and root nodulation varied with the associated host plant species. Plant tissue P and VAM colonisation were significantly correlated in all host plants. The pesticide treatments had no marked effect on plant growth, but accumulations of nutrients in pesticide-treated plants were lower than those in untreated plants. Growth and nutrient status of the legumes varied with VAM fungal colonisation.