Biol Fertil Soils (1995) 20:275-283 9 Springer-Verlag 1995 K. Udaiyan 9 S. Manian 9 T. Muthukumar 9 S. Greep Biostatic effect of fumigation and pesticide drenches on an endomycorrhizaI-Rhizobium-legume tripartite association under field conditions Received: 23 February 1994 Abstract The effects of formaldehyde fumigation and pesticide drenching with Bavistin, Cuman, Copperthom, Sulfex, Furadon, and Termix at recommended rates on ve- sicular-arbuscular mycorrhizal (VAM) colonisation and Rhizobium sp. nodules were assessed regularly for a peri- od of 90 days in the legumes Cajanus cajan, Dolichos biflorus, Vigna mungo, and V. unguiculata under field conditions. The fumigant and the pesticides initially re- duced 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 ex- cept 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 stag- es of plant growth for all species except V. unguiculata. The effect of pesticides on VAM fungi and root nodula- tion varied with the associated host plant species. Plant tissue P and VAM colonisation were significantly corre- lated in all host plants. The pesticide treatments had no marked effect on plant growth, but accumulations of nu- trients in pesticide-treated plants were lower than those in untreated plants. Growth and nutrient status of the le- gumes varied with VAM fungal colonisation. Key words VAM fungi 9Nodulation 9 Fumigation 9 Pesticide drench 9 Root colonisation 9 Spore number 9 Legumes K. Udaiyan (~) 9 S. Manian 9 T. Muthukumar 9 S. Greep Microbiology Laboratory, Department of Botany, Bharathiar University, Coimbatore-641 046, Tamil Nadu, India Introduction Most legumes are symbiotically associated with VAM fungi and rhizobia, and the effect of this dual symbiosis on plant growth is well documented. However, in cultivat- ed soils, the effect of dual symbiosis may fluctuate and the beneficial effects may be lost through cultural prac- tices. In particular, the use of pesticides is known to affect VAM fungi and rhizobia. Previous studies have shown that pesticides may have either adverse or innocuous ef- fects on rhizobial growth, nodulation, and N 2 fixation, both in vitro and under field conditions (Tu 1980; Mallik and Tesfai 1983). Generally, fumigation with formalde- hyde is toxic to VAM fungi and systemic fungicides have a more harmful effect on VAM fungi than non-systemic fungicides (Menge 1982). Nemec (1985) reported that nematicides had little or no effect on VAM fungi, but Sreenivasa and Bagyaraj (1989) reported that pesticides were toxic to VAM fungi even at recommended levels. Reports on the effects of pesticides on VAM fungi and rhizobia are mostly from pot experiments with selected species and strains (Sreenivasa and Bagyaraj 1989; Hetrick and Wilson 1991) and only limited results have been obtained under field conditions (Kapusta and Rouwenhorst 1973; Rhodes and Larsen 1981). Therefore, in the present study, the effects of selected pesticides on VAM-legume-rhizobial associations were assessed under field conditions. Materials and methods Site The experiment was conducted in the experimental field at Bharathiar University, Coimbatore. The soil was a sandy loam and had a pH of 8.5 (1:1, soil:water) and electrical conductivity of 0.4 mS cm- 1. Soil nutrient determinations (Jackson 1958) included 10.4 mg N kg -1, 1.7 mg P kg -1, and potassium 23 mg K kg 1. The soil was ploughed to a depth of 50 cm, levelled, and 2- • 2-m plots were prepared, separated by 2-m intervals. The total VAM spore