The impact of pesticides on arbuscular mycorrhizal and nitrogen-fixing symbioses in legumes

Effects of the pesticides Afugan, Brominal, Gramoxone, Selecron and Sumi Oil on growth, nodulation and root colonisation by arbuscular mycorrhizal (AM) fungi of the legumes cowpea (Vigna sinensis L.), common bean (Phaseolus vulgaris L.) and Lupin (Lupinus albus L.) were determined. The growth of all plants was inhibited by pesticide application, but this effect varied with the pesticide and plant species. Nodule formation was significantly inhibited in cowpea after 20 days of planting by all pesticides tested. Following the initial decrease, there was recovery from the inhibitory effects at 40 and 60 days after planting. Although the number of nodules on common bean and lupin did not differ from control at 20 days after planting, differences were evident during the later stages of plant growth. The pesticides significantly inhibited AM root colonisation and the number of spores in all legumes, but on the other hand, spore formation was stimulated in pesticide-treated cowpea 60 days after planting. The accumulation of N, P and K in pesticide-treated plants was lower than in control plants. Growth and nutrient status of the legumes varied with nodulation and AM colonisation. The results suggest that pesticides affect plant growth, Rhizobium/Bradyrhizobium and AM fungi at different stages of plant growth and effects varied with pesticide and plant species. (C) 2000 Elsevier Science B.V.