Earthworms indirectly reduce the effects of take-all (Gaeumannomyces graminis var. tritici) on soft white spring wheat (Triticum aestivum cv. Fielder)

Earthworms have been shown to decrease the severity of some soil borne plant diseases including take-all (Gaeumannomyces graminis var. tritici (Ggt)). However, the mechanism by which the disease is affected by earthworms remains unknown. We conducted both field and controlled-environment experiments to examine the interactions between earthworms and Ggt. The controlled-environment experiments used split-pots in all possible combinations of with or without earthworms and with or without Ggt on either side of a semi-permeable membrane. Phospholipid fatty acid (PLFA) analysis was used to study gross changes in the microbial community associated with these treatments. The results from the field experiments showed that earthworms increased the yield, but had no effect on the severity of Ggt in soft white spring wheat. Similarly, in the controlled-environment experiments Aporrectodea tuberculata increased plant growth, but had no effect on the severity of Ggt. There was a significant increase in the percent total N and a lower ON ratio regardless of earthworms in Ggt infested soil compared with non-infested soil. The PLFA analysis showed that earthworms increased the total amount of PLFA in soil and the activity of Gram negative bacteria. This indicated that there was a larger and more active microbial population when earthworms were present. The PLFA ratio of bacteria: fungi showed a significant interaction between earthworms and take-all. Earthworms increased the fungal biomass in the Ggt inoculated treatment and in contrast, promoted the bacteria when there was no Ggt. The mechanism by which earthworms reduce the severity of take-all and increase the growth of soft white spring wheat appeared to be microbially-mediated. (C) 2001 Elsevier Science Ltd. All rights reserved.