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 plan t diseases including take-all (Gaeumannomyces graminis var. tritici (Ggt)). However, the mechanism by which the disease is affected by earthworms rema ins unknown. We conducted both field and controlled-environment experiments to examine the interactions between earthworms and Ggt. The controlled-env ironment experiments used split-pots in all possible combinations of with o r without earthworms and with or without Ggt on either side of a semi-perme able membrane. Phospholipid fatty acid (PLFA) analysis was used to study gr oss changes in the microbial community associated with these treatments. Th e results from the field experiments showed that earthworms increased the y ield, but had no effect on the severity of Ggt in soft white spring wheat. Similarly, in the controlled-environment experiments Aporrectodea tubercula ta 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 rega rdless 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 th ere 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 i n 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.