Interactions between earthworms, beneficial soil microorganisms and root pathogens

The capacity of earthworms and soil microorganisms to influence plant growth is well known. However, the interactions between them have been little studied. Recent work in southern Australia has examined these associations with a view to managing both earthworms and beneficial microorganisms to promote cereal and pasture productivity. In this paper we examine the role of earthworms as vectors of beneficial soil bacteria and their capacity to influence the population dynamics and impact of microorganisms on soil and plants. The earthworms Aporrectodea trapezoides and Aporrectodea rosea are the most common species in agricultural ecosystems in southern Australia. They have been shown to spread Pseudomonas corrugata 2140R, a biocontrol agent for Take-all (a fungal disease of wheat roots caused by Gaeumannomyces graminus var. tritd) through soil with corresponding colonization of the roots of wheat seedlings at soil depths of 3 - 9 cm. In addition, A. trapezoides can spread Rhizobium through the soil and cause increased root colonization and nodulation of legume roots. Despite evidence that populations of soil microorganism are affected by earthworm activity at a spatial scale (e.g. in casts or tunnel walls), there is little evidence to demonstrate that earthworms affect populations at larger spatial scales. Take-all and Rhizoctonia bare patch are the most serious fungal root diseases of wheat in southern Australia. Laboratory trials have shown that, in the presence of the worms A. trapezoides and A. rosea, symptoms of these diseases in wheat seedlings are substantially reduced. In a further trial, the presence of A. trapezoides was associated with a 100-fold decrease in population size of Rhizobium recoverable from soil after 40 days.