Involvement of root inhibitory Pseudomonas spp. in the poor early growth of direct drilled wheat:: studies in intact cores

Wheat seedlings were grown in intact cores of soil removed from 26 sites throughout southern New South Wales where slower seedling growth in direct-drilled (DD) soil than in cultivated soil had been commonly reported. Experiments were conducted in a controlled environment using soil cultivation and fumigation to assess the role of physical and biological constraints to growth under direct drilling. Populations of various microbial groups (especially Pythium and Pseudomonas spp.) were assessed for their involvement in the growth reductions. Soil cultivation increased the growth of seedlings relative to the DD treatment in cores from 18 of the 26 sites (mean increase 43%), while sterilisation increased growth in DD cores from 25 of the sites (mean increase 68%). The growth response to cultivation was less than that to sterilisation in cores from most sites, suggesting that cultivation only partially removes the growth limitation. With the exception of one site, cultivating sterilised cores provided no additional growth benefits over the sterilised DD treatment, indicating that biological rather than physical factors per se were predominantly responsible for the reduced growth. The reduced growth in DD cores was not associated with the presence of any of the common soil-borne fungal pathogens of wheat. Averaged across sites, Pseudomonas populations in the rhizosphere of seedlings grown in cultivated cores were reduced by 50% compared with the DD treatment, while sterilisation reduced populations by 65%. An assessment of the inhibitory activity of pseudomonads towards wheat seedlings in a test-tube bioassay indicated that reduced growth in DD cores was more closely related to the deleterious activity of Pseudomonas spp. toward root growth than to their population in the rhizosphere. A close relationship existed between the inhibitory activity of Pseudomonas spp. and the response of wheat seedlings to both cultivation (r = 0.859) and sterilisation (r = 0.842).