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 th roughout southern New South Wales where slower seedling growth in direct-dr illed (DD) soil than in cultivated soil had been commonly reported. Experim ents were conducted in a controlled environment using soil cultivation and fumigation to assess the role of physical and biological constraints to gro wth under direct drilling. Populations of various microbial groups (especia lly Pythium and Pseudomonas spp.) were assessed for their involvement in th e growth reductions. Soil cultivation increased the growth of seedlings relative to the DD treat ment in cores from 18 of the 26 sites (mean increase 43%), while sterilisat ion 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 c ores from most sites, suggesting that cultivation only partially removes th e growth limitation. With the exception of one site, cultivating sterilised cores provided no additional growth benefits over the sterilised DD treatm ent, indicating that biological rather than physical factors per se were pr edominantly 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 sterilisat ion reduced populations by 65%. An assessment of the inhibitory activity of pseudomonads towards wheat seedlings in a test-tube bioassay indicated tha t reduced growth in DD cores was more closely related to the deleterious ac tivity of Pseudomonas spp. toward root growth than to their population in t he rhizosphere. A close relationship existed between the inhibitory activit y of Pseudomonas spp. and the response of wheat seedlings to both cultivati on (r = 0.859) and sterilisation (r = 0.842).