Reduced early growth of direct drilled wheat in southern New South Wales - role of root inhibitory pseudomonads

The early growth of wheat was compared under direct drilled (DD) and cultivated treatments at 21 farm sites throughout southern New South Wales in 1997, 7 sites in 1998, and 11 sites in 1999. The experiments investigated the involvement of (1) paddock history/management, (2) common soil-borne fungal pathogens, and (3) populations of various microbial groups in early growth reductions associated with DD. Cultivation increased early vegetative growth at 62% of sites with an average increase in dry matter production of 33% compared with DD plots. Reduced shoot growth under DD was closely associated (r = 0.899) with decreased development of the root system. Early growth reductions under DD did not appear related to management practices such as different sowing points, herbicide or fertiliser application, or soil pH and fertility. Soil type and previous crop species, however, influenced the incidence but not severity of early growth reductions. Reduced growth under DD was not associated with the presence of any of the common soil-borne fungal pathogens of wheat, or with rhizosphere populations of total aerobic bacteria, total fungi, aerobic spore-forming bacteria, Gram-negative bacteria, and actinomycetes. On average, Pseudomonas populations in the rhizosphere of seedlings grown in cultivated soil were reduced by 61% compared with the DD treatment at sites where early growth reductions under DD were evident. An assessment of the inhibitory activity of pseudomonads towards wheat seedlings in a test-tube bioassay indicated that reduced growth in DD plots was more closely related with the inhibitory activity of Pseudomonas spp. to root growth than their population in the rhizosphere. A close relationship existed between the inhibitory activity of Pseudomonas spp. isolated from each site and the response of wheat seedlings to cultivation (r = 0.865). These results suggest that Pseudomonas spp. with inhibitory activity to root growth are involved in the reduced early growth of DD wheat in southern New South Wales.