Nutritional constraints on displacement of 'Fusarium pseudograminearum' from cereal straw by antagonists

Abstract

Displacement of the fungus 'Fusarium pseudograminearum' from stubble by antagonists is a potential means of biocontrol of crown rot in cereals. The role of carbon and nitrogen nutrition in interactions between the pathogen and the antagonists 'Fusarium equiseti', 'Fusarium nygamai', 'Trichoderma harzianum' and the non-antagonistic straw fungus 'Alternaria infectoria' was investigated. Sole carbon source utilization patterns on Biolog plates were similar among the three 'Fusarium' species, suggesting a possible role for competition. However, carbon niche overlap was unlikely to be important in antagonism by 'T. harzianum'. Straw medium supplemented with sugars generally reduced the inhibitory effect of antagonists on growth of 'F. pseudograminearum' in dual culture, indicating that availability of simple carbon sources does not limit antagonism. Adding nitrogen as urea, nitrate or ammonium to straw medium had little effect on antagonism by 'F. equiseti' and 'F. nygamai', but ammonium addition removed the inhibitory effect of 'T. harzianum' on growth of 'F. pseudograminearum'. Displacement of 'F. pseudograminearum' from straw by all fungi in a Petri dish assay was greater when urea or nitrate was used as a nitrogen source than with ammonium. All forms of nitrogen significantly increased displacement of 'F. pseudograminearum' from straw under simulated field conditions when straws were either inoculated with 'T. harzianum' or exposed to resident soil microbes. However, in 2 out of 3 experiments urea and nitrate were more effective than ammonium. The results suggest that availability of nitrogen, but not carbon, is limiting the activities of antagonists of 'F. pseudograminearum' in straw, and the way nitrogen is applied can influence the rate of displacement and mortality of the pathogen in host residues.