Opportunistic association between Pythium species and weed residues causing seedling emergence failure in cereals

Abstract

Field trials were conducted to establish the link between weed residue management, Pythium infectivity and cereal seedling emergence failure. The herbicide glyphosate was used to manage weed-free and weed residue treatments, and the systemic fungicide metalaxyl was used to protect seedlings from pythiaceous attack. Seedling vigour and emergence were enhanced by the application of metalaxyl, and by the total suppression of weed growth. Invasion of cereal seedlings by pythiaceous fungi was greatest when seed was sown into cloddy seedbeds within 1-3 days after herbicide application. Debilitated weeds enhanced the activity of pythiaceous pathogens, promoting the invasion of cereals sown into the residues. If sowing is delayed for 3 weeks after herbicide application, pythiaceous activity will be reduced by saprophytic competitors colonizing the residues, resulting in improved seedling vigour and establishment.