Production in vitro of phytotoxic compounds by non-aggressive and aggressive isolates of Ophiostoma ulmi, the Dutch elm disease pathogen

Development in shake culture of two non-aggressive and two aggressive isolates of the Dutch elm disease fungus, Ophiostoma ulmi, was monitored in a time course study. The time sequences for the production of total biomass, spores, polysaccharides and phytotoxic compounds were followed, but none of them showed a clear correlation with aggressiveness. The same parameters were examined for nine non-aggressive and 11 aggressive isolates after 10 days growth. Of two phytotoxic compounds, a glycopeptide and the protein cerato-ulmin, only the latter was generally produced in larger quantities by aggressive isolates. However, variability in production and a number of exceptions to the trend made cerato-ulmin production an uncertain criterion for assessing the aggressiveness of an individual isolate.Immunofluorescence of sections of infected elms treated with an anti-glycopeptide serum showed intense fluorescence only in the cell walls of the fungus. Apparently the glycopeptide is abundantly present in or on fungal cell walls.The roles of the different compounds in relation to Dutch elm disease symptom development are discussed.