Control of Dutch elm disease by induced host resistance

Summary A range of fungi was screened for the ability to suppress symptoms of Dutch elm disease in European and hybrid elms under UK and Italian field conditions. The efficacy of the preventative inoculations varied Considerably between elm clones of differing resistance to the causal fungus Ophiostoma novoulmi. On susceptible English elm, Ulmus procera, preventative inoculations with conidia of Verticillium dahlias, Opbiostoma ulmi or Ophiostoma piceae (OPH form) caused no reduction in final disease levels. On U. x bollandica cv.‘Commelin’, pretreatment with V. dabliae or O. ulmi reduces final disease levels, but not significantly, On the elm clone Ca06 of U. carpinifolia, and on the hybrid clones FL025 and 196–6, significant (p0.05) symptom suppression occurred in trees pretreated with O. ulmi or V. dahliae. On the more resistant elm clones 3–14 and Lobel, only V. dahliae was an effective pretreatment (p0.05). Certain pretreatment fungi alone caused symptoms in some of the elm clones. Major difficulties in using this approach to control Dutch elm disease are discussed.The technical assistance by F. C. Baldwin, R. Brooker, S. A. Kirk and J. Rose at the Forest Research Station, Alice Holt, and by A. Fagnani and F. Ferrini at CNR, Florence, as well as statistical advice from A. J. Peace at Alice Holt, is gratefully acknowledged.The authors would like to thank the Pilkington Charitable Trust for funding the research in the UK and lsotron plc, Swindon, Berkshire, for gamma irradiating the conidial suspensions. The Italian contribution was supported by the National Research Council of Italy, special project RAISA, subproject no. 2, paper no. 2133. The authors also thank John GIBBS for helpful comments on the manuscript.