CONTROL OF DUTCH ELM DISEASE BY INDUCED HOST-RESISTANCE

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 fie ld conditions. The efficacy of the preventative inoculations varied co nsiderably between elm clones of differing resistance to the causal fu ngus Ophiostoma novo-ulmi. On susceptible English elm, Ulmus procera, preventative inoculations with conidia of Verticillium dahliae, Ophios toma ulmi or Ophiostoma piceae (OPH form) caused no reduction in final disease levels. On U. x hollandica cv. 'Commelin', pretreatment with V. dahliae or O. ulmi reduced final disease levels, but not significan tly. On the elm clone Ca06 of U. carpinifolia, and on the hybrid clone s FL025 and 196-6, significant (p < 0.05) 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 pretreatm ent (p < 0.05). Certain pretreatment fungi alone caused symptoms in so me of the elm clones. Major difficulties in using this approach to con trol Dutch elm disease are discussed.