Correlation of resistance and H2O2 production in Ulmus pumila and Ulmus campestris cell suspension cultures inoculated with Ophiostoma novo-ulmi.

The Dutch elm disease (DED) pathogen Ophiostoma novo-ulmi Buissm. elicited the production of H2O2 in cell suspension cultures of the resistant species Ulmus pumila L. This response was not observed in suspensions of the susceptible elm U. campestris Mill. H2O2 production started after a lag time of 30-40 min following inoculation, peaked between 4 and 6 h and lasted up to 24 h. Treatment of the suspensions with exogenously added H2O2 did not cause accumulation of the sesquiterpene phytoalexins mansonones nor of the coumarin scopoletin. Spore germination and growth of O. novo-ulmi were significantly delayed with different amounts of H2O2 (0.1-1 mM). These results suggest that H2O2 production is an inducible defence response which may contribute to DED resistance by delaying the growth of the pathogen at the earliest stages of infection. Whether H2O2 is involved in other elm defence responses to the pathogen is presently unknown, but its production seems to be an independent event from phytoalexin formation.