EVALUATION OF DUTCH ELM DISEASE SUSCEPTIBILITY AND PATHOGENICITY OF OPHIOSTOMA-ULMI USING MICROPROPAGATED ELM SHOOTS

Elm (Ulmus sp.) shoots from in vitro micropropagated plants were teste d with Ophiostoma ulmi culture filtrates, at different dilutions to de termine the susceptibility of different host clones to Dutch elm disea se (DED). 'Commelin', a susceptible Dutch hybrid, showed a high wiltin g index but that of American elm was higher. Although U. pumila x japo nica (Sapporo gold 2) was the most tolerant to spore inoculation, its wilting index was as high as that of 'Commelin'. The absorption capaci ty and wilting index of the elm shoots showed consistent correlation t o isolate effectiveness (as determined by inoculation experiments), to production of toxin (cerato-ulmin) and to filtrate dilution. This met hod could be used to assess the effectiveness of the different O. ulmi isolates and for the detection of elm genotypes that are highly susce ptible to Dutch elm disease.