Introduction Elms are important trees in urban environments and coastal areas due to their resistance to harsh conditions such as wind, salt, flood, and narrow root space, as well as to their attractive architecture. However, a devastating disease, the so-called Dutch elm disease (DED), appeared in Europe during the 1920s and in North America by 1930. Sudden wilting and dying of the leaves and branches was caused by Ophiostoma ulmi during the first pandemic from 1920s to 1940s, while an even more aggressive strain O. novo-ulmi is responsible for the current pandemic. Elm bark beetles, mainly Scolytus scolytus and S. multistriatus, transmit the disease by breeding in weakened and dead elms (Scheffer et al, 2008). Approximately 100 years after its first introduction into Europe, biological control of Dutch elm disease is an effective component of an integrated control strategy for this disease. Conidiospores of Verticillium albo-atrum isolate WCS850 (active ingredient of Dutch Trig®) enhance the natural defence mechanism of elms after being injected in the xylem tissue of tree, where they germinate and induces resistance (Scheffer, 1990). The isolate was proven to be non-pathogenic for all the 19 tree species that had been tested (http://www.dutchtrig.com/home)