The actual transmission of DED occurs in spring and early summer (late May to early July) at the time of early wood production. Therefore the spring maturing adult of the European beetle and the overwintering adult of Hylurgopinus rufipes are most important for transmission. In the far northern parts of the US where the susceptibility period of the elm is extended (e.g. St. Lawrence Valley), the native beetles that overwintered as larvae probably play the main role {[230]}.

Photo 48:        Transmission of Dutch elm disease occurs during twig crotch feeding of elm bark beetles on healthy elms (Courtesy of P. Svihra, University of California, Novato, CA, USA).

The beetles emerge from their breeding sites and subsequently fly to the tops of healthy elms to feed {[332],[371]}. In this way elm bark beetles contaminated (internally and/or externally) with O. ulmi  s.l. spores can transmit the fungus to a new host (Photo 48). Transmission by H. rufipes usually occurs when adult beetles move from the hibernation tunnels in the base of the trunk to the crown of the elm to feed {[482]}. Basset et al.{[367]} showed that contact between beetle and xylem for a period of 72 hours can be sufficient for transmission of DED.

Not every feeding wound results in DED infection {[381]}. In 1941 Parker et al.{[329]} reported that 13% of all cases resulted in succesful infection of susceptible elms. Webber et al.{[371]} tested the efficiency of the infection process at several sites in England and showed that 23-47.5% of the beetles in the feeding grooves carried O. ulmis.l. spores. Detectable O. ulmi s.l. contamination was found for 30% of the feeding wounds. Apparently most beetles transmit their viable spores to the elm in the feeding groove upon arrival. However, the resulting xylem infection is much lower. Eventually, 2-5% of the feeding grooves will lead to DED infection {[548]}. The infection may be a result of primary spore transfer from the beetle into the xylem vessels. More likely a secondary infection will occur. O. ulmis.l. spores first colonize the feeding wound and subsequently grow into the xylem tissue {[344]}. The moisture content in combination with the availability of nutrients in the feeding grooves provide favourable conditions for fungal growth and sporulation {[371]}.

Members of the superfamily Curculionidae are known to be associated with the ascomycete fungal orders Sphaeriales, Helotiales, Ophiostomatales, Endomycetales (the yeasts), Sordariales, the Basidiomycotina and the Deuteromycotina classes Coelomycetes (3 orders) and Hylomycetes (4 orders) {[451]}. Colonization of the feeding grooves with potential antagonistic fungi and other mycophageous organisms (e.g. mites) results in microbial competition with O. ulmi s.l. (Control:Fungus:). As a consequence the usual threshold inoculum of viable O. ulmis.l. spores required to establish DED infection (1000 spores) will be increased {[436]}.