LIFE CYCLE OF O. ULMI S.L.

The life cycle of the DED fungus can be clearly divided into two phases, i.e., the pathogenic phase and the saprophytic phase. During its entire life cycle, the fungus remains associated with the elm tree. It is unknown whether or not the DED fungus can survive in soil on its own. Invasion and spread of O. ulmi s.l. in the xylem vessels of the host tree occurs in the pathogenic phase. However, the first intimate interaction between host, vector, and fungus occurs during the saprophytic phase. The latter phase starts when scolytid beetles enter the bark to breed, and lasts until their progeny emerge. During the saprophytic phase, the DED fungus colonizes the beetle galleries in the phloem of dead or dying elm trees. The combined bark colonization by beetle and pathogen ensures the reunion of O. ulmi s.l. with a new vector generation, which is required for completion of a successful DED cycle. Although in warm summers the saprophytic phase may be as short as two months, overwintering bark beetle larvae regularly produce a saprophytic phase of 6-10 months (start in late summer and last till early summer of the next year). The following sequence of events appears to take place during the life cycle of O. ulmi s.l. {[371],[548]}.