Spore stages of O. ulmi s.l.

O. ulmi s.l. has four commonly observed spore stages, and is able to reproduce itself both sexually and asexually (Fig. 6). Starting with any type of spore, O. ulmi s.l. can multiply by yeast-like budding of earlier formed conidia (Fig. 6A), or germination (Fig. 6B) and proliferation of mycelium. The hyphae and spores of O. ulmi s.l. are predominantly uninucleate; the somatic nuclei are haploid {[530]}. Hyphal-mediated spore formation can occur directly on the mycelium (Sporothrix state, Fig. 6C) or successively through asexual fruiting bodies (Graphium state, Fig. 6D). All three asexual stages are able to complete the vegetative life cycle. Experiments described by W.C. Richards {[169]} suggest that the absence of sporulation in certain O. ulmi isolates is controlled by a single nuclear gene.

O. ulmi s.l. is a heterothallic fungus (self-sterile, male gametes can only fertilize female gametes of a sexually compatible different mycelium) {[311]}. When conditions are favorable and cells of different mating types come together, association of dissimilar nuclei is possible (Fig. 6E) and sexual fruiting bodies (perithecia), asci, and sexual (asco)spores are formed {Fig. 6F, 230,513}. The occasional sexual propagation results in the maintenance of a great genetic variability of O. ulmi s.l. {[234]}.

In the following paragraphs, the different spore stages of the fungus and the regulation of yeast-mycelium transition will be discussed in more detail.

Fig. 6: Life history of O. ulmi s.l. Beginning with any type of spore, the DED fungus can multiply by yeast-like budding (A) or germination (B) and proliferation of hyphae (mycelium). Mycelium produces successively the Sporothrix state (C) and the Graphium  state (D). Spores from any state can complete a vegetative life cycle. When cells of different mating types come together, allowing association of dissimilar nuclei (E), the development of perithecia, asci and, ascospores (F) is possible. Living trees are not necessary for O. ulmi s.l. propagation. Ascospoes, Sporothrix- and Graphium- type spores, however, contaminate elm bark beetles, which may inoculate them into healthy elms (Courtesy of W.A. Sinclair, Cornell University, Ithaca, NY, USA).