Sporothrix-type spores
O. ulmi s.l. is able to produce conidia directly on almost all the weft of aerial mycelium.
The asexual, mycelial-borne spores – which are often incorrectly named the Cephalosporium stage – are now referred to as the Sporothrix stage. Spore formation occurs terminally on short side branches or conidiophores
to which they are attached by minute denticles {[513]}. Each successive spore develops behind and to one side of the previous apex where growth has ceased
(sympodial growth, Photo 22).
The sequential production of conidia at the tip of a conidiophore results
in the formation of a characteristic cluster of Sporothrix spores embedded in a mucilaginous drop. The colorless Sporothrix spores are single-celled and show a elongated pyriform (pear-like) shape,
often with one side curved convexly {[230],[513]}.
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Photo 22: Sympodial growth of O. ulmi s.l. Each successive spore develops behind
and to one side of the previous apex where growth has ceased
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The size of the mycelial conidia varies among O. ulmi species {[513],[139]}. For O. ulmi and O. novo-ulmi (race NAN and EAN), the spores commonly measure 4-6 x 2-3 µm; O. himal-ulmi Sporothrix stage conidia range from ca. 4-14 x 2-3 µm (mean 7.0 ± 0.4 x 2.3 µm). Jensen et al. {[261]} determined the fatty acid composition of phospholipids from O. ulmi conidiospores. While the spores contained 10.6% palmitic acid (C16:0), 23.8% oleic acid (C18:1), 50.5% linoleic acid (C18:2), and 9.0% linolenic acid (C18:3), arachidonic was completely absent.
O. ulmi and O. novo-ulmi are reported to produce microhyphae and various other irregularly shaped
growth forms when growing in liquid cultures, through millipore filter membranes, and on sterilized wood sections {[96],[550],[551],[552]}. Microhyphae
are cytologically different from normal O. ulmi cells in that they often lack a delimiting wall when in contact with host
cells {[96],[556]}. Several authors suggest the existence of (micro)endoconidia for the DED fungus. Endoconidia are spores formed inside the mycelium,
resulting from division of the protoplast within the broad hypha. (Micro)endospores can also be liberated by breakdown of the enclosing cells. Subsequently, the liberated spores may enlarge into typical spores {[594]}. Already
in the early 1960s,the presence of mostly brown-colored O. ulmi microendoconidia (< 0.5 µm in diameter) in infected elm trees was reported
{[553],[554]}. Although Brotzman et al. {[557]} could not confirm the latter observation, Gowen and Marion {[550]}
found O. ulmi cells resembling these microendospores. Reduction of the conidial size
(from 4-5 x 2-3 µm to < 1 x 1.5 µm) may result from degeneration under submerged conditions of continuous conidial formation {[549]}.
Sansome et al. {[555]} described O. ulmi spores the size of ordinary conidia that occur closely packed together
within broad vegetative hyphae. Occasionally, these spores were found to be associated with the presence of very narrow hyphae (microhyphae) within
the broad vegetative mycelium. Whether or not the observed conidia are true endoconidia remains to be determined.
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