Elm bark beetles (Scolytus spp. and Hylurgopinus rufipes) carry spores of the pathogen in their bodies. When the insects feed on vascular tissues on branches of the tree, the fungus is inoculated and colonizes the xylem vessels causing its clogging and producing a vascular wilt. The first symptom is a wilting at the end of one branch, which progress downward in the crown. Symptoms may progress throughout the whole tree in a single season. When the fungus spreads from diseased to healthy trees through grafted roots, the entire crown may be affected very rapidly. Branches and stem of infected plants develop dark streaks of discoloration in the sapwood. The disease caused by O. novo-ulmi usually progresses much faster than that caused by O. ulmi sens. str.
Ophiostoma novo-ulmi: Colonies on malt extract agar after 7 days in darkness at 20 oC and 10 days in diffuse daylight greyish-white to cream-white ranging from regular striate petaloid forms to irregular lobed forms; commonly with moderate aerial mycelium aggregated into ropes to give a fibrous striate appearance or occasionally with less aerial mycelium and frosty to smooth colonies. Diurnal zonation moderate to strong. Growth on malt extract agar at 20 oC in darkness ranging from (2.8–) 3.1–4.8 (–5.7) mm day-1; growth optimum 20–22 oC; maximum 32-33 oC. Hyphae septate, 1—6 µm in diameter, submerged hyphae sometimes to 10 µm in diameter; aerial hyphae often aggregated into strands. Mycelial conidia usually abundant, Sporothrix: conidiophores mostly lateral, 10—30 (—50) µm; conidia holoblastic, borne on short denticles of 0.5—1 µm, single-celled, hyaline, very variable, ellipsoid to elongate, often tapering and slightly curved, with a small attachment collar, 4.5—14 x 2—3 µm. Mycelial conidia often aggregated into mucilaginous droplets, also budding in a yeast-like fashion; Synnematal anamorph (Graphium or Pesotum) usually absent on malt agar, generally produced only on sterilised elm sapwood (but abnormal synnemata may be produced on malt agar by degenerate colonies); single or multiple, brown-black, slender, up to 1—2 mm tall. Attached to substratum by brown rhizoid-like hyphae and composed of parallel bundles of brown septate hyphae, flaring at the top to branched hyaline hyphae producing holoblastic single-celled hyaline ovoid to ellipsoid conidia 2—6 x 1—3 µm, aggregating into a cream-white mucilaginous spore drop. The holoblastic budding yeast-like anamorph is produced in liquid cultures, and on the surfaces of solid media. Heterothallic with two compatibility types, ‘A’ and ‘B’. A-types producing brown-black protoperithecia, occasionally to frequent on on malt agar and frequent to abundant on elm sapwood agar. B-types producing ascogonia sporadically on malt agar and ascogonia or protoperithecia occasionally to frequently on elm sapwood agar. Perithecia, superficial to partially immersed, attached to the substratum by brown rhizoid-like hyphae; the base globose, black 75—140 µm wide, sparsely to moderately bristly, the bristles brow