E-Flora BC: Electronic Atlas of the Flora of British Columbia

Summary:
Nectria pithoides is distinguished "by the bright red perithecia which are decidedly barrel-shaped when dry". Other features include the yellowish stroma on bark of dead alders, and microscopic characters.

The type description is from BC.

Fruiting body:
stroma erumpent, yellowish; perithecia seated on the stroma in dense clusters 0.15-0.25cm in diameter, "individual perithecia bright red, collapsing so as to become truncate, resembling the head of a barrel" with the ostiolum appearing as a light translucent dot in the center, 0.01-0.025cm in diameter

Microscopic:
spores 6-10 x 3-4 microns, elliptic, colorless, 1-septate, with an oil droplet in each cell; asci 8-spored, 70-80 x 5 microns, cylindric