Figure 11.1–A: diagram of Arceuthobium occidentale on Pinus sabiniana. The depth of individual sinkers (s) in host growth rings indicates the age of the infection in that specific area of the host stem. The oldest portion of the endophytic system shown is in section A, where the primary haustorium (ph) extends into 2-year-old xylem. The youngest portion is in section C, where the endophyte lies entirely within the cambial zone and the most recently produced phloem and xylem derivatives of the host. Sinkers are initiated from near the tips (unlabeled arrow in section C) of bark strands (b). B: diagram of a Pseudotsuga menziesii shoot infected with Arceuthobium douglasii. The bud scale regions or segment girdles (nos. 1-5) delimit annual growth increments. Although the endophytic system occupies the entire host branch, aerial shoots (A) do not erupt until the branch is 3 years old. Aerial shoots are particularly concentrated at bud scale regions (large arrows), 1x.