Ontogeny of the Needle Cast Fungus, Hypodermella Arcuata

SUMMARY

Hyphae of Hypodermella arcuata penetrate the endodermis of Pinus monticola and tissues external to it. Stromatic tissue developing between the host's hypodermis and epidermis constitutes the first macroscopic sign of the fungus and is visible on needles in April or May the year following infection. Differentiation begins in the upper half of the stroma with an orientation of hyphae perpendicular to the dermal tissues. These hyphae appear at maturity to be paraphyses, but initially are apically attached to the upper stromatic layer. Pseudoparenchymatous tissue surrounding the centrum becomes compressed forming a wall. Unitunicate asci develop from the basal pseudoparenchymatous tissue. The host's epidermis and the upper stromatic tissue of the fungus remain over the hymenium until maturity, at which time they split, almost completely exposing the hymenium. Eight sheathed uninucleate ascospores develop in each ascus and are forcibly ejected when the mature ascocarp absorbs water. Ascospores germinate from either end by short germ tubes, form appressoria, and probably penetrate the needle directly through the cuticle and epidermis.