VIEW ARTICLE

Cytology and Histology

Histopathology of the Brown Spot Fungus on Longleaf Pine Needles. F. F. Jewell, Sr., Professor, School of Forestry, Louisiana Tech University, Ruston 71272; Phytopathology 73:854-858. Accepted for publication 28 December 1982. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-854.

The brown spot fungus, Scirrhia acicola (Dearn.) Siggers, affected only the mesophyll cells in bar spot type lesions on needles of longleaf pine, Pinus palustris Mill. The affected mesophyll cells were collapsed and flattened in a lattice pattern. Hyphae of S. acicola in symptomatic tissue were sparse and were not observed in host tissue adjacent to or beyond the symptomatic area. The amount of host tissue damage was far out of proportion to the presence of the pathogen. The production of a toxin by S. acicola is proposed as a cause for the extensive damage to the host mesophyll. In contrast, profuse hyphal development and cellular deterioration were noted in all tissues of host needles presumably killed by S. acicola. Spore-bearing structures of S. acicola developed in the outer mesophyll and at maturity were somewhat erumpent and split the epidermis. Conidia were four-celled, olive to light brown, curved, and pointed at one end but somewhat blunt at the other. Perithecia were present only on dead tissue. Ascospores were hyaline, were two-celled with one nucleus per cell, and exhibited no oil globules. Fungi consistently associated with needles infected by S. acicola were a species of Fumago, present on symptomatic areas but not on dead tissue, and species of Lophodermium and Pestalotia.