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In Vitro Resistance of Embryos of Pinus taeda to Cronartium quercuum f. sp. fusiforme: Ultrastructure and Histology. D. J. Gray, Graduate research assistant, Botany Department, North Carolina State University, Raleigh 27650, Current address of senior author: Department of Plant and Soil Science, University of Tennessee, Knoxville 37996; H. V. Amerson, visiting assistant professor, Botany Department, North Carolina State University, Raleigh 27650. Phytopathology 73:1492-1499. Accepted for publication 17 May 1983. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-1492.

Several responses of Pinus taeda to infection were noted in vitro as early as 36 hr after inoculation with Cronartium quercuum f. sp. fusiforme when infected embryos were examined with light and transmission electron microscopy. Incompatible necrosis and apposition formation were among these early host responses. Incompatible necrosis was first characterized by an electron-opaque region that was initiated at the host cytoplasm-vacuole boundary. Concurrently, the host cytoplasm became more densely stained and eventually necrotic. Wall breakdown subsequently occurred. Haustorial necrosis occurred simultaneously or after host cell necrosis with only the haustorial body affected. Also, uninfected cells adjacent to necrotic cells often became necrotic. Three seed lines of divergent field resistance were used to examine necrosis. Embryos from all three lines exhibited some degree of incompatible necrosis; however, the most resistant line developed the most necrosis thus correlating incompatible necrosis with field resistance. The degree of necrosis appeared similar in all three lines at later sample times. Host wall appositions encountered in penetrated cells of P. taeda appeared to simultaneously permit host cell survival and fungal necrosis. Both incompatible necrosis and apposition formation were regarded to be effective host responses to infection.