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Microscopy of Cultured Loblolly Pine Seedlings and Callus Inoculated with Cronartium fusiforme. W. R. Jacobi, Graduate research assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27650, Current address of senior author: Department of Botany and Plant Pathology, Colorado State University, Fort Collins 80523; H. V. Amerson(2), and R. L. Mott(3). (2)(3)Visiting assistant professor, and professor, Department of Botany, North Carolina State University, Raleigh 27650. Phytopathology 72:138-143. Accepted for publication 26 March 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-138.

Loblolly pine seedlings grown in axenic culture were readily penetrated and colonized in the hypocotyl region by Cronartium fusiforme. Germinating basidiospores with variable-length germ tubes frequently produced appressoria of various shapes and degrees of swelling. Penetration occurred directly through epidermal cells and occasionally through the guard cells or subsidiary cells of stomata. Loblolly pine callus was resistant both to cell penetration and to intercellular growth by C. fusiforme under the culture conditions utilized. Several culture media were used in efforts to alter the growth, form, and susceptibility of the callus to infection; none of these were successful. Basidiospores germinated and germ tubes grew normally on all callus tissues tested, but neither appressoria nor haustoria were observed. Two types of extracellular substances were detected primarily in senescing callus cells.

Additional keywords: Pinus taeda, fusiform rust, scanning electron microscopy, stress metabolites.