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Cytology and Histology

Host-Parasite Relationships in Karnal Bunt of Wheat. Norma L. Cashion, Wheat Program, CIMMYT, Londres 40, Apartado Postal 6-641, 06600 Mexico, D. F.; E. S. Luttrell, Department of Plant Pathology, University of Georgia, Athens 30602. Phytopathology 78:75-84. Accepted for publication 10 July 1987. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-75.

Tilletia indica, cause of Karnal bunt, is a local lesion pathogen that infects individual ovaries of wheat. Symptoms first appear at the soft-dough stage in the form of blackened areas surrounding the base of the grain and extending upward to varying degrees. In severe infections the entire grain is blackened and shriveled. This study by light and electron microscopy showed that the fungus is restricted to the pericarp, where it is entirely intercellular. Hyphae proliferate in the space formed by disintegration of the middle layers of parenchymatous cells of the pericarp during normal development of the grain and prevent fusion of the remaining outer and inner layers of the pericarp with one another and with the seed coat. The smut hyphae form a compact, hymenium-like layer over all surfaces of the pericarp tissue surrounding this cavity and give rise to short, septate sporogenous hyphal branches that produce teliospores singly from their terminal cells. Growth of the fungus ruptures the connection between the pericarp tissue surrounding the vascular bundle in the bottom of the adaxial groove in the pericarp and the nucellar projection along the length of the developing seed. The consequence is atrophy of the seed through disruption of the normal flow of nutrients from the pericarp. The endosperm is shrunken to varying degrees and cartilaginous in appearance. The embryo with attached endosperm may be easily dissected out and is germinable before or after removal from the grain. In the most severe infections, the grain is reduced to a black membranous sack of teliospores, and the shriveled embryo is dead.