Histopathology of Seed-Borne Infections - Applied Research Center ...

120 Histopathology of Seed-Borne InfectionsswflflfflssAscembBCFIGURE 5.10 Schematic diagrams to show different pathways of development of seed-borneinfections of smuts and bunts in cereals. A, Ustilago tritici (loose smut) of wheat — intraembryalseed inoculum followed by systemic infection. B, Ustilago hordei (covered smut)of barley — externally seed-borne inoculum causing systemic infection. C, Tilletia indica(karnal bunt) — externally seed as well as soil-borne inoculum, spores cause blossom infectionof individual florets. (Abbreviations: mb, embryo; ffl, first foliar leaf; fl, foliar leaves; s, seed;sc, seed coat; ss, soil surface; wfl, weakened foliar leaves.) (Adapted and redrawn fromNeergaard, P. 1979. Seed Pathology. Vols. 1 and 2. Macmillan, London.)to form promycelium. They cause secondary infection to developing caryopsis, resultingin embryal infection, which remains dormant in seeds, e.g., Ustilago tritici andUstilago nuda (loose smuts of wheat and barley), until germination (Figure 5.10A).The primary inoculum is externally seed-borne or, as in some Tilletia spp., externallyseed- as well as soil-borne, and causes seedling (coleoptile) infection at the time ofseed germination. Infection is systemic and latent in the vegetative phase andexpresses in ears. All ears of tillers of a plant and every seed in the infected eardevelop teliospores held by the persistent membrane and pericarp (Figure 5.10B),e.g., Ustilago hordei (covered smut of barley). Sometimes, as in the common bunt,individual seeds or ears may remain healthy (Tilletia tritici and Tilletia laevis). Theprimary inoculum is externally seed-borne and soil-borne. Sporidia formed in soilbecomeairborne, cause infection of individual florets,and invade ovary and ovuleforming teliospores. Grain infection is either total or partial, covered by pericarp(Figure 5.10C), e.g., Tolyposporium penicillariae (smut of pearl millet), Tilletiaindica (Neovossia indica, Karnal bunt).5.5.3.1.1 Type 1 Disease CycleThe embryal infection in wheat grains by Ustilago tritici, causal organism of loosesmut of wheat, was first reported by Maddox (1896) in Australia and later confirmedby Brefeld (1903) in Germany. Subsequently, many studies have described the course