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

124 Histopathology of Seed-Borne Infectionsto the seed surface during harvesting and threshing or are soil-borne. In nature, smutinfection occurs through air-borne sporidia produced from germinating teliosporesin soil at the time of flowering of the crop. Smut infection occurs through the stigmasbefore anthesis and is confined to individual spikelets (Bhat, 1946; Mitter andSiddiqui, 1995). The mycelium, after reaching the ovary, branches profusely betweenthe pericarp and aleurone layer. The hyphae are inter- and intracellular. A cavity,full of fungal mass, is formed inside the ovary. Teliospore formation follows, andfinally all the structures within the ovary except the ovary wall are replaced byteliospores (Mitter and Siddiqui, 1995). The smutted grains are oval or pear-shaped,bigger than healthy grains, and project beyond the glumes. The top is bluntly roundedto conical. The enclosing membrane is tough and consists of host tissue.Smut of finger millet (Eleusine coracana) due to Melanopsichium eleusinis iscaused by air-borne sporidia. Soon after infection the cells in the ovary wall divide,making it multilayered. This is followed by the disintegration of cells at differentsites, resulting in the formation of small lysigenous cavities. The cavities enlarge,become ovate or spherical, and a thick felt of mycelium borders each cavity, filledwith a mucilaginous fluid. Teliospores are formed from the hyphae around the cavityin a centripetal manner. The sorus is usually multilocular due to the developmentof two, three, or even four cavities full of spores. Each cavity or locule remainsdistinct, separated from the others by the host tissue, even at maturity (Thirumalacharand Mundkur, 1947).A similar disease cycle occurs in rice bunt, caused by Tilletia horrida(Chowdhury, 1946), and karnal wheat bunt, caused by Tilletia indica (Neovossiaindica). In T. indica the primary infection, which takes place in individual florets,under favorable conditions, causes secondary and tertiary spread of the pathogenwithin and between spikelets through mycelium or secondary sporidia produced oninitially infected florets (Dhaliwal et al., 1983; Bedi and Dhiman, 1984). In the caseof T. indica entry of hyphae is reported through the ovary wall.5.5.3.2 Uredinales (Rusts)Some rust fungi that cause serious diseases in crop plants are known to be seedborne(Alcock, 1931; Savile, 1973; Neergaard, 1979; Halfon-Meiri, 1983). Alcock(1931) reported Uromyces betae on seed balls of sugar beet as yellow spots, soricontaining uredospores (Table 5.3). Hungerford (1920) found abundant sori of uredosporesand teleutospores of Puccinia graminis var. tritici on wheat seeds.Puccinia calcitrapae var. centaureae (=Puccinia carthami), a macrocyclic autoeciousrust, has been convincingly shown to be transmitted directly from seeds(Figure 5.12). Uredo- and teleutospores of P. calcitrapae var. centaureae occur insidethe crevices, scarred ends, and surface of safflower cypsils (Figure 5.13A to D).Roughness of spore surface also contributes to their adherence to cypsil surface. Theteleutospores, larger in size and two-celled, are easily distinguished from uredospores(Figure 5.13B). The teleutospores germinate, forming sporidia, whichcause seedling infection (Halfon-Meiri, 1983). Schuster and Christiansen (1952),Schuster (1956), and Zimmer (1963) have also reported that seed or soil infestedby teleutospores supply the initial inoculum for seedling infection.