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

122 Histopathology of Seed-Borne Infectionsoccurs just before the ripening of the grains, hyphae are confined to the endosperm,especially at the base (Pedersen, 1956). Ohms and Bever (1956) also observed thehighest percentage of infected embryos with a large amount of hyphae when inoculationswere made at anthesis in winter wheat. Inoculation made after anthesisshowed a decreased percentage of infected embryos with a small amount of hyphae.The embryo count method is commonly used in routine seed health testing forthe detection of loose smut mycelium in wheat seed lots. In extracted embryos, themycelium takes trypan blue stain, but the older mycelium is brown and does notstain. The smut mycelium is broad and branched (Khanzada et al., 1980; Mathurand Cunfer, 1993).Loiselle and Shands (1960) compared the location of U. nuda in grains ofresistant and susceptible barley cultivars. They found that in grains of resistant plantsmycelium was confined to the chalaza and the parenchyma associated with thevascular bundle while in grains of susceptible cultivars, it occurred in the integument(seed coat and pericarp), aleurone, endosperm, and embryo. Batts and Jeater (1958)and Popp (1959) found three different reactions when resistant and susceptiblecultivars were artificially inoculated by different races of U. tritici. Mycelium spreadin all tissues of the embryo of susceptible cultivars. The scutellum and plumular budwere regularly infected, but the coleoptile, epiblast, radicle, coloerhiza, and suspensorwere less frequently infected. In resistant varieties, only the scutellum wasconsistently infected and the plumule bud had no infection. Other structures wereless frequently infected. All parts of the embryo were free of infection in totallyresistant or immune varieties. Popp (1959) further demonstrated that only infectionof plumule bud is directly correlated with smut infection in plants grown frominfected seed (Table 5.4). Based on the above criteria, a reasonably accurate predictionof percentage of smut infection in field plants can be made. Based on recordingsof scutellum infection, however, Khanzada et al. (1980) found that the laboratorycounts of the embryo infection in wheat seed closely correlated to the number ofsmutted plants in the field.Batts and Jeater (1958) and Malik and Batts (1960b) determined the developmentof the fungus in infected germinating seed, seedling, and plant in susceptible cultivarsof wheat and barley, respectively. The mycelium becomes active at the time of seedgermination and is carried upward by the elongation of the plumule (epicotyl). Themycelium spreads in the crown node in which the development of all parts of theadult plant (nodes, internodes, and ears of each tiller with its leaf sheath) takes place.The smut mycelium permeates the whole structure including the ears (Figure 5.11B).During further development, stems elongate and the mycelium already present inthe ears is carried up. By the time the ears emerge out of the leaf sheath, all of itsparts except the rachis are completely replaced by spores.Wallen (1964) has reported that the cultivar Keystone barley in Canada isextremely embryo susceptible but mature plant resistant. The embryal infection isnever transferred to the plant.5.5.3.1.2 Type 2 Disease CycleCommon bunt or stinking smut is caused by two closely related pathogens, T. tritici(T. caries) and T. laevis (T. foetida). Bunt balls are seed- as well as soil-borne. The