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

82 Histopathology of Seed-Borne InfectionsSeeds after harvest and threshing or after natural dissemination are devoid ofthe above surroundings and are directly exposed to the environment that they cometo occupy such as storage areas and soil. Seeds have a well-formed cuticule, surfacewax deposits (when present), seed coat, or seed coat and pericarp (one-seeded dryindehiscent fruits) with protective layers, hilum, pits or micropores, and cracks. Themicropyle is narrow or open to various extents. Exposed hilum, micropyle, and raphe(when present) are features of true seeds. The latter two do not occur on the surfaceof one-seeded fruits, and the scar left upon separation of the fruit is at best analogousto the hilum. Since seeds are stored under dry conditions, no free moisture isavailable. The chief determinants for the development of fungi in and on seeds duringstorage are temperature and available moisture (water) of the grain. Storage fungican grow with restricted water availability (Christensen and Kaufman, 1969; Jainet al., 1994). The soil environments of seeds are highly variable, ranging from dryto water-logged soils.4.2 NATURE OF THE PATHOGENThe seed-borne fungi may be parasitic or saprophytic and, according to Dickinsonand Lucas (1977), may be biotrophs or necrotrophs. Biotrophs cause minimal damageto the host, including seed tissues, and are in fair harmony with the host.Biotrophs have a narrow host range and are usually obligate parasites. Necrotrophscause apparent damage to the host cells and have a wide host range. They secreteenzymes and bring about the disintegration of cell components, resulting in celldeath. The released cell contents are used by such pathogens for their growth.Basically, the mode of nutrition is like that of saprophytes. The necrotrophic fungi,depending upon time of infection and humidity, cause superficial or deep infection,whereas the biotrophs generally establish in deeper tissues including the embryo.The majority of seed-borne fungi are known to be necrotrophs. The obligate parasitesthat belong to Peronosporaceae, Albuginaceae, Erysiphales, Ustilaginales, and Uredinalesare biotrophs. Many intermediate conditions occur between the true necrotrophsand biotrophs. Maude (1996) believes that necrotrophs, which degrade tissuesas they spread, are rarely transmitted to the embryo through the mother plant.Another quality of the pathogen that may determine its passage during infectionof the ovule and seed in the field is the nature of disease in the plant and themechanism of transmission for becoming seed-borne. Neergaard (1979) has listedeight disease cycles for seed-borne pathogens taking into consideration the locationof primary inoculum in seed, course of disease development, and reinfection of ovuleand seed. The infection may be systemic, local, or organospecific. The systemicinfection may follow a vascular or a nonvascular course predominantly.4.3 INFECTION IN DEVELOPING SEEDSSince the ovules and developing seeds are present inside the ovary, the passages fortheir invasion need to be recognized at two levels: (1) routes leading to internalovary infection and (2) ovary to ovule and seed infection. Infection passages of a