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

102 Histopathology of Seed-Borne InfectionsFIGURE 5.1 (Color figure follows p. 146.) Seeds of soybean (Glycine max) showing slightto very severe symptoms, purple seed stain, caused by Cercospora kikuchii.maturity, may remain superficial unless the environment (humidity and temperature)is favorable to pathogens for a long period. Early infections of necrotrophs andnonsystemic pathogens may cause failure of ovule and seed development or deepinfections. Pirson (1960) found a relationship between the time of inoculation ofStagnospora nodorum in winter wheat and the amount of infection, expressed bythe number of conidia produced in the spikes and the reduction of grain weight.Early inoculation, June 10, led to a reduction in grain weight of about 40% whilelate inoculation, July 10, had little or almost no effect. Djerbi (1971), using infectionof Fusarium culmorum to the caryopsis of wheat, also observed that the extent ofinvasion reflects the actual time of inoculation or infection. When infection takesplace at an earlier stage, colonization may be deep, reaching the integument,endosperm, and the surroundings of the embryo, but if infection takes place nearmaturity of the caryopsis, only the outer layers of the pericarp will be invaded.Ponchet (1966) reported the occurrence of the mycelium of S. nodorum beneaththe testa of diseased wheat kernels. However, Agarwal et al. (1985), while examiningthe seeds of cultivars Svenno and Starke-II infected by S. nodorum that were categorizedas bold, loose and cracked pericarp, shriveled, and discolored, found myceliumrestricted to the outer layers of the pericarp in bold infected seeds, but in seeds withloose and cracked pericarp, shriveled and discolored types, profuse mycelium occurredin pericarp, and it extended to other parts in order of severity (Figure 5.2A). Thediscolored seeds carried the mycelium in all parts, including the embryo.A similar trend in spread of infection has been observed in wheat kernels infectedby Bipolaris sorokiniana (Figure 5.2B, C) and Drechslera tetramera (Figure 5.2D,E) (Yadav, 1984), maize by Botryodiplodia theohromae (Singh et al., 1986b), chiliby Colletotrichum dematium (Chitkara, Singh, and Singh, 1990), barley by Drechsleragraminea (Thakkar et al., 1991), sorghum by Curvularia lunata and Phomasorghina (Rastogi, Singh, and Singh, 1990, 1991), sunflower by Rhizoctonia bataticola(Godika, Agarwal, and Singh, 1999), and mustard by Albugo candida (Sharma,Agarwal, and Singh, 1997).