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

5Location of Fungal Hyphaein SeedsContamination of seeds by smut propagules (Remmant, 1637; Tull, 1733), andaccompanying infection of the ergot pathogen, Claviceps purpurea (Hellwig, 1699)were reported long before internal seed infection was observed. Frank (1883) wasprobably the first to report that the mycelium of Colletotrichum lindemuthianum,the cause of anthracnose in beans, often penetrated the bean (Phaseolus vulgaris)seed cotyledons. The embryo infection of wheat seed by Ustilago tritici, loose smutfungus, was recorded by Maddox in 1896 and confirmed by Brefeld (1903). Heald,Wilcox, and Pool (1909) reported the internal seed-borne mycelium of Diplodiazeae in endosperm and embryo of maize (Zea mays). Neergaard (1979) providedsuch information under seed and fruit component subheadings, namely (1) embryoinfection; (2) endosperm infection; (3) seed coat infection; (4) pericarp infection;(5) bract infection; and (6) contamination of seed coat and pericarp. He pointed outthat the location of pathogen within the embryo is dependent on the species, perhapseven the race or strain of the pathogen and the variety or cultivar of the host species.Agarwal and Sinclair (1997) followed Neergaard’s categories in their book. Maude(1996), while describing the internal inoculum of seed, stressed the nutritional statusof the fungus, necrotroph or biotroph. Accordingly, the necrotrophic fungi are generallylocated in the seed and fruit coat, and deeper penetration, i.e., endosperm andembryo infection, is infrequent. Biotrophs, on the other hand, are located in theembryo.5.1 SEVERITY OF INFECTION AND LOCATIONHistopathology of infected categorized seeds of different crops by pathogens rangingfrom well-known necrotrophs to biotrophs has shown that the extent and amount offungal inoculum in seed tissues are directly correlated with the severity of seedinfection. In symptomatic infected seeds (Figure 5.1), severity can be determinedby the coverage of seed surface by fungal inoculum, such as number and distributionof microsclerotia, pycnidia, aceruli, dormant mycelium (with or without propagules),and degree of discoloration (Figure 5.1) and shriveling. Asymptomatic infectioncannot be evaluated by visual observation. In symptomatic infected seeds, weakinfections are usually confined to seed coat and pericarp, whereas heavy infectionmay invade all parts including the embryo.The severity of infection of any pathogen in an affected cultivar may depend onthe stage at which the ovule and seed become infected and also on its environmentafter the infection has taken place. Late infections, when the seed is reaching101