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

Location of Fungal Hyphae in Seeds 107TABLE 5.1Characteristics of Hyphae of Cercospora sojina, Colletotrichum truncatum,and Phomopsis sp. in Seed Tissues Carrying Mixed InfectionHyphal MorphologyStain ReactionSafranin–Light Green Trypan BluePathogenColorWidth (mm)OilGlobuleCercospora sojina Dark brown 0.8–1.6 + Light green BlueColletotrichum Brown 3.0–11.0 + Green BluetruncatumPhomopsis sp. Hyaline 3.8–8.7 – Red or green BlueNote: + = presence of oil globules; – = absence of oil globules.Based on Kunwar, I.K., Singh, T., and Sinclair, J.B. 1985. Phytopathology 75: 489–592.extensively ramified in deeper tissues. Hyphae of both the fungi occurred on thesurface of the plumule and radicle, but only F. oxysporum was seen in inner layers(Sharma, 1992).5.5 COLONIZATION OF SEED TISSUESThe histopathology of seeds infected by fungi belonging to Oomycetes, Ascomycetes,Basidiomycetes, and Deuteromycetes is described for each genus separately.Seed infection of endophytes is given separately.5.5.1 OOMYCETESTerrestrial Oomycetes, mostly members of Peronosporales, are either facultative orobligate plant parasites. The hyphae of facultative parasites grow indiscriminatelyinto and through the affected cells of the host, whereas those of obligate parasitesare usually intercellular forming haustoria into the host cells. Species of Phytophthora,Plasmopara, Peronospora, Peronosclerospora, Sclerospora, Sclerophthora,and Albugo are known to be seed-borne (Richardson, 1990). The information onlocation of Oomycetes in seeds is presented in Table 5.2.5.5.1.1 PhytophthoraAseptate and branched mycelium without haustoria of Phytophthora parasitica var.sesami, causing sesamum blight, occur in the seed coat, endosperm, and embryo ofinfected sesame seeds (Sehgal and Prasad, 1966). Dubey and Singh (1999) reportedP. parasitica var. sesami mycelium, chlamydospores, and oospores in seed components.The distribution of mycelium in seed varies and depends on the severity ofinfection. It is superficial, confined to the seed coat in seeds with weak infection,but seeds with deep infection carry mycelium in the seed coat, endosperm, andembryo (Figure 5.4 A, B). Severely infected seeds fail to germinate, but weakinfection is transmitted to seedling and plant.