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

Location of Fungal Hyphae in Seeds 141establishes in seeds of beet and spinach. Huang, Hanna, and Kokke (1985) observedV. albo-atrum throughout the stems of alfalfa, but it occurred only sporadically inthe peduncle, pedicels, pods, and seeds. Artificial inoculation of stigmas of healthyplants of cultivar Vernal of Medicago sativa (alfalfa) caused infection of the stigmaand upper part of style. The infection remained confined to these parts throughoutthe fruit and seed development. Under humid conditions, the fungus in the remnantsof style attached to the mature pod, rarely invaded the pod and seed coat (Huang,Hanna, and Kokke, 1985). Christen (1982) found V. albo-atrum hyphae in alfalfaseeds, particularly small seeds obtained from artificially inoculated greenhouseplants of susceptible cultivars. SEM photomicrographs showed the mycelium withinand between hourglass cells of the seed coat.Artificial inoculation by injecting spore-mycelium suspension of V. albo-atrumin stems of Lupinus albus and L. luteus showed variability in the colonization offruit and seed. The pathogen was discontinuous in tissues of fruit, including funiculusin L. albus. It was confined to the seed coat, but not seen in vascular elements. InL. luteus, mycelium occurred in vascular bundles of the seed coat, within the tracheidbar and the funicular trace in infected seeds. Profuse mycelium occurred in the spacebetween the seed coat and embryo and from there it spread to the cotyledons (Parnisand Sackston, 1979).5.5.4.1.10 TrichotheciumTrichothecium roseum, a well-known saprophyte, is common in seeds of crop plants.It causes seed rot and seedling blight in incubation tests in maize kernels (Singh,Singh, and Singh, 1985), seeds of cowpea (Varma and Singh, 1991), and rape andmustard (Sharma, 1989). The seeds are asymptomatic or symptomatic. In cowpea,rape, and mustard, symptomatic seeds show discoloration and shiny pinkish myceliumand/or conidia on the seed surface. In maize kernels, thin, shining hyalinehyphae colonized the basal cap, pericarp, endosperm, and embryo. The infectedembryo was well formed, but thin with lysigenous cavities containing abundantmycelium and bicelled spores of T. roseum.In symptomatic seeds of cowpea, T. roseum occurred in the seed coat, endosperm,and embryo. The mycelium was inter- and intracellular in hourglass cells and formeda thick mat in parenchyma cells and spongy parenchyma in the hilar region. Theembryonal infection occurred in the peripheral layers of cotyledons, radicle, and allover in the plumule and seminal leaves (Varma and Singh, 1991). In rape and mustardthe mycelium of T. roseum was confined to the epidermis and subepidermis of theseed coat in symptomless and bold weakly discolored seeds. In heavily discoloredseeds, however, it occurred in the seed coat, endosperm, and embryo. The myceliumwas inter- as well as intracellular and caused depletion of cell contents (Sharma,1989). The symptomatic seeds in cowpea and heavily discolored and shriveleddiscolored seeds of rape and mustard failed to germinate.5.5.4.2 Coelomycetes5.5.4.2.1 ColletotrichumColletotrichum species, which cause anthracnose and die-back diseases, have mostlyGlomerella and occasionally Physalospora or some other genus as the sexual stage.