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

136 Histopathology of Seed-Borne InfectionsInfected embryos remain thin, and those with heavy infection possess mycelium inall parts. Occasionally, embryos develop induced premature xylogenesis in themesocotyl and lysogenous cavities (Yadav, 1984). More deleterious effects on wheatembryo are caused by Drechslera tetramera than by B. sorokiniana.The inter- and intracellular mycelium of Bipolaris maydis occurs in the basalcap, closing tissue, and pericarp in weakly infected maize kernels, but it is abundantin the pericarp, basal cap, closing tissue, aleurone layer, endosperm, and embryo inheavily infected seeds (Singh, Singh, and Singh, 1986a).Drechslera graminea, which causes leaf stripe disease in barley, is seed-borne.According to Vogt (1923), Genau (1928), and Platenkamp (1975), the hyphae areconfined to the pericarp. Thakkar et al. (1991) found that moderate to heavily infectedseeds are symptomatic, and the expanse of mycelium and its effect on embryo arevariable. In moderately infected seeds, the hyphae may be confined to the pericarp(husk) and vascular strand of seed, or they may invade persistent lodicules (Figure5.15A, B), all layers up to the aleurone layer (Figure 5.15C, D), or penetrate theendosperm and rarely the embryo (Figure 5.15E to G). The hyphae spread interandintracellularly in all parts of heavily infected kernels. In very heavily infectedseeds, hyphae freely traverse from one part to another, binding the tissues and makingit difficult to separate husk, pericarp, aleurone layer, and even endosperm. Heavilyinfected seeds may have a small well-formed embryo, a shriveled poorly differentiatedembryo, or undifferentiated embryonal mass. The mycelium invades all partsexcept seminal leaves and the plumule bud in small embryos, all parts of shriveledembryos, and formless embryos. Barley kernels with deep infection of D. gramineafail to germinate (Thakkar, 1988).Bipolaris oryzae, a serious pathogen that causes brown leaf spot in rice, occursin the pericarp, seed coat, and endosperm (Fazli and Schroeder, 1966). Paul (1987)noted B. oryzae in the husk (palea and lemma), pericarp on the embryal side, andhilar region in weakly infected kernels. It spread all over the pericarp and testa, butnot in the endosperm and embryo in moderately infected seeds. In shriveled heavilyinfected seeds, B. oryzae occurred in all parts. The fungus formed pseudosclerotiabetween pericarp and testa and between testa and aleurone layer. Hyphae occurredin the peripheral layers of the endosperm and in the scutellum and other tissues ofthe embryo.5.5.4.1.4 CylindrocladiumCylindrocladium black rot caused by Cylindrocladium crotalariae is widespread inpeanut-growing areas in the United States. Porter et al. (1991) found a high degreeof correlation between the incidence of disease in the field and seed infection.Hyphae of C. crotalariae ramified both inter- and intracellularly in cells of the seedcoat in discolored seeds. Seeds with dark brown seed coat carried hyphae in cotyledonsalso. In these seeds, abundant hyphae occurred in the seed coat, between theseed coat and the cotyledons, and in cells of cotyledons (Porter et al., 1991). Porteret al. (1991) have proposed cleaning seed lots by removing shriveled and discoloredseeds to check the disease transmission.