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

132 Histopathology of Seed-Borne InfectionsAlternaria infection, established as dormant mycelium, is reported in seed coatfor Alternaria brassicae in Brassica (Chupp, 1935), Alternaria brassicicola inBrassica (Boek, 1952; Domsch, 1957), and Alternaria longipes in tobacco (Heursel,1961), and in the pericarp of carrot for Alternaria dauci and Alternaria radicina(Soteros, 1979; Netzer and Kenneth, 1969). Atkinson (1950) alone recorded Alternariaraphani in the seed coat and embryo of radish.Dormant mycelium of Alternaria alternata (Alternaria tenuis) occurs in the seedcoat and pericarp in asymptomatic and weakly symptomatic seeds. Moderately toheavily infected seeds carry fungal mycelium in all parts — the seed coat or pericarpand seed coat (one-seeded dry fruits), endosperm, and embryo in the case of Capsicum,Glycine, Brassica, Eruca, Helianthus, and Cumin (Table 5.5). In weaklyinfected seeds of soybean inter- and intracellular hyphae occur in all the layers ofthe seed coat. Rarely do hyphae traverse the cells of endosperm and peripheral layersof the cotyledon. Moderately and heavily infected seeds carry a thick mat of hyphaein the region of seed coat parenchyma and endosperm; the two zones are indistinct.Hyphae also occur in all the parts of the embryo. In the hilar region, hyphae areseen outside as well as inside the hilium. When the hyphae are internal, they occurin the parenchyma and the tracheid bar (Kunwar, Manandhar, and Sinclair, 1986a).In Hordeum vulgare the husk, persistent lodicules, pericarp, and endosperm areinfected (Thakkar, 1988). The mycelium usually occurs in layers of pericarp andtesta outside the thick cuticle of the endosperm in sunflower seeds. Rarely in heavilyinfected seeds, the mycelium penetrates the cuticle and invades the endosperm andembryo (Singh, Mathur, and Neergaard, 1977; Godika, Agarwal, and Singh, 1999).In Alternaria sesamicola, the cause of sesame blight, the mycelium readilyspreads in soft parenchymatous layers of the seed coat. The epidermal cells, whichhave calcium oxalate crystals, remain free of infection in weak and moderatelyinfected seeds (Figure 5.14A, B). In severely infected seeds, abundant myceliumoccurs in all parts of the seed showing host cell disintegration (Figure 5.14C, D).Conidia formation occurs in spaces in the seed and also on the seed surface. Suchseeds were probably infected early during development or had prolonged humidenvironments after being infected.In A. brassicicola, the cause of leaf spot and blight of crucifers, infected seedsare asymptomatic and symptomatic in Brassica juncea, B. campestris, and Erucasativa. In asymptomatic infected and weakly infected seeds, the hyphae are confinedto the seed coat. However, in moderate and heavily infected seeds (bold discoloredand shriveled discolored), the hyphae occur in the seed coat, endosperm, and embryo.The infection is maximum (100%) in the seed coat and gradually declines in theendosperm and embryo (Sharma, 1989; Sharma, 1992). Among the Brassica spp.,the severity of seed infection was greater in mustard (B. juncea) than in rapeseed(B. campestris). Knox-Davies (1979) has found that in cabbage seeds naturallyinfected by A. brassicicola, the testa is usually colonized in the hilum area.5.5.4.1.2 CurvulariaCurvularia lunata is a common seed-borne fungus, recorded in seeds of field cropsall over the world (Richardson, 1990). It is one of the fungi that caused grain molddisease in sorghum (Rastogi, Singh, and Singh, 1990). The affected seeds are