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

Location of Fungal Hyphae in Seeds 139fungal mycelium, but it occurred only in 56.8% of the endosperms. The embryoswere free of infection (Rangnathaiah and Mathur, 1978).5.5.4.1.8 FusariumFusarium spp. are a common associate of seeds of a large number of crop plants(Richardson, 1990). They cause vascular wilts, primarily of annual vegetable andflowering plants. Djerbi (1971) has given an excellent account of Fusariumculmorum infection in wheat kernels. Infection may occur only in the outer layersof the pericarp, in the layers of pericarp and seed coat, and in all parts of the caryopsisincluding the thick cuticle of the endosperm and the aleurone layer, where themycelial cushions may be formed. The degree of invasion of kernel tissues reflectsthe time of infection. If infection takes place during the early stages of kerneldevelopment, colonization may reach deeper tissues. If infection takes place nearmaturity, only the pericarp is invaded (Djerbi, 1971).Fusarium moniliforme was initially reported to be confined to the pericarpbetween the brown cap and the pedicel in maize kernels (Sumner, 1966). Singh andSingh (1977) observed abundant mycelium of F. moniliforme in the pericarp(Figure 5.17A, B) and parenchyma layers outside the brown sclerotic cap in maizeseeds (Figure 5.17A, D). Only rarely does the mycelium penetrate the endospermon the sides (Figure 5.17C) as well as around the brown cap. Singh, Singh, andSingh (1985) in a study of maize kernels from tribal areas in Rajasthan, India, foundhyphae of F. moniliforme in the pericarp, butt, endosperm, and peripheral layers ofscutellum. The hyphae also penetrated the placentochalazal region. Mathur, Mathur,and Neergaard (1975) have also reported the presence of F. moniliforme in thepericarp, endosperm, and embryo in sorghum.Fusarium oxysporum is common in seeds of Fabaceae (Table 5.5). Velicheti andSinclair (1991) have reported the hyphae of F. oxysporum over the seed surface, inthe hilar region, and seed coat of soybean. Sharma (1992) has reported that thesoybean seeds infected by F. oxysporum are asymptomatic or symptomatic, dependingon the degree of infection. The symptomatic seeds are reddish brown. In asymptomaticseeds, the hyphae are confined to the seed coat and the hilar stellate parenchyma.In symptomatic seeds, however, the infection occurs in all the layers of theseed coat, stellate parenchyma, and rarely, in the tracheid bar, aleurone layer, andperipheral two to three layers of cotyledons on the abaxial surface in weakly infectedseeds. In moderately infected seeds, inter- and intracellular mycelium occurs in allcomponents. Heavy colonization is found in the seed coat with mycelial mat in theparenchymatous region. In cotyledons the infection is more on the abaxial than theadaxial surface. Infection in the embryonal axis is rare. In heavily infected seeds,aggregation of mycelium occurs in different components including the embryonalaxis. Chlamydospores, microsclerotia, and microconidia may be present on the seedsurface, in spaces between the components, and in the lysigenous cavities in cotyledonsand the hypocotyl root-shoot axis. Mycelium is inter- and intracellular andalso seen in the vascular elements of the seed coat and cotyledons (Sharma, 1992).A similar trend in the spread of mycelium of F. oxysporum has been observedin the seeds of Cyamopsis, Cajanus, and Vigna (Varma, 1990; Bhatia, Singh, andSingh, 1996a; Sharma, 1996).