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

Location of Fungal Hyphae in Seeds 115the common host, and the disease appears in the form of tumor-like swellings onaerial parts including flowers. The infection is partial and scattered on different parts.The infected fruits are hypertrophied, and they are called galls. The fungal myceliumand chlamyspores are found only in infected tissues (Gupta, 1962; Pavgi andMukhopadhyay, 1972; Rao, 1972; Singh, 1991).The healthy cremocarp is symmetrical and comprises two mericarps. The hypertrophiedand galled fruits are symmetrical or asymmetrical depending on whetherthe infection is complete or partial. The ridges on the surface of these fruits areinconspicuous, and the affected pericarp lacks differentiation. Its cells are uniformlyenlarged with prominent air spaces and abundant chlamydospores. The vittae arenot recognized and the vascular bundles are dispersed with poor thickening of xylemelements. The cells in the inner epidermis are tangentially stretched and thin-walled.The cells in carpophore are also hypertrophied, and its vittae and vascularbundles are inconspicuous. The pedicel and stylopodium, which remain with fruit,show hypertrophy of cells, prominent air spaces, distinct but reduced amount ofvascular zone, indistinct vittae, and abundant mycelium and chlamydosphores. Inpartially infected fruits, the uninfected part shows normal anatomical features(Figure 5.7).The size of locules and the development of ovules are directly correlated withthe intensity of infection. In severely infected and fully hypertrophied fruits, theovules abort. In moderately hypertrophied fruits, the ovules develop to varyingextents and have endosperm and embryo, but the latter is weakly developed. Fruitswith chlamydospores confined only to the pericarp usually have well-formed seedswith a developed embryo. Such seeds germinate normally (Gupta, 1962).5.5.2.2 ClavicepsSpecies of Claviceps cause ergot disease in diverse graminaceous crops and grasses.The disease cycle (Figure 5.8) follows an almost uniform pattern. The path ofinfection and development of stroma and sclerotia under natural and experimentalconditions are known for Claviceps purpurea in rye (Tulsane, 1853; Campbell,1958), C. paspali in Paspalum dilatatum (Brown, 1916; Luttrell, 1977), C. fusiformisin pearl millet (Thakur, Rao, and Williams, 1984; Roy, 1984), and C. sorghi insorghum (Bandyopadhyay et al., 1990). The development of sclerotia follows twodifferent courses.Initial infection by ascospores (primary) and conidia (secondary) takes placethrough the stigma. The invading hyphae grow down the style (Figure 5.9A) andcolonize the ovary. In ovaries of rye infected by C. purpurea and pearl millet byC. fusiformis, the development begins at the base of ovary (Figure 5.9B, C) with theappearance of the sphacelial stage (Weihing, 1956; Roy, 1984). The affected tissuesof the ovary produce sweet viscid fluid in which conidia remain embedded. It isexuded as honeydew. The apical part of the ovary resists disintegration, and it iscarried at the top of the sphacelial stroma. The downward colonization of the hostdoes not proceed beyond the plate of cells at the upper limit of rachilla (Figure 5.9C),where a tissue (plate) of four to six layers separates the infected ovary from the