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

Location of Fungal Hyphae in Seeds 143showed drying of leaves from tip to base and died in 2 to 3 weeks. Only 2 to 3%of the plants survived.5.5.4.2.2 AscochytaAscochyta rabiei and A. fabae var. lentis cause chickpea and lentil blight, respectively.Infected seeds show discolored areas. Lesions vary from small to large withmycelium and pycnidia often seen on the seed surface of chickpea (Maden et al.,1975) and occasionally in lentil (Singh, Khare, and Mathur, 1993). In chickpea thelesions are localized, and abundant mycelium occurs in the region of the lesions,but the adjoining clean areas are free of infection. Seeds with small superficial lesionscarry mycelium in the seed coat and on the surface of cotyledons, whereas thosewith deep lesions contain profuse mycelium in the seed coat (Figures 5.18A, B) andcotyledons (Figure 5.18C). Pycnidia are common in the seed coat (Figure 5.18B)and hyphae often become intracellular in vascular elements (Figure 5.18D). Bothsuperficial and deep infections have equal potential to cause seedling infection(Maden et al., 1975; Vishnuawat, Agarwal, and Singh, 1985).The hyphae of A. fabae f. sp. lentis travel vertically as well as horizontally inthe palisade cells, hourglass cells, and parenchymatous cells of the seed coat inmoderately infected seeds. Hyphae are inter- and intracellular in parenchyma. Inheavily infected seeds, the fungal mycelium occurs in the seed coat, the spacebetween the seed coat and the cotyledons, and, rarely, the space between the cotyledonsand the embryonal axis. Heavy accumulation of mycelium also takes placein the hilar tissues (Singh, Khare, and Mathur, 1993).Ascochyta pisi, which causes blight, leaf, and pod spot in pea, is carried asmycelium in the seed coat in infected seeds; 75% of cotyledons and 40% of plumuleare also infected in such seeds. Infection may be caused by mycelium spreading inthe pod cavity or directly by lesions on the pod surface (Dekker, 1957).5.5.4.2.3 BotryodiplodiaBotryodiplodia theobromae is seed-borne in a large number of crops (Kumar andShetty, 1983; Richardson, 1990). It causes stem rot and seed rot in maize. Singh,Singh, and Singh (1986b) found that the seed surface in weak to moderately infectedmaize seeds develops black streaks or pinhead-like microsclerotia, but heavilyinfected seeds are almost black. B. theobromae hyphae are confined to the pericarpand basal cap in weakly infected seeds, but occur in the pericarp, basal cap, closingtissue, aleurone layer, and, rarely, in other layers of the endosperm and placentochalazalregion in moderately infected seeds. It occurs in all tissues, including theembryo in heavily infected kernels (Kumar and Shetty, 1983; Singh, Singh, andSingh, 1986b). Heavy infection causes disintegration of tissues and depletion ofreserve food material.The decoated (woody testa removed) rubber seeds infected with B. theobromaehave brown to black discoloration progressing from micropylar to chalazal end(Varma, Singh, and Singh, 1990). The expanse of mycelium differed quantitativelyin different components in seeds showing different degrees of discoloration. Thefungus colonized the tegmen, perisperm, endosperm, and embryo in all the symptomaticseeds, but it was recorded only rarely in asymptomatic seeds. In moderately