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

Penetration and Establishment of Fungi in Seed 85inflorescence axis is directly infected from the mother plant. The mycelium subsequentlyinvades floral primordia (Figure 4.2A) and enters anthers and ovaries. However,secondary infection by conidia may take place through the stigma, style, orovary (Prabhu, Safeeulla, and Shetty, 1983). Muralidhara Rao, Prakash, and Shetty(1987) also found the fungal mycelium of P. sorghi in systemically infected maizeplants at all growth stages, including seeds. Fungal infection through silk and itsestablishment in seeds also occurred.Plasmopara halstedii infection in systemically infected plants of sunflowerproceeds from the receptacle to the ovary through its base. The hyphae reach thefuniculus and cause seed infection (Cohen and Sackston, 1974; Doken, 1989). Doken(1989) has shown that P. halstedii hyphae move from the receptacle to the ovarythrough intercellular spaces in the pedicel at an early ovule development stage.The mycelium of endophytic fungi also spreads intercellularly (Sampson, 1933;Philipson and Christey, 1986; Siegel, Leach, and Johnson, 1987). Using light andelectron microscopy, Philipson and Christey (1986) have provided an excellentaccount of endophyte infection in Lolium perenne. The endophyte progresses intercellularlyinto inflorescence primordium and shows acropetal growth of its hyphaeinto successively formed primordia of spikelets, florets, ovary, placenta, and ovule.In the ovary wall, hyphae rarely invade the vascular system.In barley seedlings and plants systemically infected by Drechslera graminea,the fungal hyphae spread through intercellular spaces, finally causing ear, floret, andpistil infection (Figure 4.2B to F) while the ear is still in the boot leaf stage (Thakkar,1988). The infection reaches the base of the ovary and ovule prior to the differentiationof vascular elements in these structures (Figure 4.2E, F).4.3.1.2 Indirect Infection from OutsideWhen transferred from other infected plants or from a local infection on the sameplant to the ovary or fruit, fungal spores and conidia result in indirect infection. Theinoculum may be transferred through various dispersal agencies such as wind, water(rain, irrigation), and insects. The routes for internal infection vary, depending onthe site of receipt of inoculum.4.3.1.2.1 Stigma and StyleDuring the first quarter of the 20th century, several authors reported fungal infectionthrough the stigma (Figure 4.3A) and its establishment in the seed, viz., loose smutsof barley and wheat (Lang, 1910, 1917) and anther mold of red clover, caused byBotrytis anthophila (Silow, 1933). This has been denied by later workers (Batts,1955; Jung, 1956; Campbell, 1956; Malik and Batts, 1960; Bennum, 1972). Jung(1956) inoculated stigmas of 61 plant species with numerous fungi but never foundhyphal entry into the ovary. Bennum (1972) reinvestigated B. anthophila infectionin Trifolium and found that hyphae from the stem enter the ovary and reach thefuniculus. She also observed hyphae growing acropetally from the receptacle intothe style but never saw this in the stigma.Ergot disease of grasses caused by Claviceps spp. is a classic example of blossominfection. Claviceps purpurea infects floral tissues prior to fertilization or within the