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

Penetration and Establishment of Fungi in Seed 83pathogen to reach inside the ovary may be exclusive or follow a particular coursepredominantly together with other alternate courses. The former condition seems tooccur only rarely.4.3.1 ROUTES FOR INTERNAL OVARY INFECTIONThe infection may either reach the pistil directly from the mother plant through thevascular supply or the parenchyma, primarily through the intercellular spaces of thepedicel, or take place indirectly from outside using stigma-style, ovary or fruit wall,and other floral parts, including nectaries, as sites for the receipt of inoculum.Investigations carried out using artificial inoculation and histological techniques,including SEM, have improved our understanding of the course of hyphae duringpenetration and growth in the tissues of the pistil (Marsh and Payne, 1984; Chikuoand Sugimoto, 1989; Neergaard, 1989; Kobayashi et al., 1990).4.3.1.1 Direct Infection from Mother PlantSystemic plant infection of most vascular and nonvascular pathogens enters theflower and fruit through the pedicel (Figure 4.1A, B). Local infection below theflower, if it becomes systemic, can also cause infection via the pedicel (Lawrence,Nelson, and Ayers, 1981).4.3.1.1.1 Entry through Vascular SupplyVascular infection of wilt pathogens, Fusarium and Verticillium species, reaches thepistil via the vascular supply. Rudolph and Harrison (1945) isolated F. moniliforme,F. oxysporum, and F. scirpi from vascular bundles from all parts of the cotton plant,including boll and seed. Snyder and Wilhelm (1962) found that V. albo-atrum movedthrough the vascular elements of the mother plant into the flower and fruit stalk insugar beet and spinach. Verticillium dahliae also follow a similar mode of flowerand fruit infection in these crops (Van der Spek, 1972). Parnis and Sackston (1979)found mycelium of V. albo-atrum in vessels of the testa of Lupinus luteum seedsand believed that it spread through the funicular tissue. Infection via the vasculartissue (xylem) of the mother plant is the usual route for infection of garden stock(Mathiola incana) seed by F. oxysporum f. sp. mathiolae (Baker, 1948).Kingsland and Wernham ((1962) report that F. moniliforme invades the rudimentaryears in corn through vascular tissues of the stalk. But Lawrence, Nelson,and Ayers (1981) have found that sweet corn plants showing systemic or localinfection of F. moniliforme and F. oxysporum carry hyphae of the two fungi inintercellular spaces. The xylem vessels were found occluded in stem and leaves, butno hyphae were seen. The fungus moved through the parenchyma tissue of the stalkinto the cob and subsequently the pedicels of florets. Klisiewicz (1963) has observedmycelial tufts of F. oxysporum f. sp. carthami in infected receptacles of safflowerheads. Hyphae traversed through the abscission zone of the cypsil and were associatedwith them, but not limited to the xylem. Halfon-Meiri, Kunwar, and Sinclair(1987) have found colonization of achenes of Ranunculus asiaticus by Alternariafrom the mother plant through the vascular system.