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

110 Histopathology of Seed-Borne InfectionsNeergaard (1979) reported the occurrence of Phytophthora cinnamomi myceliumin the embryo of avocado pear seed. Kumi et al. (1996) also observed thatcacao seeds from symptomatic (black) and asymptomatic pods from infected treesof Theobroma cacao yielded Phytophthora from more than 90% of embryos, butthe recovery was only 4% from seed coat. Such seeds either failed to germinate orproduced seedlings that usually died soon after germination or, if they survived,were stunted and developed leaf spots.5.5.1.2 PeronosporaLeach (1931) found mycelium and oospores of Peronospora farinosa (Peronosporaschachtii) in the seed coat of sugar beet and Melhus (1931) found mycelium andoospores of P. viciae in the seed coat of pea. The presence of oospore crust onsoybean seeds, first described by Johnson and Lefebvre (1942), is well known, Suchseeds are produced in infected pods (Hildebrand and Koch, 1951; Mckenzie andWyllie, 1971) and cause systemic infection in seedlings (Lehman, 1953; Zad, 1989).Roongruangsree, Olson, and Lange (1988) found that seeds encrusted by P. manshurica,contain, in addition to the oospores, thick- and thin-walled mycelium onthe seed surface. Only thin-walled mycelium occurred in the seed coat betweenpalisade cells, hourglass cells, and thin-walled parenchyma layers (Figure 5.4 C).The oospores, thick-walled mycelium on seed surface and mycelium in seed coat,retain viability. Hildebrand and Koch (1951) also observed fragments of myceliumof P. manshurica within the palisade and hourglass cells, but considered them to besenescent.Peronospora ducomati, the causal agent of downy mildew of buckwheat(Fagopyrum esculentum), is also internally seed-borne (Zimmer, McKeen, andCampbell, 1992). The commercial seed is in fact an indehiscent, one-seeded fruit,the achene. According to Zimmer, McKeen, and Campbell (1992), oospores ofP. ducomati occur in the remnant of persistent calyx on the inside of the seed coat(probably the pericarp) and in the spermoderm layer (the seed coat). The seed-borneoospores constitute the primary inoculum that causes systemic invasion of the seedlingsat the time of germination.5.5.1.3 PlasmoparaPlasmopara halstedii (P. helianthii) is an obligate parasite that causes downy mildewof sunflower. Seeds (cypsils) with severe infection contain mycelium in all partsincluding the embryo (Novotelnova, 1963; Cohen and Sackston, 1974). Doken(1989) observed hyphae of P. halstedii in the pericarp and testa (seed coat) of seedsfrom systemically infected sunflower plants. No infection was seen in the embryoin these seeds. The infection entered the ovary from the receptacle, and the fungalcolonization was especially dense in the hilum region (the point of fruit separation).5.5.1.4 Sclerospora, Peronosclerospora, and SclerophthoraIn 1920, Weston was the first to show that the mycelium of Sclerospora is internallyseed-borne, followed by Arya and Sharma in 1962. Weston (1920) reported the