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

Seed Infection by Nematodes 239conditions. The developing pods are invaded after the fruiting pegs have penetratedthe soil. The nematodes enter the pod near the point of connection with the peg(Figure 8.5D). They invade the peg and feed on the parenchyma cells, which collapselater. The nematodes enter the exocarp, feed on the parenchyma cells, and migrateto the base of the mesocarp (Figure 8.5A, E). They remain confined to the sclerenchymatousmesocarp for quite some time, and its cells (in contact with nematodes)become discolored. As the mesocarp breaks down, the nematodes reach the endocarp.They enter the seed through the micropyle, invade the seed coat (Figure 8.5F) andembryo (Figure 8.5C), and lodge on the surface of the cotyledons. At maturity thetesta carry eggs and nematodes in the testa (Figure 8.5B, H). In the testa, nematodesmay also occur in vascular bundles (Figure 8.5G).Venter, McDonald, and van der Merwe (1995) have reported that Ditylenchusafricanus Wendt et al. (peanut pod nematode) is both seed- and soil-borne. Thenematodes survive in the testa and embryo of the seed and in the hull. Ditylenchusangustus has been found in dried seeds of rice, located mainly in the germ portion(Prasad and Varaprasad, 2002).8.2.5 APHELENCHOIDES BESSEYI CHRISTIE (WHITE TIP NEMATODEOF RICE)The disease was first described in the United States and Japan, but it is now knownin most of the rice-growing countries of the world (Franklin and Siddiqui, 1972;Franklin, 1982). The nematode is an ectoparasite that is carried beneath the hull inthe rice kernel. In India the disease has also been reported on Setaria italica, thefox tail millet (Lal and Mathur, 1988) and Panicum melacium (Gokte et al., 1990).Its other important host is strawberry in the United States and Australia and severalother flowering plants in Hawaii, the Philippines, and Japan (Franklin, 1982).Detailed histological observations are lacking. The infested seeds, upon reachingsoil, absorb moisture and the preadult nematodes emerge and move in the thin filmof water to reach different plant parts. They feed ectoparasitically on vegetativetissues, migrate to the growing panicle, puncture the inflorescence, reach the florets,and develop beneath the glumes. The nematode is amphimictic, and its densityincreases before anthesis. As the grain dries, second-stage larvae undergo anhydrobiosisand persist in the paddy kernel (Huang and Huang, 1972; Nandkumar et al.,1975). Nandkumar et al. (1975) have observed that in dry seed the nematodes remaincoiled up inside the palea and on the surface of the lodicules, which become paperyas the kernel matures.Aphelenchoides besseyi was detected beneath the glumes in anhydrobiotic statein P. miliaceum. The maximum number of nematodes obtained from a seed was 16,and the average number was 1.8 (Gokte et al., 1990).8.2.6 APHELENCHOIDES ARACHIDIS BOS (TESTA NEMATODE OFGROUNDNUT)The testa nematode of groundnut (A. arachidis) is a facultative endoparasite of roots,hypocotyl, pods, and testa (Bos, 1977). The nematode penetrates young roots and