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

Seed Infection by Nematodes 2318.2 HISTOPATHOLOGYOf the genera that cause internal infection of seeds, Ditylenchus, Aphelenchoides,and Pratylenchus are soil- as well as seed-borne. They are facultative endoparasitesand produce most stages in soil. Usually the preadult stage (fourth stage) is infective.Other stages, except egg and first-stage larvae, are also known to cause infection.Anguina shows more marked adaptation to parasitism. They are obligate parasitesand normally complete their development only after invading the inflorescence, but,rarely, galls containing adults may be formed on leaves. Second-stage larvae thatremain in seed galls are released in soil. These become associated with the hostseedlings on which they feed ectoparasitically until they invade the floral parts.8.2.1 ANGUINA TRITICI (STEINBUCK) CHITWOOD(EAR COCKLE DISEASE)The ear cockle disease of wheat caused by Anguina tritici was the first plantnematode disease that was discovered. It was observed by Needham, a Dutch clergyman,in 1743. The nematode is known to infect bread wheat in most wheatgrowingcountries of the world (Southey, 1972). It also infects emmer wheat (Triticumdicoccum), spelt wheat (Triticum spelta), rye (Secale cereale), oat (Avenasativa), and certain grasses. The nematode produces seed galls, which are the majorsource of inoculum.8.2.1.1 Disease Cycle (Figure 8.1)The galls contain second-stage infective quiscent larvae. On sowing, the galls, alongwith healthy seed, reach the soil. The galls absorb moisture, and the larvae becomeactive and are released. They move in a thin film of water and reach different partsof the seedling, including roots and parts of seed, aerial parts (shoot), and spacesbetween coleoptile and leaf and between those of the leaf bases. The nematodes thatbecome lodged in seed tissues are of little consequence in disease development, butthose that reach aerial parts are then carried up with the growing seedling. The larvaefeed ectoparasitically, affect growth, and cause various symptoms in plant parts.After the plant enters the reproductive phase, the larvae in large numbers lie aroundthe differentiating florets in the spike enclosed by the boot leaf. Subsequently, theyenter the primordia of floral structures at the early stages of development. Theinfected structures (stamen and/or carpel) develop into galls, which are green initiallyand turn brown-black at maturity.After becoming endoparasitic, the second-stage larvae molt and differentiateinto adult males and females. The nematode is amphimictic. The females lay eggsinside the galls. The eggs undergo embryogenesis and produce second-stage larvaein the green galls. These larvae remain in quiescent form in dried galls. The totallife period is between 106 to 113 days (Swarup, Dasgupta, and Koshy, 1989). Onlyone generation is completed in a crop season.