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

92 Histopathology of Seed-Borne Infectionsplace through the stomata, micropores, and pits and natural cracks in the seed surface.Micropores and cracks develop during ripening and desiccation of the seed (Yaklich,Vigil, and Wergin, 1986).Rudolph and Harrison (1945) isolated F. moniliforme, F. oxysporum and F. scirpifrom xylem isolated from seeds and concluded that the infection reached throughthe xylem. The hyphae of Alternaria sp. in achenes of Ranunculus asiaticus invadethe ovule and seed through the ovular vascular supply via the funiculus, raphe, andchalaza, but the hyphae that escape from the pericarp and/or funiculus in the locularcavity penetrate the seed coat directly (Halfon-Meiri, Kunwar, and Sinclair, 1987).Vaughan et al. (1988) determined the routes of entry of A. alternata into ovulesand seeds in artificially inoculated pods of soybean cultivars Union, PI 181550 andWilliams. The fungal hyphae cover the seed surface and enter the micropyle. Entrydirectly through the cuticle and seed epidermis and in the case of cultivar Williams,through micropores on the seed coat, also occur. Variation occurs in seeds of soybeancultivars for the presence and nature of pitting, and also the features of the micropyle,closed or open type (Yaklich, Vigil, and Wergin, 1986; Wolf, Baker, and Bernard,1981). Kulik and Yaklich (1991) have found that hyphal infection of Phomopsisphaseoli via the micropyle is far more prevalent in seeds of cultivars with highincidence of infection. Seeds of cultivars with high infection have open micropyleand seed coats with multiple pits, whereas those with low infection have closedmicropyle, and the seed coats lack pits. The hyphae of P. longicolla from localizedpod infection in soybean enter the seed through the funiculus or directly throughthe seed coat (Roy and Abney, 1988).D. bryoniae infection in the pistil of Cucumis sativus follows the path of thepollen tube and reaches the ovary mainly in the transmitting tissue. Invasion of theovules takes place through the funiculus, and occasionally the hyphae transversedirectly from the inner ovary wall to the ovule and seed surface in the chalazal regionand enter through the epidermis (Figure 4.5C). Thereafter, the mycelium spreadsthroughout the ovule (Figure 4.5D).The mycelium in infected ovaries of wheat and barley by U. tritici and U. nuda,respectively, grows centripetally to enter the testa and other seed tissues (Batts, 1955;Malik and Batts, 1960). The young ovule in barley receives infection of Drechsleragraminea through the funiculus, hyphae moving in intercellular spaces. However,during grain development, the hyphae may also trespass from the inside of the ovaryon to the seed surface, which they penetrate directly (Thakkar, 1988).Styer and Cantliffe (1984) observed penetration of the pericarp through smallrandom cracks in the surface of cultivar Sh 2 of sweet corn by F. moniliforme, andSmart, Wicklow, and Caldwell (1990) observed the same in Dekalb hybrid ¥ L12of corn by A. flavus.4.4 AVENUES OF INFECTION IN THRESHED SEEDSAfter harvest, seeds are generally stored under dry conditions. Fungal inoculum ofpathogens (field fungi) infesting the seed surface rarely develop further and remainquiescent during storage. However, storage fungi, particularly species of Aspergillusand Penicillium, can tolerate low available moisture and are more thermotolerant.