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

Penetration and Establishment of Fungi in Seed 93In a favorable environment, such fungi are able to grow and cause internal infection.In true seeds, avenues for infection include (1) seed surface through the cuticle,natural openings, cracks, or injuries caused during threshing; (2) hilum that iscovered by the cuticle but with fissures; (3) micropyle, particularly the open type;and (4) accessory structures, e.g., hairs, wings, aril, and caruncle. Little informationis available on the role of accessory structures in seed infection.Seenappa, Stobbs, and Kempton (1980) found infection of Aspergillus halophilicusin dried red peppers, stored at 70% relative humidity. The fungal conidiagerminated, germ tubes entered the stalks through the stomata and pericarp throughcrevices caused by mechanical injury. It colonized the inner fruit wall, producedconidiophores and conidia, which reached the seed surface. These conidia germinatedforming hyphae with appressoria and infection pegs entering the seed coat.The germination of spores of Alternaria brassicicola on the seed coat, hilum, andmicropyle took place on artificially inoculated cabbage seeds (Knox-Davies, 1979).Extensive mycelial growth occurred on a seed with damaged testa.One-seeded dry fruits such as caryopsis, cypsils, achenes, and cremocarp mayalso get infected during the postharvest period. In these seeds (sensu lato), thepericarp, adhering bracts, separation scar (analogous to hilum), remnants or scarsof the style and stigma, and other persistent structures such as the seed cap in sugarbeet and spinach provide points of entry. Such seeds lack the micropyle, hilum,raphe and chalaza as exposed areas. Mycock, Lloyd, and Berjak (1988) noted thatA. flavus var. columnaris penetrates maize caryopsis through lesions in the pericarpand peduncle (caryopsis base) under suitable storage conditions.4.5 MECHANISM OF PENETRATION OF OVARY, FRUIT,AND SEED SURFACESThe penetration of fungal hyphae into the ovary and fruit and the ovule and seedsurfaces is similar to the processes observed during their entry into vegetative parts(see Dodman, Barker, and Walker, 1968; Heath, 1980; Kulik, 1987). It may bemechanical or enzymatic or both. The germ tube may enter directly without anyspecial manifestation or penetrate after forming appressoria, cushions, and pegs. Thelimited information based on artificial inoculations or natural infection of the fruitwall reveals that the germ tubes from fungal propagules may invade the surfaceusing more than one mode of entry. Similarly, there is evidence that there may bedifferent modes of entry on surfaces of different cultivars (Bassi, Moore, and Batson,1979).The formation of appressorium at the site of infection on the fruit surface seemsto be quite common. Batts (1955) and Malik and Batts (1960) report that as thewheat and barley ovaries were inoculated with spores of U. tritici and U. nuda,respectively, the tips of the promycelia, upon coming into contact with the epidermalcells of the ovary wall, developed appressorium-like swellings. Beneath the appressoriuma bulbous swelling developed, through which the penetrating hypha passedand entered the cell immediately below it. Binyamini and Schiffmann-Nadel (1972)found that in avocado fruits infected by C. gloeosporioides, the spores germinateand germ tubes enter the thick wax layers above the cuticle and form dark appressoria