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

94 Histopathology of Seed-Borne Infections(Figure 4.7A, B), which persist in fruits until the fruits harden. During softening ofthe fruits, thin hyphae develop from the appressoria and penetrate the cuticle andcell wall. Appressoria formation and subsequent infection by narrow germ tubeshave been reported in ethylene-degreened Robinson tangerines by Colletotrichumgloeosporioides (Brown, 1977) and sugar beet flowers and seed bolls by C. dematiumf. spinaciae (Chikuo and Sugimoto, 1989).Penetration of the fruit surface of Carica papaya by Colletotrichum gloeosporioidestakes place after appressoria formation (Figure 4.7C, D) or directly by thegerm tube. The penetration by appressoria and infection peg formation is the mostcommon mode of entry (Chau and Alvarez, 1983). Similar mode of entry of hyphaeof Capsicum capsici occurs on cotton bolls in which appressoria are formed on thecuticle of the epidermal cells, multicellular hairs and stomata (Figure 4.6 A to D)(Roberts and Snow, 1984). The hyphae of Fusarium moniliforme enter directlythrough small cracks and by appressoria formation on intact areas in sweet corn(Styer and Cantliffe, 1984).The stigmatic surface is usually penetrated directly by fungal hyphae (Luttrell,1977; Bandyopadhyay et al. 1980; Prakash, Shetty, and Safeeulla, 1980; Neergaard,1989). Snow and Sachdev (1977) have observed direct penetration of wax, epidermalcells, and epidermal hairs on cotton bolls by Diplodia gossypina.Bassi, Moore, and Batson (1979) observed differential behavior of Rhizoctoniasolani on tomato fruits of a susceptible cultivar, C-28, and those of a resistant cultivar,PI 193407. The mycelial growth was extensive with cushion formation on C-28fruits (Figure 4.7E), but it was sparse on those of PI 193407. The penetration wasby multiple infection pegs under infection cushions in the former (Figure 4.7F) andrare with individual hyphae in the latter.Appressoria formation takes place on seeds of Capsicum before the seed coatis penetrated by Aspergillus halophilicus (Seenappa, Stobbs, and Kempton, 1980).The direct or indirect (through the development of appressoria or cushions)penetration of the fruit or seed surface by fungal pathogens may be mechanical orenzymatic. Histological studies suggest that if the cuticle and/or cell wall at the pointof penetration is depressed inwards, the penetration is accomplished by physicalforce. Lack of such depressions and the symptoms of digestion of cuticle are takenas evidence of an enzymatic penetration (Kolattukudy, 1985). Enzymatic hydrolysisof the cuticle and cell wall will certainly weaken these barriers and permit the entryof infection hyphae. Secretion of cutinase by fungi during surface penetration ofplant parts has been demonstrated (Shaykh. Soliday, and Kolattukudy, 1977; Dickman,Patil, and Kolattukudy, 1982). Fusarium solani f. sp. pisi, C. capsici and C.gloeosporioides, which are common pathogens infecting fruits and seeds, secretecutinase. Dickman, Patil, and Kolattukudy (1982) isolated cutinase from papaya fruitsinfected by C. gloeosporioides and further demonstrated that exogenous applicationof cutinase helped fungal pathogens, which are incapable of infection because oftheir inability to penetrate the cuticle, to cause infection. Mycosphaerella sp., whichinfects papaya fruits only when the cuticular barrier is mechanically breached,infected fruits with an intact cuticle when the surface was pretreated with cutinasefrom C. gloeosporioides (Dickman, Patil, and Kolattukudy, 1982). Similar informationon the penetration of the ovule and seed surface is lacking.