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

Location of Fungal Hyphae in Seeds 119affected tissues secrete honeydew (Luttrell, 1977). The extramaterical stromaexpands, fills the space between the lemma and palea, and protrudes above theirtips. In the base of the sclerotium in P. dilatatum, the outline of the ovary is distinct.A loose plectenchyma fills the space occupied by the ovary.The mature sclerotia in all cases are dark colored, and the remnants of the apicalpart of the ovary, particularly the fragments of stigmatic cells together with conidialpouches, persist (Luttrell, 1977; Roy, 1984). Anatomically, the mature sclerotium ismade up of dark colored (brown) rind or cortex of relatively thick-walled irregularplectenchyma on the surface and uniform compact plectenchyma inside in the caseof C. paspali. The sclerotia are also composed of dark cortex in C. purpurea, butinternally it is differentiated into an outer region of fine hyphal cells and an innerregion of isodiametric cells (Campbell, 1958).5.5.2.3 SclerotiniaSclerotinia spp., particularly Sclerotinia sclerotiorum, cause destructive diseases ofvegetable and flower crops. The early symptoms on infected plants are a white fluffymycelial growth in which sclerotia, white at first but ultimately black and hard, areproduced. Sclerotinia sclerotiorum sclerotia may be attached to the seeds or mayoccur mixed with them (Neergaard, 1979). Tollenaar and Blieholder (1971) observedmycelial strands of S. sclerotiorum in the thick-walled and thin-walled layers of thepericarp and in the seed coat of sunflower kernels.Sharma (1992) detected S. sclerotiorum in seeds of Eruca sativa from Rajasthan,India. Infected seeds are symptomatic with black mycelium and sclerotia on theseed surface. The infection is confined to the seed coat, usually the seed epidermis,and only rarely on the subepidermal cells. The infection occurs as thick myceliumand microsclerotia, which are either submerged in the epidermis or emergent.5.5.2.4 DidymellaDidymella bryoniae, the cause of internal fruit rot in Cucumis and Cucurbita, invadesthe pistil through the stigma and style in Cucumis sativus. The fungus infects theovules and developing seeds superficially or internally (Neergaard, 1989). In pumpkinseeds, the fungus is located in the seed coat and rarely in cotyledons. It occursin all the layers of the seed coat and is usually prominent in the chlorenchyma andthe inner epidermis (Lee, Mathur, and Neergaard, 1984).5.5.3 BASIDIOMYCETES5.5.3.1 Ustilaginales (Smuts and Bunts)The seed-borne smuts and bunts attack the ovary and sometimes other parts of floretsor the inflorescence (Table 5.3). Three main disease cycles are found in these fungi.The primary infection is intraembryal and causes systemic, latent infection in theplant, expressing itself in the ovary or developing grains in which abundantteliospores (chlamydospores) are formed. The exposed teliospores are disseminatedby the wind and germinate on the stigma and ovary wall in florets on healthy plants