Root rot caused by Phytophthora nicotianae and P. citrophthora is one of the most serious and economically significant diseases of citrus (Graham and Timmer, 1994; Timmer and Menge, 1988). Phytophthora spp. can infect almost all parts of the citrus plant causing root rot, gummosis, and foot rot of the trunk; damping-off of seedlings; leaf blight; and brown rot of fruit. The use of resistant rootstock is an effective way to control Phytophthora infection. Quantitative evaluation of root rot susceptibility has indicated that trifoliate orange [Poncirus trifoliata (L.) Raf.] and ‘Swingle citrumelo’ (P. trifoliata × C. paradisi Macf.) are tolerant rootstocks (Widmer et al., 1988).
Viroids are infectious agents composed of covalently closed, single-stranded RNA molecules that undergo autonomous replication and have the ability to induce diseases in susceptible hosts (Diener and Raymer, 1971; Semancik, 1979). Citrus viroids are classified into: Citrus exocortis viroid (CEVd), Citrus bent leaf viroid (CBLVd), Hop stunt viroid (HSVd), Citrus viroid III (CVd-III), Citrus viroid IV (CVd-IV), and Citrus viroid V (CVd-V) (Duran-Vila et al., 1988). Citrus viroids cause two economically important diseases, exocortis caused by CEVd and cachexia caused by HSVd (Roistacher, 1991). Exocortis was first described in 1948 as a bark-shelling or scaling disorder in trees grown on trifoliate orange and some of its hybrids (Benton et al., 1949; Fawcett and Klotz, 1948). Later, this disease was demonstrated to be graft-transmissible (Benton et al., 1950; Fawcett and Klotz, 1948). Cachexia was first described in 1948 (Childs, 1950) with specific disease symptoms of discoloration, gumming, and browning of phloem and bark cracking on ‘Orlando’ tangelo (C. paradisi Macf. × C. reticulata Blanco).
In Brazil, Rossetti et al. (1980) observed that exocortis-infected ‘Hamlin’ orange [C. sinensis (L.) Osb.] on Rangpur lime (C. limonia Osb.) rootstock resisted Phytophthora infection, but there were no consistent differences between the effects of mild and severe exocortis strains on Phytophthora lesions. Later, it was determined that the severe strains were exocortis and mild as were other viroids such as CVd-III.
Infection resulting from a pathogen induces changes in the host plant, including reinforcement of the cell wall by the formation of an impermeable deposit (Rossetti et al., 1980), accumulation of low-molecular-weight antifungal substances such as phytoalexins (Afeck and Sztejnberg, 1988), and the appearance of new pathogenesis-related proteins (Afeck and Sztejnberg, 1989; Antoniw et al., 1980). Some of the changes in the infection zone suggest that phenolic compounds may be the first line of defense (Matern and Kneusel, 1988) either as toxic products (free phenolic acids or their oxidation products, quinines) or as impedance in the invaded tissue (Bonhoff et al., 1987). An increase in phenols provides resistance for plants against fungal, bacterial, and viral infections (Matern and Kneusel, 1988). Soluble, low-molecular-weight phenolics are considered to have greater fungicidal activity compared with high-molecular-weight compounds (Harborne, 1989). Friend (1981) suggested that cell wall-bound phenolic acids alter the cell wall, creating a physical and chemical barrier against invading fungi. In citrus, phenolic compounds are produced in response to pathogen attack and stress factors such as ultraviolet radiation (Bonhoff et al., 1987; Broers and Jacobs, 1989; Manthey et al., 2000; Robbins, 1990). The production of antifungal phenolic compounds has been demonstrated in citrus fruits (Rodov et al., 1994), peels (Dubery et al., 1999), leaves (Harborne, 1989), and roots (Feldman and Hanks, 1968). Two antifungal compounds, xanthoxylin and scoparone, were found in the bark of lemon after infection with P. citrophthora (Afeck and Sztejnberg, 1988, 1989, 1990, 1993, 1995; Afeck et al., 1986). Scoparone accumulation was found to be responsible for resistance against collar rot caused by P. citrophthora. However, scoparone levels did not show strong relation to root rot resistance in response to infection by P. nicotianae (Aucamp et al., 2000).
In this study, we investigated the possible suppression of Phytophthora in citrus resulting from the presence of citrus viroids. Through high-performance liquid chromatography (HPLC), we evaluated the associated changes in the level of phenols in healthy compared with viroid-infected, Phytophthora-infected, and both Phytophthora and viroid-infected plants and investigated the levels of sporangia formation in each category of these plants.
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