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1.7. Symptoms of Black Root Rot

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1.7. Symptoms of Black Root Rot Chapter 1 . Literature Review 1.1. General Introduction T. basicola, a filamentous fungus, is a widespread, soil-borne plant pathogen. It has been reported to attack more than 200 ornamental and agricultural plants causing the disease, black root rot (Otani, 1962). In many hosts, the pathogen invades the epidermis and root cortex reducing the plant's capability to absorb nutrients (King and Presley, 1942). Although the pathogen was first detected in Australia on tobacco in 1942 (Simmonds, 1966), it has spread on a number of field crops in Australia via movement of spores attached to foot-wear or machinery wheels (Honess, 1994). The most serious and widespread epidemic of black root rot in Australia is cotton black root rot, which developed in New South Wales during the 1990s (Nehl et ai., 2004). It was estimated in 2004 that 97% of the cotton farms regularly surveyed in northern NSW have been infested by the pathogen and that by the year 2011, 95% of the cotton fields will be infested (Nehl et ai., 2004). Although this disease does not kill seedlings directly, yield losses up to 50% have been recorded in severely affected crops (Nehl, 2002). The incidence of black root rot could have an enormous impact on the Australian economy as Australia is the world's third largest cotton exporter (Dowling, 2003). Past research on T. basicola has concentrated on finding better methods for controlling the root rot disease it produces on cultivated crops. Control strategies based on cultural practices, biocontrol agents, chemical fungicides, and genetically determined host resistance have not yet solved the loss of yield. As a soil-borne root pathogen, it has remained as an unsolved problem in agricultural systems in Australia and worldwide. While the steps in the infection of host plants by T. basicola have been well documented (Hood and Shew, 1997c, b; Mims et al., 2000), little is known about the genes involved in 1 plant-pathogen interactions. Moreover, no molecular biology tools have been developed for studying the infection process of T. basicola. The application of molecular genetic analysis to investigate mechanisms of the infection process will lead to the identification of resistance mechanisms that can be used to aid resistance breeding in host plants. The focus of this review is to provide some background information on the host range and specificity of T. basicola, factors that promote and suppress black root rot, the steps that are involved in the infection process, the symptoms of black root rot and the current management procedures available for this disease. Understanding this area will allow the appreciation of T. basicola, the causal organism of black root rot, as a model organism for soil fungi because of its capability to interact with a wide variety of plants and its significant impact in the agricultural field. This review also highlights some of the pathogenicity genes that have been characterised during different stages of the infection process by filamentous fungi. 1.2. T. basicola 1.2.1. Characterisation of fungi Fungi are eukaryotic microorganisms. They are heterotrophic (not autotrophic) meaning that they are unable to manufacture their own food by photosynthesis (nonphotosynthetic). Unlike animals (also heterotrophic), which ingest then digest, fungi digest by producing exoenzymes, then feed by absorbing simple nutlients small enough to pass through the fungal cell wall and across the plasma membrane (Hardham and Mitchell, 1998). Some fungi are unicellular, such as yeast, and others are multicellular known as filamentous fungi or moulds (Atlas, 1995). The filamentous fungi consist of a mass of filaments or vegetative cells that connect to each other resulting in multicellular branching tubular filaments known as hypha. Integrated masses of hyphae are called mycelia. Hyphae 2 contain individual compartments formed by crosswalls called septa, but in some fungi, hyphae are coenocytic (aseptate) (Atlas, 1995). The hyphae are surrounded by cell walls conlposed of chitin, 1,3-p-glucan and 1,6-p-glucan, mannan and proteins. However, cell wall composition varies markedly between species of fungi. The fungal cell wall is a highly dynamic structure subject to constant change, for example, during cell expansion and division in yeasts and during spore germination, hyphal branching and septum formation in filamentous fungi (Adams, 2004). Fungi propagate by spores as a result of asexual and sexual reproduction (Atlas, 1995). Many of the fungal pathogens of plants (and humans) belong to the group of filamentous fungi that exhibit dimorphism (capable of reversible transitions between yeast and hyphal forms) (Gow et al., 2002). Filamentous fungi use polar growth. After a period of isotropic or symnletric expansion, filamentous fungi switch permanently to apical extension and a germ tube emerges. New material is added exclusively to the apex, resulting in an extending tubular cell, or hypha. Branches also emerge from the main hypha, establishing additional axes of polarity (Momany, 2002). T. basicola is a filamentous soil-borne fungus that attacks plants causing the soil-borne root disease black root rot (Shew and Meyer, 1992). Soil-borne fungi are fungi that are detected in soil and roots of plants. Plants are the primary producers of the ecosystem and thus are the major source of food to all animals as well as many microorganisms. Losses in crop production due to plant diseases average 13% worldwide and severely limit production of food. The 11,000 different diseases documented in plants, are caused by 120 genera of fungi, 30 types of viruses and eight genera of bacteria (Montesinos et al., 2002). 3 1.2.2. Host range T. basicola is a very common fungus. Plant health authorities in over 46 countries have reported it as a pathogen infecting 137 plant species in 33 plant families (Otani, 1962). It is found in undisturbed habitats as well as in crops and can be commonly found in both uncultivated and cultivated soils (Yarwood, 1974, 1981). The fungus is commonly found on plants from the Fabaceae, Solanaceae, Malvaceae and Curcubitiaceae families (Shew and Meyer, 1992). Important agricultural hosts include cotton, tobacco, beans, carrots, soybeans, peanuts and lettuce. T. basicola also infects ornamental plant species which include sweet peas and pansies. Although the fungus is a facultative parasite, it can also associate with hosts in a non-pathogenic manner (Yarwood, 1974). 1.2.3. Host specificity Host-pathogen specificity as defined by Zhou and Hyde (2002) is a relationship in which a fungus derives its nutrition from a live host plant during some phase of its life cycle, and is restricted to a particular host or a group of related species, but does not occur on other unrelated plants in the same habitat. Many fungal pathogens are host-specific and this view has been supported by many reviews (Wolpert et aI., 2002; Zhou and Hyde, 2002). Host-pathogen specificity has also been reported to exhibit different degrees of genetic specificity (Kirchner and Roy, 2002). For example, some soil-borne plant pathogens exhibit low specificity, infecting a wide range of host genotypes (Kirchner and Roy, 2002). The molecular mechanisms that are involved in fungal host range specificity are not fully understood. However, a number of virulence determinants have been identified to be host specific. For example, some isolates of the ascomycete, Nectria haematococca contain pisatin demethylase and pea pathogenicity (PEP) genes that are found on dispensable pisatin demthylase chromosomes that encode the ability to detoxify phytoalexins of pea 4 whereas, other strains encode other genes present on non-pisatin demethylase-containing chromosomes that cause pathogenicity on other hosts such as ripe tomato and mature carrots (Funnel and VanEtten, 2002). Evidence of host specificity in T. basicola isolates has been reported to be related to differences in pathogenicity (Keller and Shanks, 1955~ Lloyd and Lockwood, 1963; Tabachnik et a!., 1979; O'Brien and Davis, 1994; Bai et a!., 1996). Keller and Shanks (1955) found that T. basicola strains that attacked tobacco did not attack poinsettia and vice versa. Lloyd and Lockwood (1963) reported that T. basicola strains from orange, pea and poinsettia were moderately to highly pathogenic on bean and pea but nonpathogenic on tobacco plants. In contrast, strains from tobacco were highly pathogenic on tobacco but nonpathogenic on bean plants. One tobacco strain was mildly pathogenic on pea plants. Tabachnik et al. (1979) reported that although host specificity was found for some isolates of T. basicola, isolates obtained from various hosts were not necessarily specific for these hosts. The specificity they observed could be due to differences in stimulation of spore germination, or differences in pathogenicity. Differences observed in pathogenicity between different isolates are due to differences in the genotype between the isolates. Results from RAPD analysis conducted by Punja and Sun (1999) support the possibility of the occurrence of genetically distinct strains of the pathogen that may be more adapted to specific hosts. Specific host preferences among isolates of T. basciola have previously been demonstrated experimentally. Specificity of hosts by T. basicola may occur at the plant species or at the cultivar level. At the species level, the tobacco species Nicotiana debneyi was shown to be completely resistant to a wide range of T. basicola strains while another tobacco species N. tabacum showed varying levels of susceptibility to the disease (Bai et al., 1996). Cultivar specificity has been demonstrated on lettuces (Lactuca sativa L.) with the 5 cultivars Monaro and Centenary being resistant, while others like Classics and Yatesdale being highly susceptible to infection by T. basicola (O'Brien and Davis, 1994). In a more detailed and thorough study, Punja (2004) used 27 isolates of T. basicola to assess the degree of pathogenicity on detached bean leaves as host tissues. His results also demonstrated that there were pronounced differences in pathogenicity among isolates.
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