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Germinating seed and seedlings of lucerne are infected and killed rapidly, with the disease appearing as damping off. Older plants are also infected, with characteristic reddish-brown or black root lesions developing into root rot disabling some or all of the root system. Mature plants may be killed, especially following repeated infection in poorly drained areas, but more often exhibit chlorosis and reduced growth (Erwin, 1954; Schmitthenner, 1964). Infected plants are prone to winter injury, including frost heaving.
In chickpea, Phytophthora root rot is characterized by chlorosis and desiccation of foliage, decay of lateral roots, small brown lesions that develop into extensive dark-brown lesions on the taproot, and the development of girdling lesions with reddish-brown margins extending to the collar (Vock et al., 1980).
Control of Phytophthora root rot of lucerne is possible in most situations by combinations of improved water management, use of resistant cultivars and crop rotation. Assuring good drainage is the first step in almost all situations. This can be accomplished by selecting well-drained soils for establishment of lucerne crops or grading fields to eliminate low spots (Erwin and Ribeiro, 1996), by installing drainage tile or ditches to remove water, by subsoiling to break up impervious soil layers, and by avoiding excessive irrigation (Lehman et al., 1968).
Phytophthora root rot in chickpea in Australia has been partially controlled by using moderately resistant varieties, by seed applications of fungicides such as metalaxyl, by alternative crop rotations and by reduced irrigation regimes (Myatt et al., 1993).
Lucerne and chickpea cultivars vary widely in their tolerance of Phytophthora root rot, and selections with useful field resistance are available in most regions where root rot is a problem (Lehman et al., 1967; Irwin, 1974; Irwin and Maxwell, 1980; Frosheiser, 1980; Brinsmead et al., 1985; Lowe et al., 1987; Dale and Irwin, 1991a; Ansar et al., 1995). However, resistance is relative and losses may still be heavy in seedling stands and in chronically poorly drained soils (Erwin and Ribeiro, 1996). If Aphanomyces euteiches is also present, then it may be necessary to use cultivars with dual tolerance (Holub and Grau, 1990b). In the future, new resistant cultivars produced by genetic engineering may be available (Masoud et al., 1996). A genetic linkage map has been developed for lucerne grown in northern Australia (Musial et al., 2005). Quantitative trait loci (QTLs) involved in resistance to P. medicaginis were identified in a backcross population. This genetic linkage map provides an entry point for future molecular-based improvement of disease resistance in lucerne in Australia.
There has been a recent advance in chickpea breeding for resistance to Phytophthora root rot with the evaluation of wild Cicer species for resistance to P. medicaginis. Cicer echinospermum accessions have shown significantly greater resistance to P. medicaginis than chickpea genotypes, both in the glasshouse and in the field (Knights et al., 2003). C. echinospermum accessions can be readily crossed with chickpea to give fertile progeny and so provide a readily available source of resistance for this pathogen. However, while this source of resistance provides extended scope in breeding programmes, it is incomplete and chickpea with this resistance remain partially susceptible under extreme disease pressure. Thus, other disease control methods in conjunction with plant resistance should not be overlooked.
Crop rotations of 3 years or longer are necessary to reduce P. medicaginis populations in fields because of the long-lived oospores (Pratt and Mitchell, 1975; Stack and Millar, 1985b). Populations may quickly recover to damaging levels after a rotation unless drainage is improved and resistant cultivars are employed. Chemical control of Phytophthora root rot is possible, but is seldom economically feasible. The Oomycete-active fungicides will limit development of root rot in seedlings and in established plants, but will not eradicate the pathogen from infested soil. It may be feasible to provide critical short-term protection to seedling stands by fungicidal seed treatments before sowing (Rhodes and Myers, 1989). The usefulness of metalaxyl to control P. medicaginis may soon be reduced with the discovery of an isolate of the fungus insensitive to this fungicide (Stack and Millar, 1985a). Protection of seedlings can be obtained under experimental conditions by seed treatment with biological agents such as the bacterium Bacillus cereus (Silo-Suh et al., 1994). Myatt et al. (1993) found that Pseudomonas cepacia (7 strains) and Pseudomonas fluorescens (2 strains) were able to limit or delay chickpea seedling disease caused by P. medicaginis in a pasteurized soil. However, commercial application of this type of control has yet to be demonstrated.
An integrated disease management programme incorporating host resistance and including cultural, chemical or biological methods needs to be implemented to best control P. medicaginis.
There is apparently no specific information available on the economic impact of P. medicaginis on a regional scale. It is widespread in many lucerne production areas and reduces stand density, winter hardiness (Tu, 1980; Basu and Butler, 1991; Wiersma et al., 1997) and forage yield of individual plants and entire fields (Lowe et al., 1987). A 75% stand reduction in the first year after sowing a susceptible cultivar in naturally infected fields (Gray et al., 1983) is perhaps representative of severe damage. In a different study, individual infected and healthy plants were monitored for 2 years; forage yield of root-rotted plants was reduced by 68% (Gray et al., 1988). In Wisconsin, USA, lucerne cultivars with good field resistance to Phytophthora root rot yielded 40% more than susceptible cultivars and combined resistance to Phytophthora and Aphanomyces increased yield still more (Wiersma et al., 1995). Estimating damage from comparisons of susceptible and resistant cultivars probably underestimates total loss, because even 'resistant' cultivars may be infected and damaged, especially under very wet conditions (Erwin and Ribeiro, 1996).
In eastern Australia, Phytophthora root rot caused by P. medicaginis is regarded as one of the major constraints to lucerne production on poorly drained soil types (Irwin, 1974; Rogers et al., 1978).
P. medicaginis is a major disease of chickpea in Australia (Vock et al., 1980; Irwin and Dales, 1982; Brinsmead et al., 1985; Knights et al., 2003). Large areas of chickpea in a field can be killed causing growers to abandon part or all of affected crops (Dale and Irwin, 1990b). Yield losses can exceed 50% for individual crops and reach 20% on a regional basis in years with above average rainfall (Knights et al., 2003).