Allelochemicals Biologica... - Name

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MARK A. BERNARDS, LINA F. YOUSEF, ROBERT W. NICOL
rhizosphere secondary chemicals in general may be serving as signal molecules for
plant pathogens in a manner analogous to Rhizobium-legume and tree-mycorrhizae
symbioses (Peters and Long, 1988; Larange et al., 2001). The ability of rhizosphere
phytochemicals to fulfill a molecular need for the pathogen or to induce cellular
pathways important for pathogenesis or virulence is a relatively unexplored avenue of
plant-microbe allelopathy, even though this has obvious implications for diseases in
economically important plants.
Anecdotal evidence suggests that ginseng crops suffer from a replant syndrome
and a common farming practice is to avoid planting consecutive ginseng gardens in
the same area. Although there is no data on the cause of the replant problem in
ginseng, it has been suggested that pathogens could be involved (Reeleder et al.,
1999). In fact the ginseng replant problem bears a similarity to the situation in apple
orchards. Using our results, a replant syndrome in ginseng gardens could be postulated
to have the same basic ingredients (i.e., phytochemicals, pathogens and antagonists)
present in the model of the apple replant syndrome. For example, it is well
established that the “dominant causal agents” (Mazzola, 1998) of the apple replant
syndrome are a complex of soilborne fungal pathogens, especially species in the genera
Cylindrocarpon, Fusarium, Phytophthora and Pythium (Mazzola 1999; Isutsa
and Merwin, 2000) including C. destructans, P. cactorum and Py. irregulare (Braun,
1991; 1995; Mazzola, 1998). Although pathogens play a primary role, antagonists
have also been implicated through results obtained from their application as biocontrol
microbes (Utkhede et al., 2001) or surveys of orchard soil (Mazzola, 1999). Finally,
it has been suggested that an important component of apple replant syndrome is the
induction of virulence in microbes by root exudates (Szabo and Wittenmayer, 2000).
Similarly, unidentified allelochemicals may also be involved in the asparagus replant
syndrome (Peirce and Colby, 1987; Hartung and Stephens, 1983; Pedersen et al.,
1991). Therefore, a ginseng replant syndrome, like that of apple and asparagus,
could be due to a phytochemical-mediated alteration in the soil microbial population
resulting in the stimulation of a complex of pathogens coupled with an inhibition of
beneficial microbes. In other words, allelopathy may play a role in year-to-year soilborne
crop disease cycles as well as replant syndromes.
6. CONCLUDING REMARKS
Plant disease researchers often focus on causal agents in relative isolation. Our research
further demonstrates that other host-derived factors may also play an important role
in the etiology and severity of plant diseases and replant syndromes. That is, allelopathy
may contribute to plant diseases through the ability of certain microbes to grow and/
or survive preferentially in the phytochemical profile of the rhizosphere surrounding
specific crop roots. Because pathogenicity is a common nutritional mode in microbes,
it is not surprising that pathogens are adapted to the chemical environment of their
host plants. Manipulation of rhizosphere chemistry, therefore, either through genetic
modification of plants, identification and use of microbe-influencing genotypes or the