Effects on plant community composition through interactions among herbivores, microbes and plants are an increasing area of interest in research in plant community ecology. This is a major theme of research for NPARL Research Plant Pathologist, Anthony Caesar, since discovering that insect/plant pathogen synergisms are fundamental to effective biological control of herbaceous, perennial invasive plants. In research, at every stand of Euphorbia esula/virgata (leafy spurge) surveyed, foreign and domestic, that was subject to root-attacking insects, a set of soil borne plant pathogens was isolated from the damaged root tissue of the plants.
The same situation has been found with regard to Centaurea spp. and more recently insect/plant pathogen interactions have been found occurring with Carderia (=Lepidium) spp., which exhibit root galls, typically caused by Ceutorhynchus spp. This has confirmed the central importance of fungal or bacterial synergists in successful biocontrol. Harnessing fungi found to be partners in synergistic interactions would enable their application at the time of release or after insects have become established to enable the effective synergism.
There is ample evidence that information gained from the study of these interactions can form the interface between biological control and restoration, which should be considered as part of biological control. Investigations of how plant communities affect microbial communities has been supplemented with studies on how, in turn, microbial communities can affect the composition of plant communities.
Parallel studies are the investigation of the comparative ecology of an invasive species in its native and invaded range. For my program this means study of microbial factors associated with the effects of insects or how specific plant pathogens (such as rusts or smut fungi) impact the target in its native range. The elucidation of key biotic factors associated with the sparse density and scattered distribution of an invasive species in its native range is a major goal. In contrast, invasives achieve high density and rapid spread in the invaded range, and significant disease is rare in the invaded setting. Dr. Caesar found that soil borne plant pathogens are keystone factors limiting the vigor and distribution of invasive plant species in their native ranges.
Because most highly invasive species in rangelands are herbaceous, deep-rooted perennials, a prolonged and sustained attack is necessary to cause mortality and thereby reduce stand density of the target species. Such a sustained and prolonged attack in the form of insect/microbial synergisms is constantly evident in the native range of the invasive species causing the sparse and restricted populations of the invasives there. Synergisms effective in the native range can provide microbial agents that can be used in prerelease testing of candidate agents for capacity to synergize. The particular value of a candidate agent should be assessed by the effectiveness of the synergisms it may engage in upon release. Additionally, candidate rusts and smuts can be assessed for their overall effects in the native range to provide clues on their likely impact once introduced for biological control.
Soil microbial community structure associated with biocontrol can be compared to that of both weed-invaded and uninvaded soil to assess the nature of the feedback and patterns of plant community succession. Of central interest is whether microbial community structure might also predict the success of attempts to reestablish native species. Further, comparing microbial community structure in the native range with that of the above three states can provide insight into whether the status of the soil negative feedback might predict when insects should be released for successful insect plant pathogen synergism, and thus impact, to occur.
Retrospective studies are the review of past successes often for broad trends such as which insect taxonomic groups have been successful in biological control of weeds. For my research retrospective studies consist of the review of successful programs for clues on whether and how plant pathogens may have contributed to successful weed biocontrol.
Past biocontrol studies contain valuable clues, which when examined in a new light and placed in proper context with more recent findings, can be used to improve biological control. New avenues of research to improve the screening of candidate agents for the capacity to synergize with plant pathogens are a major prospective benefit of retrospective studies. Studies early in the history of biological weed control have already provided data and principles leading to the conclusion that insect/pathogens synergisms caused control of some major target weed species such as prickly pear and St John' s wort. As described above, this correlates with recent findings that confirm the importance of synergisms in successful biocontrol of leafy spurge
Practical outcomes would be the utilization of important soilborne plant pathogens occurring on diseased stands in the native and invaded ranges or found associated with insect damage (usually foreign) for invoking synergisms and for inclusion as part of prerelease testing of candidate agents for the capacity to synergize with the pathogens. Testing agents for synergistic ability can ultimately reduce the costs of screening, host range testing, release, redistribution and monitoring of new agents in biocontrol programs by restricting released agents to those with clear evidence of impact in prerelease studies. Increasing overall impact as early as possible through insect/pathogen synergistic action can additionally reduce the hazard of nontarget effects. An ultimate goal of biocontrol should be the restoration of a healthy, productive and sustainable plant community and the themes discussed above seek to find means to achieve that goal.
Contributing Scientist: Anthony Caesar(Plant Pathologist)
Soilborne fungi associated with root galls of Lepidium draba caused by Ceutorhynchus spp. By: Anthony J. Caesar (Plant Pathologist) Nearest Neighbor Analysis of the Effects of the Rust Fungus Uromyces scutellatus on Euphorbia spp. in Europe By: Anthony J. Caesar (Plant Pathologist) Biocontrol of exotic invasive plants is based on the Enemy Release Hypothesis. This entails searching within the native range of the invasive for insects or pathogens that damage the "target" plant species.
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Isolation of fungi from insect-damaged roots of Lepidium draba in Switzerland, Hungary and Austria revealed that this species was often infected with one or more soilborne fungi. Plants with evident stunting and/or chlorosis and reddening of leaves nearly always exhibited root damage by one or more insects.
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Soilborne fungi associated with root galls of Lepidium draba caused by Ceutorhynchus spp.
By: Anthony J. Caesar (Plant Pathologist)
Nearest Neighbor Analysis of the Effects of the Rust Fungus Uromyces scutellatus on Euphorbia spp. in Europe
By: Anthony J. Caesar (Plant Pathologist)
Biocontrol of exotic invasive plants is based on the Enemy Release Hypothesis. This entails searching within the native range of the invasive for insects or pathogens that damage the "target" plant species.