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The continuing development of fungicide resistance in plant and human pathogens necessitates the discovery and development of new fungicides. Discovery and evaluation of natural product fungicides is largely dependent upon the availability of miniaturized antifungal bioassays. Essentials for natural product bioassays include sensitivity to microgram quantities, selectivity to determine optimum target pathogens, and adaptability to complex mixtures. Experimental accuracy and precision must be stable between assays over time. These assays should be relevant to potential pathogen target sites in the natural infection process of the host and applicable to the agrochemical industry. Bioassays should take advantage of current high-throughput technology available to evaluate dose-response relationships, commercial fungicides standards, modes of action, and structure activity studies. The focus of this chapter is the evaluation of natural product based fungicides for agriculture and we will provide a review of bioautography prescreens and microtiter assays (secondary assays). Also presented is more detailed information on newer techniques such as the detached leaf assays for evaluating fungicides against strawberry anthracnose ( Colletotrichum spp.) and field plot trials for gray mold ( Botrytis ) and anthracnose control in strawberry.

Many allelopathic compounds in their native or processed forms have potential for development as viable components of plant-parasitic nematode management strategies. Allelochemicals have been identified that possess differing levels of activity against a wide range of plant-parasitic nematodes. In general, these compounds are less toxic to nontarget species, and less persistent in soil than chemical nematicides. Operative mechanisms for plant-parasitic nematode control with allelopathic compounds include nematicidal activity, nematostatic activity, and nematode behavior modification. Allelochemicals are sometimes produced in large quantities in plant material or as agricultural waste, making the use of rotation crops, cover crops, and organic amendments effective means for production and/or distribution of the active compounds. A greater understanding of the effects of soil microbes and environmental conditions on allelopathic compounds is necessary to improve their efficacy for control of parasitic nematodes. Use of allelochemicals for nematode control will require that growers know specifically what types and population levels of nematodes are present in their production fields. Development of improved production and incorporation methods for rotation and green manure crops, and appropriate application methods for processed allelochemical compounds, will also enhance the efficacy and consistency of these compounds for nematode control.

Increasing attention has been given to the role and potential of allelopathy as a management strategy for crop protection against weeds and other pests. Incorporating allelopathy into natural and agricultural management systems may reduce the use of herbicides, insecticides, and other pesticides, reducing environment/soil pollution and diminish autotoxicity hazards. There is a great demand for compounds with selective toxicity that can be readily degraded by either the plant or by the soil microorganisms. In addition, plant, microorganisms, other soil organisms and insects can produce allelochemicals which provide new strategies for maintaining and increasing agricultural production in the future.

Pathogenesis can have both detrimental and beneficial impacts on plant fitness. As such, pathogens are important forces that influence the structure and dynamics in natural and manipulated plant ecosystems. Plant production and numbers within a community are constrained by environmental limitations, which are often mediated through plant interference. Competition for resources and allelopathy (chemical interactions) are the two most important ways that plants interfere with each other. This chapter reviews the effects of pathogens on the competitiveness and allelopathic ability of their hosts. In most cases, pathogens reduce the competitive ability of their host, making the host prone to displacement by neighbouring, resistant plants. However, pathogens may simultaneously increase the allelopathic ability of their hosts, thereby offsetting their loss in competitiveness to varying degrees. Evidence for enhanced allelopathy by infected plants comes in two forms: (i) pathogens stimulate the production of secondary metabolites by plants, many of which are implicated in allelopathy (eg phenolics), and (ii) field, glasshouse and bioassay studies showing that infected plants may suppress their neighbours more than healthy plants, under conditions of low competition. By conferring the benefit of increased allelopathy on their hosts, pathogens may maintain a self-advantage through increasing the survival chances of their hosts and ultimately themselves. The enhanced allelopathy of infected plants supports the ‘new function’ hypothesis, which suggests that pathogens evolve toward a mutualistic relationship with their host through the appearance of strains with beneficial effects on the host in addition to their detrimental effects.

Allelochemicals offer ample scope for ecologically safe and effective weed control in aquatic and wetland systems. This could be attributed to the absence of soil interface in aquatic habitats that contributes largely for rapid degradation of allelochemicals. Simpler strategies involving allelopathy especially for small holder farms, low input agriculture and aquatic environments with appreciable results have been reported. Such strategies include use of allelopathic cultivars, organic manures and plant products. Though allelopathic suppression of weeds could not be construed as an alternate to replace synthetic herbicides, it can fit in an integrated weed management program very well as a prime component. Such strategies are reviewed. Further, a specific case study for the use of plant product along with insect agents for controlling water hyacinth in India and different steps involved in selecting allelopathic plant products for aquatic weed control are discussed.

The release of root exudates from plants encourages the formation of beneficial bacterial communities in the root zone capable of generating secondary metabolites that improve plant health and crop yield. Metabolites with antibiotic or lytic action have been identified, while others are known to induce systemic disease resistance in the host plant, or interfere with the nutritional requirements of phytopathogens. However, despite existing positive relationships between bacterial communities and their plant hosts, man-made attempts at applying bacteria for biocontrol purposes have met with limited success. Inconsistent performance of biocontrol bacteria in the field may be due to the variable expression of genes involved in the biocontrol action, or simply the resistance of established soil communities to a sudden and inundative influx of adventive bacterial species or strains. Regardless of the inherent capacity of ‘naturally occurring’ soil microbial ecosystems to buffer anthropogenic interference, crop management systems are regularly used to distort agro-ecosystems through, for example, the use of tillage operations, alternate cropping systems, monoculture, crop rotation length, fertilizer and organic amendments, and various crop protection chemistries. The management of soil microbial communities for disease control appears to involve, in part, the creation of short term chaos in the microbial community through the application of such perturbation stresses. While hope remains that bacterial communities with biocontrol activity will one day be used as an adjunct to, or replacement for, agrichemical crop protectants, reliable biological controls that moderate pathogen attack remain elusive. In the interim, disease suppressive soils may be encouraged to form through the use of modest perturbation stresses that promote microflora species’ diversity and functionalities underpinning natural bioantagonism.

Allelopathic bacteria encompass those rhizobacteria that colonize the surfaces of plant roots, produce, and release phytotoxic metabolites, similar to allelochemicals, that detrimentally affect growth of plants. Practical application of this group of bacteria to agriculture could contribute to biological weed management systems that have less impact on the environment than conventional systems by reducing inputs of herbicides. Allelopathic bacteria have been investigated for potential as inundative-type biological control agents on several weeds. Because allelopathic bacteria generally do not attack specific biochemical sites within the plant, unlike conventional herbicides, they offer a means to control weeds without causing direct selective pressure on the weed population, therefore, development of resistance is not a major consideration. Additionally, the use of allelopathic bacteria appears to be environmentally benign relative to herbicides. These characteristics make allelopathic bacteria an attractive approach for managing crop weeds in a sustainable manner, even within the boundaries of conventional agriculture systems. However, recent evidence suggests that indigenous allelopathic bacteria might be exploited under certain crop and soil management practices that are inherently part of sustainable agricultural systems. The development of “weed-suppressive” soils in diverse sustainable systems is encouraging because indigenous populations of allelopathic bacteria may develop in several soils and environments using similar practices. The recent demonstrations of apparent weed-suppressive soils may lead to development of specific management strategies for the establishment and persistence of native allelopathic bacteria directly in soils conducive to annual weed infestations.

American ginseng ( Panax quinquefolius L.) is a perennial herb valued for the medicinal properties of its large, fleshy tap root. These medicinal properties are purported to be due to the triterpenoid saponins, or ginsenosides, that accumulate to 3–6% of the root dry weight. We asked the question: what is the ecological role of ginsenosides in Panax species? In addressing this question, we have determined that ginsenosides, like other saponins, possess fungitoxic properties, although different fungi and oomycotan organisms appear to be differentially affected by them in vitro. In order to play an allelopathic role, however, ginsenosides must be present in the soil at biologically active (i.e., ecologically relevant) concentrations. Results to date support the hypothesis that ginsenosides are phytoanticipins and serve as host resistance factors. The success of certain pathogens (e.g., Pythium cactorum, Pythium irregulare, Cylindrocarpon destructans ) on ginseng may arise from their ability to detoxify or otherwise utilize ginsenosides as a nutritive source or growth stimulating factor, while other soil borne organisms appear susceptible to their fungitoxic properties. Ginsenosides have been isolated from rhizosphere soil and root exudates suggesting that these compounds are involved in allelopathic interactions between the host plant and soil fungi. Ultimately this allelopathic interaction may influence the fungal diseases of ginseng.

Antimicrobial sesquiterpenoids, 8a-angeloyloxycichoralexin and guaianolides such as cichoralexin and 10a-hydroxycichopumilide were isolated and identified from the root of chicory ( Cichorium intybus ). These sesquiterpenoids exhibited antifungal activities against Pyricularia oryzae, Pellicularia sasaki and Alternaria kikuchiana . Ether soluble phenolics from the chicory root were found to exhibit nematicidal activity. The addition of dry chicory root powder to noodles, a boiled fish paste, and cocoa- and coffeecakes, during food processing, provided some protection from food spoilage organisms, and this product may have value as a natural food preservative.

Allelochemals induced in mycorrhizal plants play an important role in disease resistance. Mycorrhizal associations are the most important symbiosis systems in terrestrial ecosystems and offer many benefits to the host plant. Arbuscular mycorrhizal associations can reduce damage caused by soil and root - borne pathogens.