- Agronomic: peanuts
- Vegetables: peppers
- Pest Management: biological control, genetic resistance, integrated pest management, cultivation
We found that adult beet armyworms (BAW) oviposited more and larvae fed significantly more and performed better on white mold infected plants than on healthy plants. The volatile profile released by white-mold infected peanuts was significantly different from those emitted by undamaged plants. Also, peanut plants infected with the white mold and then exposed to BAW damage released all the volatiles emitted by healthy plants fed on by BAW, and those emitted by plants in response to white mold infection alone. The BAW larval parasitoid Cotesia marginiventris, landed more frequently on infected than on healthy plants exposed to BAW damage.
Herbivorous insects and plant diseases present a continuos threat to the agricultural environment because they reduce yield and the quality of crops. In particular, plant diseases, caused by infectious viruses, bacteria, phytoplasmas, fungi and nematodes, present serious problems in agriculture. These problems include reduced yield, lower product shelf-life, decreased aesthetic and nutritional value. In addition to the direct damage to the plant, attack by some pathogen strains and insect species also results in the production and accumulation of secondary metabolites and toxins that can cause disease in humans and animals. Control of plant diseases and herbivorous insects is vital for providing an adequate supply of food, feed, and fiber to cope with the increasing human population and its demands. In Florida alone, more than a dozen plant disease epidemics occurred from 1970-1990 (Kucharek 1990). Growers currently spend large sums of money to control pathogens and insects that attack their crops. Nevertheless, crop and commodity losses due to disease and herbivore damage cost billions of dollars each year. In the United States, it is estimated that the yearly economic losses are approximately 9.1 billion dollars for plant diseases, and approximately 7.7 billion dollars for insect damage, all this after the application of control measures practiced under modern agriculture (Agrios, 1997). Thus, information about how insect and pathogen pests interact with crops, and how this interaction affects the economic value and quality of agricultural products is important for establishing the economic thresholds for managing pest populations, minimizing pest damage, developing new methods of insect and pathogen prevention and control, and improving host-plant resistance and other mechanisms for tolerance to insect and pathogen pests.
Plants play an active role in the interactions taking place in their ecosystem, they possess a number of chemical defense mechanisms that may be triggered by herbivore and/or pathogen attack. These chemical defenses can directly modify the development and survival of the attacking organism (e.g. phytoalexins, proteinase inhibitors) (Mur et al. 1997), or serve as attractants to natural enemies of the pest (i.e., release of herbivore-induced synomones) (Turlings and Tumlinson 1991, Turlings et al. 1991, 1993). Oxilipins are oxygenated fatty acids found in higher plants that activate transcriptional genes in response to herbivory or pathogen infection. Jasmonic acid is an oxylipin that is derived from oxidation of linolenic acid and is dramatically increased after insect damage, turning on genes necessary for the production of phytoalexins and proteinase inhibitors (reviewed in Choi et al. 1994). Arachidonic acid, a fatty acid found in the fungus Phytophtora infestans (potato late blight), has also been found to activate phytoalexin-encoding genes but these are different from those activated by jasmonic acid in potato discs (Choi et al. 1994). This finding may be an indication that the biochemical pathways involved in plant defense against herbivores is different from those involved in the defense against pathogens.
In addition to the production of internal defense compounds in response to insect and pathogen attack, plants may also produce volatile substances that are released externally. Plants release a mixture of such compounds in response to attack by herbivores (McCall et al. 1994, Loughrin et al. 1995, Röse et al. 1996, Pare and Tumlinson 1997). These chemical signals are attractive to parasitoids of the pests (Turlings et al. 1991, 1993, Röse et al. 1998) and, since both the emitter (plant) and the recipient (parasitoid) benefit from these infochemicals, they are categorized as synomones. In addition to direct herbivore feeding, some plants also release synomones in response to herbivore oral secretions or regurgitate when this is either applied topically to a mechanically-damaged leaf or fed through the stem of excised plants. The production of herbivore induced synomones by plants raises questions about the processes involved in the induction and production of such chemicals. Compounds responsible for eliciting the emission of plant volatiles have been isolated and identified in recent years. Examples of these are volicitin, a component found in Spodoptera exigua regurgitant (Alborn et al. 1997), and beta-glucosidase, found in oral secretions of Pieris bassicae (Mattiacci et al. 1995), which elicit volatile emissions in corn and cabbage, respectively.
Pathogens and pathogen derived substances can also elicit the production and release of volatiles from the affected plant hosts. Case in point, Brassica rapa seedlings were found to release volatile products of glucosinolate degradation when infected by the fungus Alternaria brassicae (Doughty et al. 1996). In another study with beans (Phaseoulus vulgaris L.) release of volatile linolenic acid derivatives ensued 15-24 h post-inoculation with Pseudomonas syringae pv. phaseolicola (Croft et al. 1993). It has also been suggested that ethylene, a volatile phytohormone, is involved in the induction of systemic acquired resistance (SAR), which confers protection against subsequent pathogen and herbivore attacks. The production of ethylene in plants is induced by various factors such as, mechanical wounding, exogenous auxin
applications, and herbivore and pathogen attack. Chaudhry et al. 1998 found that the production and release of ethylene increased in tobacco plants 48 h post-inoculation with cucumber mosaic virus (yellow strain). Additionally, it was observed that the increase in ethylene was positively correlated with an increase in the concentration of the enzymes required for its synthesis (ACC
synthase and ACC oxydase) (Chaudhry et al. 1998). Pathogen derived compounds such as cellulysin, which is a crude cellulose extract from the fungus Trichoderma viride, have been found to induce volatile production in tobacco, lima bean, and corn plants (Piël et al. 1997, Koch, et al. 1999). Piel et al. 1997 found that a 50 Fg/ml concentration of cellulysin elicited the emission of hexenyl acetate, ocimene, linalool, nonatriene, indole, bergamotene, beta-farnescene, nerolidol and tridecatetraene similar to those elicited by applications of jasmonic acid. These compounds have also been reported to be produced by plants in response to insect damage, however, caryophyllene which is a compound induced in corn by insect damage, was not present in the emissions of this plant in response to cellulysin, and bergamotene, another herbivore-induced compound, was only present in relatively small proportion. Similarly, coronatin, a phytotoxin isolated from Pseudomonas syringae bacteria, also elicits the release of volatiles in plants. Coronatin toxicity in plants exhibits symptom similar to those observed with high doses of jasmonic acid, such as chlorosis, accelerated senescence, and ethylene release (Weiler et al. 1994, Boland et al. 1995). However, treatment of plant cell cultures with coronatin did not induce an increase in endogenous jasmonates even though it was in some cases more active than jasmonates, and the coronatin structure strongly resembles that of 12-oxo-phytodienoic acid, which is a precursor of jasmonates (Weiler et al. 1994). Thus, it was concluded that coronatin is not an elicitor of plant responses but rather a close analogue of the octadecanoic precursor of jasmonates.
Many insect herbivores use volatile isothiocyanates to locate their brassicaceae hosts, and their parasitoids in turn use these chemicals to locate their herbivore hosts. So, the released volatiles emitted in response to attack by pathogens may play a role in oviposition site selection by herbivore females and in the host-searching process by natural enemies (Doughty et al. 1996). Plant volatiles also affect pathogens. The germination and growth of white mold (Sclerotium rolfsii) are stimulated by the release of methanol and other volatile compounds emanated from moist peanut hay. Volatiles from ground-up healthy corn kernels resistant to Aspergillus flavus, however, have been found to inhibit the growth and aflatoxin production in colonies of A. flavus (Zeringue et al. 1996). In Cotton the lipoxygenase-derived volatile trans-2-hexenal inhibited, while alpha and beta pinene stimulated the growth of A. flavus (Zeringue and McCormick 1989, 1990). Other compounds like 3-methyl-1-butanol and 3-methyl-2-butanol were found to decrease fungal growth but increase aflatoxin production (Zeringue et al. 1990). In the case of coronatin-induced volatiles in beans, the compounds emitted were trans-2-hexenal, which has high bactericidal activity and cis-3-hexenol, which is also bactericidal but only at much higher doses (Croft et al. 1993). Furthermore, these compounds were released in larger quantities from resistant varieties compared with susceptible ones (Croft et al. 1993).
Enhancing resistance to herbivores and disease in plants is an excellent management option and is often very cost-effective and environmentally safe. This approach, however, depends on our ability to identify and characterize the sources for resistance in crop species and in closely related plants. It is now known that plant volatiles play an important role in plant defense against both herbivorous and pathogenic organisms and thus may have a significant role in the regulation of
the behavior, development, and survival of these organisms. Therefore, the study of the
chemically-mediated interactions; the identification of the chemical compounds involved in the mediation between pathogen and pests with their host plant species; the effect of these interactions on agricultural ecosystems; and the environmental implications are critical for the
development of ecologically sound integrated management programs.
Detection of food or feed spoilage due to fungal growth was, for many years, conducted through the use of human noses, the recent development of electronic sensors has provided a far more sensitive alternative (Schnürer et al. 1999). This practice could potentially be used for the detection of pathogens and pathogen- or herbivore-damaged plants. Knowledge on the effect of plant emitted volatiles on insect behavior will enable the utilization of such chemicals for the detection of infected plants in the field, or even pathogen themselves by detection of pathogen-produced volatiles, by means of biological or mechanical devices. This will also enable the implementation of attractants for recruitment of parasitoids of herbivorous insects. Comparison of plant volatile profiles induced by pathogens and insects, alone and in combination, will give us a better idea, based on the nature of the compounds produced, of whether plant defense against these agents share the same biochemical pathways. Furthermore, the results obtained from such investigations will provide the basis for additional studies on the value of these plant-derived volatiles as possible antimicrobial agents, and for the identification of plant varieties with an enhanced chemical arsenal against pests.
The overall objective of this project is to investigate the production of volatile compounds by plants under pathogen attack, and to evaluate the effect of simultaneous pathogen/herbivore challenge on the volatile emission by host plant. The specific research objectives are to:
1) Analyze, identify, and compare compounds from head space collections from diseased and healthy plants.
2) Determine the effect of pathogen defense induction on the production and release of herbivore-induced volatiles by the host plant
3) Evaluate the effect of volatiles emitted from healthy, diseased, herbivore-damaged, and the combination of disease and herbivore damage on insect herbivore performance and on the oviposition site selection by adult herbivores and on the host searching behavior of parasitoids .