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Activating plant defenses against pathogen and insect pests is a wonderfully enticing concept. While induced resistance has been studied for decades in the laboratory, this information has not translated to consistent pest control in the field. Plant activators are compounds that can activate plant defense responses and control disease without directly impacting the pathogen. Additionally, many of these products claim to increase plant health and yield and have a lower environmental impact. Plant activators could be a valuable tool in an integrated pest management program by delaying initial pesticide applications or they could be alternated with chemical control. Furthermore, some activators are certified for organic use and could be implemented in organic growing systems. Though several plant activating compounds are commercially available, their utilization by growers is quite limited. This is mainly due to inconsistent effectiveness of such products under field conditions. This research has evolved into four distinct projects, listed below.
Impact of plant growth-promoting rhizobacteria (PGPR) on pepper production and green peach aphid infestations.
Plant growth-promoting rhizobacteria (PGPR) are known in various cropping systems to increase plant growth and vigor, as well as induce resistance to pathogens and pests. A commercial soil amendment containing a mixture of two species of Bacillus PGPR (Bacillus subtilis and Bacillus amyloliquefaciens) was evaluated for impact on germination and initial growth of bell pepper plants, efficacy against the green peach aphid, Myzus persicae Sulzer, and yield enhancement. Studies in the greenhouse revealed that pepper germination rate and dry weight of seedlings grown with or without Bacillus spp. did not differ significantly. In the field, the PGPR did not significantly reduce aphid populations compared to control plants, whereas imidacloprid was highly effective. An increase in yield compared with control plants was observed in the 2003 season, but not the following two seasons. Aphid pressure was high in 2003, and plants grown in the presence of Bacillus spp. exhibited substantial tolerance to aphids. That is, there were significantly higher populations of the green peach aphid on both control and PGPR-treated plants compared with imidacloprid-treated plants. However, fruit yield in the Bacillus spp. treatment was significantly greater than yield in the control treatment and similar to yield in insecticide-treated plots. Bacillus PGPR could be useful in a M. persicae management program for pepper plants grown in locations with consistently high aphid pressure.
Herman, M.A.B., Nault, B.A., and Smart, C.D. 2007. Effects of plant growth-promoting rhizobacteria on bell pepper production and green peach aphid infestations in New York. Crop Protection. 27(6): 996-1002.
Induction of plant defense gene expression by plant activators and Pseudomonas syringae pv. tomato in greenhouse-grown tomatoes.
Plant activators provide an appealing management option for bacterial diseases of greenhouse-grown tomatoes. Two types of plant activators, one that induces systemic acquired resistance (SAR) and a second that activates induced systemic resistance (ISR), were evaluated for control of Pseudomonas syringae pv. tomato and effect on plant defense gene activation. Acibenzolar-S-methyl (ASM, SAR-inducing compound) effectively reduced bacterial speck incidence and severity, both alone and in combination with the ISR-inducing compound. Application of ASM also led to elevated activation of salicylic acid and ethylene-mediated responses, based on real-time PCR analysis of marker gene expression levels. In contrast, the ISR-inducing compound (made up of plant growth-promoting rhizobacteria) inconsistently modified defense gene expression and did not provide disease control to the level of ASM. No negative cross-talk was observed by combining the two activators as control of bacterial speck was similar to or better than ASM alone.
Herman, M.A.B., Davidson, J.K., and Smart, C.D. 2008. Induction of plant defense response pathways by plant activators and Pseudomonas syringae pv. tomato on greenhouse-grown tomatoes. Manuscript submitted to Phytopathology March 2008.
Defense gene expression patterns of three SAR-induced tomato cultivars in the field.
This study examined effects of acibenzolar-S-methyl (ASM), a plant activator that induces systemic acquired resistance, on defense response activation in three field-grown tomato cultivars in New York. Salicylic acid, ethylene and jasmonic acid-mediated responses were monitored by following expression of a marker gene for each pathway using quantitative real-time PCR over the course of two ASM applications. ASM induced the salicylic acid and ethylene, but not jasmonic acid, pathways in all cultivars tested. All three cultivars demonstrated a much more significant response of the salicylic acid and ethylene defense response pathways following the second ASM application.
Herman, M.A.B., Restrepo, S., and Smart, C.D. 2008. Acibenzolar-S-methyl induction of plant defense genes in three tomato varieties in the field. Physiological and Molecular Plant Pathology. In Press, accepted manuscript.
Impact of plant activators and copper on bacterial speck and gene expression in field-grown tomatoes.
Two types of plant activators and copper were evaluated for control of bacterial speck disease and tomato defense gene activation over three field seasons. Acibenzolar-S-methyl (ASM), which activates systemic acquired resistance, controlled Pseudomonas syringae pv. tomato as well as copper and no negative effects on yield were observed. The plant growth-promoting rhizobacteria (PGPR, activator of induced systemic resistance) compound reduced bacterial speck symptoms relative to the untreated control though did not consistently control to the level of ASM and copper. Alone or in combination with ASM, the PGPR compound provided some boost in yield in one of the three years of the trial. All treatments negatively impacted pathogen growth. Response of ASM-treated plants was dependant on disease pressure; the salicylic acid and ethylene-mediated responses were activated to detectable levels only under high disease pressure. Despite providing some disease control, no priming of phytohormones signaling networks was observed in PGPR-treated plants. Implications of these findings on field management strategies and phytohormones signaling interactions are discussed.
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