In this section, a selection of scientific articles is offered. Please click on the titles to view the full articles.
Genetic and virulence variability among Erwinia tracheiphila strains recovered from different cucurbit hosts (pdf)
Authors: E. Saalau Rojas, P.M. Dixon, J.C. Batzer, M.L. Gleason, Iowa State University, Ames, IA
Journal: Phytopathology (manuscript submitted in November 2012 – NOT TO BE REPRODUCED)
Abstract: The causal agent of cucurbit bacterial wilt, Erwinia tracheiphila, has a wide host range in Cucurbitaceae including economically important crops such as muskmelon (Cucumis melo L.), cucumber (Cucumis sativus), and squash (Cucurbita spp.). Genetic variability of 69 E. tracheiphila strains was investigated by rep-PCR using BOXA1R and ERIC1-2 primers. Fingerprint profiles revealed significant variability associated with crop host; strains isolated from Cucumis spp. were clearly distinguishable from Cucurbita-isolated strains regardless of geographic origin. Twelve E. tracheiphila strains isolated from muskmelon, cucumber, or summer squash were inoculated onto muskmelon (cv. Athena) and summer squash (cv. Early Summer Crookneck) seedlings, followed by incubation in a growth chamber. Wilt symptoms were assessed over three weeks, strains were re-isolated, and rep-PCR profiles were compared to the inoculated strains. Wilting occurred significantly faster when seedlings were inoculated with strains that originated from the same crop host genus (P< 0.001). In the first run of the experiment, cucumber and muskmelon strains caused wilting on muskmelon seedlings at a median of 7.8 and 5.6 days after inoculation (dai), respectively. Summer squash seedlings wilted 18.0, 15.7, and 5.7 dai when inoculated with muskmelon-, cucumber-, and squash-origin strains respectively. In a second run of the experiment, cucumber and muskmelon strains caused wilting on muskmelon at 7.0 and 6.9 dai, respectively, whereas summer squash seedlings wilted at 23.6, 29.0 and 9.0 dai when inoculated with muskmelon-,cucumber-, and squash-origin strains, respectively. Our results provide the first evidence of genetic diversity within E. tracheiphila and suggest that strain specificity is associated with plant host. This advance is a first step toward understanding the genetic and population structure of E. tracheiphila.
Development and Life Table of Acalymma vittatum(Coleoptera: Chrysomelidae), a Vector of Erwinia tracheiphila in Cucurbits (pdf)
Authors: Christa Ellers-Kirk and Shelby J. Fleischer, Department of Entomology, The Pennsylvania State University, University Park, PA
Journal: Environmental Entomology 35(4): 875-880 (2006)
Abstract: Acalymma vittatum (F.) is a cucurbit herbivore specialist and the vector of Erwinia tracheiphila (E. F. Smith) Holland, the causal agent of bacterial wilt in cucurbits. We determined the temperature-dependent development, survivorship, longevity, sex ratio, and fecundity of this vector. Egg-to adult development was modeled as y = e(0.225 x T) - e[0.225 x 36.017 - (36.017 - T)/4.425], which suggests a maximum development rate of 4.29%/d at 32°C. Linear extrapolations suggest a lower threshold of 13°C and 432 DD needed for A. vittatum development. Survivorship of immature stages, which ranged from 60% at 27°C to 4%at 33°C, was strongly inßuenced by temperature, and no beetles survived to the adult stage at 36°C. Sex ratios did not deviate from 1:1. Adults were long-lived, with continuous egg production, which ranged from 0 to 4 eggs/female/d, after an 8-d preovipositional period at 27°C. Life table statistics were generated using these data. Together, these phenology models and life table information can be used to further develop integrated pest management programs for both A. vittatum and E. tracheiphila in cucurbits.
ELISA Versus Immunolocalization to Determine the Association of Erwinia tracheiphila in Acalymma vittatum (Coleoptera: Chrysomelidae) (pdf)
Authors: Carlos Garcia-Salazar, F. E. Gildow, S. J. Fleischer, D. Cox-Foster and F. L. Lukezic
Journal: Environmental Entomology 29(3): 542-550 (2000)
Abstract: DAS-ELISA, immunohistochemistry and electron microscopy were used to investigate the association of the causal agent of bacterial wilt, E. tracheiphila, within the beetle Acalymma vittatum. After a 24-h acquisition period, a high percentage of individuals tested positive for E. tracheiphila antigen using both immunohistochemistry (100%) and DAS-ELISA (70-60%). Both assays showed that the antigen remained in beetles long after the initial acquisition, with the percentage declining during incubation. Using ELISA, the percentage decreased to 4.7% within 3 days after acquisition, then increased to 10% within 10 days and remained at 10% for 30 days. Immunoperoxidase assays of paraffin embedded gut sections were more sensitive, and showed that 95% of the beetles harboured the pathogen after 10 days and 20% after 30 days. E. tracheiphila antigen was present throughout the digestive tract soon after acquisition, but only small clusters of E. tracheiphila were observed along the alimentary canal 3 days after transfer onto clean plants (cucumber or squash). After 10 and 30 days on clean plants, E. tracheiphila antigen reaction was stronger and clusters of bacteria were more numerous, primarily in the posterior midgut and anterior portion of the hindgut. Scanning and transmission electron microscopy photomicrographs confirmed the presence of bacterial cells resembling E. tracheiphila associated with the intima of the hindgut 1 and 30 days after acquisition. This demonstrated the sensitivity of immunohistochemistry for detecting E. tracheiphila within its vector, and suggests a long-term extracellular endosymbiotic association of E. tracheiphila with the alimentary canal of A. vittatum.
Epiphytic Survival of Erwinia tracheiphila on Muskmelon (Cucumis melo L.) (pdf)
Authors: E. Saalau Rojas and M. L. Gleason, Department of Plant Pathology, Iowa State University, Ames 50010
Journal: 2012 The American Phytopathological Society; Plant Disease / Vol. 96 No. 1
Abstract: Erwinia tracheiphila, the causal agent of bacterial wilt of cucurbits, is transmitted by striped (Acalymma vittatum) and spotted (Diabrotica undecimpunctata howardi) cucumber beetles. Transmission occurs when infested frass with E. tracheiphila is deposited on plant surfaces with fresh feeding wounds. However, it is unclear whether the pathogen can survive as an epiphyte on leaves. Experiments were conducted in controlled environments to monitor E. tracheiphila survival on muskmelon (Cucumis melo) leaves under various temperature and moisture conditions. In the first experiment, muskmelon seedlings that had been spray inoculated with a rifampicin-resistant strain of E. tracheiphila were incubated at 10, 15, 20, 25, 30, or 35°C (±2°C) at ≥95% relative humidity, and E. tracheiphila populations were monitored for 72 h. In the second experiment, E. tracheiphila was monitored during alternating 12-h wet and dry periods, or continuous wet or dry conditions for 48 h at 20°C. Survival of E. tracheiphila on wet muskmelon leaves depended on temperature (P < 0.01), with the greatest survival at 10 and 15°C and least at 30 and 35°C. Leaf wetness also impacted survival; an initial 12-h dry period resulted in a 1,000- to 10,000-fold reduction in population size, followed by stabilization of the surviving population. These results demonstrate that E. tracheiphila can survive on muskmelon leaves under a wide range of environmental conditions, suggesting that epiphytic populations might serve as a reservoir of inoculum for infections.
Feasibility of Delaying Removal of Row Covers to Suppress Bacterial Wilt of Muskmelon (Cucumis melo) (pdf)
Authors: E. Saalau Rojas, M. L. Gleason, and J. C. Batzer, Department of Plant Pathology, and M. Duffy, Department of Economics, Iowa State University, Ames 50010
Journal: Plant Disease, 95:729-734 (2011)
Abstract: Bacterial wilt, caused by Erwinia tracheiphila, is a major disease of cucurbit crops in the United States. Management of the disease relies on controlling two vector species, striped (Acalymma vittatum) and spotted (Diabrotica undecimpunctata) cucumber beetles. Six field trials were conducted at Iowa State University research farms during 2007, 2008, and 2009 to assess the efficacy of delayed removal of spunbond polypropylene row covers to control bacterial wilt on muskmelon (Cucumis melo). Treatments were (i) row cover removed at anthesis (conventional timing of removal), (ii) covers removed 10 days after row cover ends were opened at anthesis, (iii) covers removed 10 days after bumble bee hives were inserted under row covers at anthesis, and (iv) a noncovered control. In two field trials during 2007 and 2008, the delayed-removal row-cover treatments significantly suppressed bacterial wilt throughout the growing season and enhanced yield compared with the noncovered and removal-at-anthesis controls. In Gilbert in 2008, however, bacterial wilt suppression was equivalent among all three row-cover treatments. No bacterial wilt was observed during three trials in 2009, and there was minimal difference in marketable yield among treatments. Net returns were compared using partial budget and sensitivity analyses. Melon prices and occurrence of bacterial wilt had a strong impact on net returns. Using row covers increased production costs by 45%. In site years in which bacterial wilt occurred, delaying removal of row covers resulted in the highest returns. When bacterial wilt was absent, however, the delayed-removal row-cover treatments had the lowest returns. Results of the sensitivity analysis indicated that delaying removal of row covers for 10 days could be a cost-effective component of an integrated bacterial wilt suppression strategy for muskmelon where bacterial wilt occurs 350% of production seasons.
Potential of Entomopathogenic Nematodes for Biological Control of Acalymma vittatum (Coleoptera: Chrysomelidae) in Cucumbers Grown in Conventional and Organic Soil Management Systems (pdf)
Authors: Ellers-Kirk CD, Fleischer SJ, Snyder RH, and Lynch JP.
Journal: Journal of Economic Entomology 93(3):605-612 (2000)
Abstract: Acalymma vittatum (F.) is the primary insect pest of fresh-market cucumber and melon crops in much of the eastern United States because of their herbivory and interactions with several diseases, most notably bacterial wilt. A study was conducted to determine how soil management affects viability and infectivity of an entomopathogenic nematode that may be used for the control of A. vittatum. Dose-mortality curves under laboratory conditions suggested several Steinernema spp. as potential biocontrol agents. Field injections combined with soil bioassays showed that Steinernema riobravis Cabanillas, Poinar & Raulston (Rhabditus: Steinernematidae) longevity exceeded A. vittatum immature development time in both conventional and organic soil management systems. Mean root length densities of cucumbers increased in both soil management systems with the inclusion of nematodes. Soil management alone also influenced A. vittatum larval survivorship, with higher survival rates in the organic compared with the conventional soil management system. A 50% reduction in A. vittatum larval survival rates in both soil management systems, as determined by adult A. vittatum emergence, demonstrated the potential of incorporation of entomopathogenic nematodes for integrated pest management of diabroticites in commercial cucumber production.
Projecting Insect Voltinism Under High and Low Greenhouse Gas Emission Conditions (pdf)
Authors: Shi Chen, Shelby J. Fleischer, Patrick C. Tobin, and Michael C. Saunders, Department of Entomology, the Pennsylvania State University, University Park, PA.
Journal: Environmental Entomology, 40(3): 505-515 (2011)
Abstract: We develop individual-based Monte Carlo methods to explore how climate change can alter insect voltinism under varying greenhouse gas emissions scenarios by using input distributions of diapause termination or spring emergence, development rate, and diapause initiation, linked to daily temperature and photoperiod. We show concurrence of these projections with a ﬁeld dataset, and then explore changes in grape berry moth, Paralobesia viteana (Clemens), voltinism that may occur with climate projections developed from the average of three climate models using two different future emissions scenarios from the International Panel of Climate Change (IPCC). Based on historical climate data from 1960 to 2008, and projected downscaled climate data until 2099 under both high (A1ﬁ) and low (B1) greenhouse gas emission scenarios, we used concepts of P. viteana biology to estimate distributions of individuals entering successive generations per year. Under the low emissions scenario, we observed an earlier emergence from diapause and a shift in mean voltinism from 2.8 to 3.1 generations per year, with a fraction of the population achieving a fourth generation. Under the high emissions scenario, up to 3.6 mean generations per year were projected by the end of this century, with a very small fraction of the population achieving a ﬁfth generation. Changes in voltinism in this and other species in response to climate change likely will cause signiﬁcant economic and ecological impacts, and the methods presented here can be readily adapted to other species for which the input distributions are reasonably approximated.
Serological Estimates of the Seasonal Dynamics of Erwinia tracheiphila in Acalymma vittata (Coleoptera: Chrysomelidae) (pdf)
Authors: Fleischer, Shelby J., de Mackiewicz, D., Gildow, F. E. and Lukezic, F. L.
Journal: Environmental Entomology 28(3): 470-476 (1999)
Abstract: Serological assays were used to estimate the proportion of Acalymma vittata (F.) that harbored Erwinia tracheiphila (E. F. Smith) Holland, the causal agent of bacterial wilt in cucurbits. These proportions were related to the proportion of A. vittata that transmitted disease in single beetle caged bioassays. The serological assays classified beetles as harboring the bacteria when the titer was above ≈105 cells per beetle. From 7.1 to 10.7% of the A. vittata captured as they emerged from soil that had been in cucurbits the previous year tested positive for the presence of E. tracheiphila. Also, from 0 to 8.3% of beetles captured on squash traps during this time of beetle emergence tested positive. This provides strong serological evidence for A. vittata serving as the primary overwintering reservoir for E. tracheiphila. During the growing season, the proportion of beetles testing positive with serological assays varied and ranged up to ≈53, 78, and 39% in 1995, 1996, and 1997, respectively. These serological proportions were 3.6-5.1 times higher than the proportion of beetles that transmitted disease in single beetle caged bioassays, and explained 44-49% of the variation in the proportion of beetles that were able to transmit disease in the caged bioassays. We suggest the proportion of A. vittata that harbored at least some E. tracheiphila cells may be >5 times higher than the proportion of beetles that can, alone, transmit disease in a short time. We discuss these data as supporting the hypothesis of beetle aggregation behavior as an important component of bacterial wilt epidemiology.
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