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Funded Project |
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Funding Program:
Regional IPM Competitive Grants - Northeastern |
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Project Title:
Identification of Factors Associated with Onion Bacterial Diseases to Facilitate the Development of an IPM Program |
Project Directors (PDs):
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Lead State: PA Lead Organization: The Pennsylvania State University |
| Research Funding: $179,788 |
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Start Date: Aug-15-2011 End Date: Aug-15-2013 |
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No-Cost Extension Date: Aug-14-2014 |
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Pests Involved: Bacterial diseases |
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Site/Commodity: Onions |
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Summary:
During the past 5 to 10 years, bacterial bulb decay has assumed much greater importance in the Northeast region. In PA and NY, annual losses range from 5 to 40%. However, disease losses are variable, both among and within fields; in many cases, the full extent of disease losses is not evident until harvest or thereafter. This proposal seeks to determine the factors that are correlated with more extensive losses from bacterial diseases. We hypothesize that the presence of inoculum of several important bacterial pathogens in soil, transplants and possibly weeds and thrips is strongly correlated to disease incidence and severity. In addition, we hypothesize that high levels of nitrogen fertilization, thrips-induced injury and wind-induced injury contribute to the incidence and severity of bacterial diseases. Our approach is to conduct intensive small plot surveys of factors related to the presence of inoculum in commercial fields and to determine the extent of bacterial disease development in those fields during the growing season, at harvest and following harvest. In addition, replicated on-farm research trials will be carried out to assess the relationship between nitrogen levels, thrips-induced injury and wind-induced injury and resulting bacterial decay in onions. Currently, small-scale diversified fresh market onion growers as well as large-scale onion producers lack effective tools with which to manage bacterial diseases in onions. The goal of this proposal are to identify important relationships and utilize such knowledge to develop IPM-based recommendations to reduce the likelihood of severe losses from bacteria pathogenic to onions.
Objectives: Currently, small-scale diversified fresh market onion growers as well as large-scale onion producers do not have adequate means of managing bacterial diseases in their onion crops. The overall goal of this project is to gain a better understanding of the factors contributing to bacterial diseases in commercial production fields, and thus facilitate more targeted management practices as part of an integrated pest management program. 1. Identify factors associated with bacterial disease incidence in commercial onion fields in order to facilitate the development of an integrated pest management program for onion bacterial diseases in the Northeast region. We hypothesize that infested transplants, weeds, soil, and thrips are sources of bacterial inoculum in commercial fields. Furthermore, we hypothesize that injury from thrips, wind whipping and high levels of soil nitrogen contribute to bacterial disease incidence and severity. These factors were selected for evaluation based on preliminary evidence of their association with the incidence and severity of onion bacterial diseases in PA and/or NY. Using intensive small plot surveys of commercial fields as well as several replicated on-farm component research trials we will determine the relationship of these factors (independent variables) to onion bacterial disease incidence and severity (dependent variable) and test our hypotheses. This information will be used to augment current management practices as well as further facilitate the development of targeted research-based integrated pest management plans. Proposal |
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Interim Report: Oct-12-2012 |
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Impacts Over the course of two field seasons, an extensive data set on potential factors associated with onion bacterial diseases has been collected from over 58 and 55 fields in Pennsylvania and New York, respectively. More specifically in NY, 22 and 33 fields were sampled in 2011 and 2012 respectively. These represented 7 and 8 growers and 6 and 7 different onion cultivars in 2011 and 2012, all grown on muck soils. In PA, 28 and 30 fields were sampled, one field per grower. All the fields represented onion cv. Candy grown on plastic mulch with drip irrigation on mineral soils. A comprehensive analysis of these data sets promises to elucidate the factors most closely associated with onion bacterial diseases. Identification of these factors will facilitate the development, augmentation and/or implementation of targeted management practices that reduce losses due to bacterial diseases without compromising yield. In PA in 2011, in-field losses from bacterial diseases ranged from 0 to 50.4% with additional storage losses ranging between 2.1 and 41.6%. In NY, losses were considerably less ranging between 0 and 17.3%. Data collection is not complete, but the incidence of bacterial bulb decay at harvest is even lower in 2012. Preliminary analysis of 2011 data in PA correlated warmer mid-season soil temperatures and visual disease ratings to increased in-field losses due to bacterial disease at harvest. More losses also occurred when onions were grown on black as opposed to white plastic; however, this practice is not recommended due to increased pressure from thrips. The bacterial pathogens commonly associated with symptomatic onions at harvest in PA were also associated with onion transplants collected at planting. In New York, preliminary analysis of 2011 data indicated a relationship between soil levels of available nitrogen-nitrogen collected in mid-season (July 7-9 leaf stage) and incidence of bacterial bulb decay at harvest; as soil nitrogen increases bacterial bulb decay increases. More than 78 samples of muck-land soils were collected from fields planted to onions near the time of planting. These samples were analyzed for the presence of bacterial pathogens found earlier to be associated with onions culled by grower-shippers during grading following storage. Samples of seeds and seedling transplants also were analyzed for the presence of bacterial pathogens found earlier to be associated with harvested and stored onions. No bacterial pathogens of consequence were isolated from 12 batches of seed collected from growers. From seedling transplants, many strains of Pantoea agglomerans were isolated. When tested for pathogenicity in the laboratory or greenhouse, all except for two failed to cause symptoms of pathogenicity. Two strains from one batch of transplants caused symptoms of center rot. From the over 78 samples of muck-land soil analyzed, strains of Burkholderia cepacia (causes sour skin) and Enterobacteria cloacae (causes Enterobacter bulb decay) were isolated frequently, while strains of Pantoea ananatis were isolated somewhat less frequently. These survey results suggest that several of the important bulb pathogens of onion overwinter in muck-land soil in New York. Results of this research thus far have been disseminated in PA and NY via presentations at several vegetable grower meetings/conferences including the Oswego Onion Twilight Meeting, Empire State Fruit and Vegetable Expo, Northeast Vegetable Growers Meeting and New Holland Vegetable Day attended by over 250 growers and interested stakeholders. In addition, an article was published in the popular trade magazine Onion World . At the end of the 2011 season, collaborating growers in PA received individual reports sharing their farm specific data and how it ranked compared to other farms participating in this project further increasing grower knowledge and awareness about onion bacterial diseases. |
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Outcomes Intensive survey plots were established in 28 and 30 onion fields in Pennsylvania and in 22 and 33 onion fields in New York in 2011 and 2012, respectively. In each field, three plots approximately 1 bed (4 to 6 rows/bed) by 30ft long were established. Soil temperature at the 2-in. depth was collected during the growing season in two plots per farm. Bacterial species associated with onion transplants, weeds, and soils were isolated and identified using a combination of culturing and molecular techniques. Mid-season visual bacterial disease and thrips damage assessments were made. Soil inorganic and potentially mineralizable nitrogen levels were assessed in each plot at planting and harvest. At onion bulbing, foliar tissue was harvested for assessment of total nitrogen. At harvest, yield assessments were made on the center 15 ft of bed. Onions were visually assessed for bacterial disease and graded (>4.0 in. colossal, 3.0-4.0 in. jumbo, 2.25-3.0 in. medium, <2.25 in. small). At total of 30 healthy (asymptomatic) onions were dried, placed in storage and reassessed for bacterial disease after four months. Bacterial pathogens were isolated and identified from a sub-set of symptomatic onion bulbs collected at harvest. Twelve seed lots in NY were also sampled for potential bacterial pathogens. In addition in NY, two on-farm small-plot field trials were conducted in 2011 and 2012 in muck onion fields to evaluate the effect of soil nitrogen on bacterial bulb decay. In 2011, 75 lb per acre and 125 lb per acre rates of applied nitrogen were compared and in 2012, 50, 100 and 150 lb per acre of nitrogen were compared. Publications Beer, S. V., Asselin, J.-A. E., Bonasera, Zaid, A. M., J. M. and Hoepting, C. A. 2012. Better understanding bacterial onion diseases in New York. Onion World 28: (4) 18-22. Beer, S. V., Asselin, J.-A. E., Bonasera, Zaid, A. M., J. M. and Hoepting, C. A. 2012. Research yields greater understanding of bacterial diseases of onion in New York. 2012 Empire State Fruit & Vegetable Expo Proceedings. http://www.hort.cornell.edu/expo/2012proceedings.php. Carr, E. A., Zaid, A. M., Bonasera, J. M., Lorbeer, J. W., and Beer, S. V. 2012. Progression of the center rot pathogen Pantoea ananatis from onion leaves to bulb infection and enhanced pathogenicity assays. Plant Disease: (In revision). Hoepting, C.A. 2012. Exploring the relationship between nitrogen and bacterial diseases of onions. In: Proceedings of the 2012 Empire State Fruit and Vegetable Expo, Cornell Cooperative Extension and New York State Vegetable Growers Association (online): http://www.hort.cornell.edu/expo/proceedings/ 2012/Onions/Onion%20Nitrogen%20and%20Bacterial%20Hoepting.pdf. Syracuse, NY, USA. January 24-26, 2012. Hoepting, C.A. and S.V. Beer. 2012 Exploring the relationship between nitrogen, plant spacing and bacterial diseases of onion in New York: Reduced nitrogen and closer spacing could result in less rot. Onion World 28(7): in press. Pfeufer, E.E., M.A. Mansfield, J. Stoltzfus and B.K. Gugino. 2012. Identification of factors associated with bacterial diseases of onion: A case study of two Pennsylvania farms. Phytopathology 102: S1.7 Zaid, A.M., Bonasera, J. M. and Beer, S. V. 2012. OEMA new medium for rapid isolation of onion-pathogenic and onion-associated bacteria. Journal of Microbiological Methods (2012) http://dx.doi.org/10.1016/j.mimet.2012.09.031. Zaid, A.M., Bonasera, J. M. and Beer, S. V. 2011. First report of Enterobacter bulb decay of onions caused by Enterobacter cloacae in New York. Plant Disease 95: 1581. |
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Interim Report: Oct-23-2013 |
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Impacts Over the course of two field seasons, an extensive data set on potential environmental and production factors associated with onion bacterial diseases has been collected from over 58 and 55 fields in Pennsylvania and New York, respectively. More specifically in NY, 22 and 33 fields were sampled in 2011 and 2012. These represented 7 and 8 growers and 6 and 7 different onion cultivars in 2011 and 2012, all grown on muck soils. In PA, 28 and 30 fields were sampled, one field per grower. All the fields represented onion cv. 'Candy' grown on plastic mulch with drip irrigation on mineral soils. A comprehensive analysis of these data sets is beginning to elucidate the environmental and crop management factors most closely associated with onion bacterial diseases. Identification of these factors will facilitate the development, augmentation and/or implementation of targeted management practices that will reduce losses due to bacterial diseases which range between 5 and 40% annually in PA and NY without compromising yield. Presentations at local, regional and national meetings as well as one-on-one interactions with growers through on-farm research have increased grower awareness and knowledge about onion bacterial diseases and the results of this research are starting to be implemented on the farm through adjustments in the use of plastic mulch to reduce soil temperatures and closer monitoring of soil and crop fertility. |
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Outcomes A total of 235 bacterial isolates were recovered from 192 samples of the four most common weed species (crabgrass, redroot pigweed, purslane and lambsquarters) collected from commercial onion fields in 2011. Nearly 60% of the culturable bacteria isolated were potential pathogens of onion. Pseudomonas marginalis was commonly cultured from all types of weeds, while Pantoea agglomerans was commonly associated with crabgrass and purslane. Pectobacterium carotovorum was common only to redroot pigweed. Based on pathogenicity tests, all the bacterial isolates from lambsquarters were able to cause onion bulb rots while other weeds were found to have mixed populations of pathogenic and nonpathogenic bacteria on their surfaces. By sampling common weeds are onion fields in 2011, it was shown that weeds are a potential source of bacterial inoculum that causes diseases in onion bulbs. A rep-PCR method was developed to facilitate our ability to track bacterial strains within the same bacterial species. This will enable us to further link bacterial isolates collected from the surface of transplants, weeds, and soil from the on-farm survey plots to those isolated from symptomatic onion bulb tissue at harvest or after storage and further facilitate the development of management strategies to reduce these potential sources of bacterial inoculum. At-planting and at-harvest soil nitrogen, leaf and bulb tissue nitrogen, soil temperature and other factors were analyzed in a multivariate linear regression model. A strong negative relationship between leaf tissue nitrogen at midseason and total harvest losses was suggested, while a positive relationship was also identified between pre-harvest soil temperatures and bacterial disease incidence. These results relate the importance of reducing soil temperatures through use of alternative plastic mulches, ensuring adequate soil fertility early in the season and taking measures to reduce the impact of inoculum sources in the production system. A follow-up replicated field trial was conducted in 2013 to evaluate nitrogen source (calcium nitrate vs. ammonium sulfate) in combination with fertilizer application timing (pre-solstice or full-season). No differences in disease incidence or marketable yield were suggested from plots fertilized with different nitrogen forms however, onions fertilized with ammonium sulfate had significantly higher sulfur content, a key component in onion pungency (P < 0.0001) and not desirable for sweet Spanish-type onions. Collectively, plots inoculated with Pantoea spp. (the causal agents of center rot) had significantly higher disease incidence than uninoculated plots (P = 0.009), and center rot incidence and marketable yield trended based on interactive effects between inoculation status and fertilization timing (P < 0.16). With funding leveraged from the Pennsylvania Vegetable Growers Association, two additional field trials were conducted in 2013 to evaluate the effects of inoculum pressure and onion maturity at harvest on harvest and post-harvest losses due to bacterial diseases caused by several pathogens. The results of this research will provide growers with additional information on the relationship between the timing of harvest and bacterial diseases losses so they can make more informed decisions about when to harvest fields under pressure from bacterial diseases. The data from these two trials is still being analyzed and the first post-harvest storage assessments will begin in October 2013 during the next reporting cycle. With funding leveraged from the New York State Integrated Pest Management Program, a field trial was conducted to evaluate the effect on bacterial bulb decay and yield of plant and row spacing, and bulb area in large-scale production of direct seeded and transplanted onions. An on-farm small-plot research trial was conducted using transplanted onions (c.v. Candy) and included 13 planting configurations. Plant size and maturity trends were generally opposite of what was found previously in studies with small-scale onion production: in this study, plant height, number of leaves per plant and maturity generally increased as plant population increased. Despite this, yield and bulb size distribution were similar to previous results. There was a general trend that total yield, boiler, small and medium sized bulb weight, and economic return increased as plant population increased. Within each row spacing (15", 10" and 7.5"), these parameters increased as the plant spacing decreased from 8" to 6" to 4". Although less dramatic, within each plant spacing (4", 6" and 8"), these parameters increased as row spacing decreased from 15" to 10" to 7.5". Generally, yield increased as plant population increased because there were more bulbs per area to contribute to yield. Our data suggests that plant spacing is a stronger predictor of yield and bulb size than row spacing or bulb area. Bacterial bulb decay 9 weeks after the onions were pulled from the field ranged from 0.8 to 3.6% incidence and no significant differences occurred among planting configurations.There were no significant correlations between incidence of bacterial bulb decay and plant size, maturity, yield and bulb size distribution. There was no relationship between bacterial bulb decay and plant population. Repeating this study is certainly warranted in order to generate robust data from which to make recommendations for planting configurations to manage bacterial diseases in large-scale onion production. Publications: Hoepting, C.A. and S.V. Beer. 2012. Exploring the relationship between nitrogen plant spacing and bacterial diseases of onion in NY: Reduced nitrogen and closer spacing could result in less rot. Onion World, Columbia Publishing, 28(7): 18-21. Hoepting, C.A. 2012. An IPM approach to managing bacterial diseases of onions. In: Proceedings of the 2012 Great Lakes Fruit, Vegetable and Farm Market EXPO (online): http://www.glexpo.com/summaries/2012 summaries/onion.pdf. Grand Rapids, MI, USA. December 4-6, 2012. Pfeufer, E.E., M.A. Mansfield, and B.K. Gugino. 2013. Environmental and management factors associated with bacterial rots of onion in Pennsylvania. Phytopathology 103:S2.173. Pfeufer, E.E. and B.K. Gugino. 2013. Environmental factors and production practices associated with bacterial diseases of onion. 2013 Mid-Atlantic Fruit and Vegetable Convention Proceedings, PA Vegetable Growers Association, Richfield, PA. Pp. 171-174. Presentations: Pfeufer, E.E. 2013. Environmental factors and production practices associated with bacterial diseases of onion. 2013 Mid-Atlantic Fruit and Vegetable Convention, Hershey, PA: January 30, 2013 (70 attendees). Pfeufer, E.E. 2013. Environmental factors and production practices associated with bacterial diseases of onion. Lancaster County Onion Growers Meeting. Leola, PA: March 20, 2013 (80 attendees). Hoepting, C.A. 2013. An IPM approach to managing bacterial diseases of onion. 62nd Annual Muck Crops Research Conference. Bradford, Ontario, Canada: April 4, 2013 (75 attendees). Hoepting, C.A. 2013. An IPM approach to managing bacterial diseases of onion. University of Wisconsin Extension and WPVGA Grower Conference. Stevens Point, WI, USA: February 6, 2013 (12 attendees). Hoepting, C.A. 2012. An IPM approach to managing bacterial diseases of onion. Great Lakes Fruit, Vegetable and Farm Market EXPO. Grand Rapids, MI, USA: December 5, 2012 (130 attendees). Understanding the environmental and on-farm management factors that are contributing to onion yield losses associated with bacterial disease incidence has been the focus of Emily E. Pfeufer's Ph.D. research in the Department of Plant Pathology and Environmental Microbiology at Penn State. It is anticipated that she will graduate in Spring/Summer of 2014. A research poster presentation given by Emily Pfeufer titled "Common weeds as potential sources of bacterial pathogen inoculum causing onion bulb rots in Pennsylvania" resulting from this research project was awarded 1st place in the Biological Sciences Division at the 2013 Penn State Gamma Sigma Delta Research Competition and 2nd place in the 2013 Penn State Graduate Exhibition Research Competition. She was also selected through a competitive process to present in the I.E. Melhus Graduate Student Symposium titled "What's in our toolbox to minimize the risk of plant disease?" at the 2013 Annual American Phytopathological Society Meeting in Austin, TX. This is one of the most prestigious awards that graduate students can receive from our national society. |
Report Appendices
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Final Report: |
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Impacts Exclusion of bacterial inoculum from commercial onion fields is one of the most fundamental bacterial disease management strategies. In PA, pathogenic bacteria were associated with both onion transplants and several common weed species while in NY, the primary onion bacterial pathogens were associated with the soil. Further studies are already being conducted in both PA and NY to identify strategies to reduce these potential sources of bacterial inoculum. In PA, in addition to general weed management, pre-plant treatments to manage transplants bacterial populations are being evaluated. In NY, soil amendments are being evaluated for their ability to reduce bacterial populations in the soil. Over the course of two field seasons, an extensive data set on over 40 environmental and production factors associated with onion bacterial diseases has been collected from over 58 and 55 fields in PA and NY, respectively. Statistical analyses have indicated significant associations between early season foliar plant nutrition (specifically nitrogen and carbon) as well as soil nitrate and application timing and bacterial disease incidence at harvest and during post-harvest storage. Replicated trials to evaluate the application timing and reduced nitrogen rates on marketable yield and bacterial disease incidence are being planned for 2015 and 2016. In addition, growers have already begun to implement practices to reduce soil temperatures at bulbing, another factor closely associated with increased bacterial disease losses. Some growers are using biodegradable mulches that enable the onion bulbs to emerge through the plastic and remain on the surface, while others are mechanically slashing the plastic at bulbing to encourage increased air circulation and reduce soil temperatures underneath the plastic mulch. In NY, some small-scale growers are using reflective silver mulch for the same reason. Over 15 publications ranging from refereed and trade journal articles to proceedings and meeting abstracts have been authored and published during the course of this project. Multiple articles have been published in the trade journal Onion World which has a national stakeholder audience. Several refereed journal articles were published by PD Beer et al. while support from this project provided additional incentive. In addition, several refereed journal publications are in preparation based on the results detailed in Emily E. Pfeufer's Ph.D. dissertation. Over 16 presentations have been given at local, regional, national and international meetings reaching a wide audience of close to 1000 stakeholders. Additional presentations are planned for the upcoming joint National Onion Association and National Allium Research Conference in Scottsdale, AZ in December 2014. An Onion Bacterial Disease Symposium is planned for the Empire Producers' Expo in Syracuse, NY in January 2015 where Drs. Emily Pfeufer, Steven Beer and Ron Gitaitis (University of Georgia) will be speakers. The extension responsibilities of the PIs and the additional funding leveraged will continue to facilitate the dissemination of project results and achieving the intermediate and long-term project outcomes described in the project logic model (Appendix 1 Complete Report). In addition, since the inception of this project, the scope of the Multi-State Western Regional Project W1008 (now W2008) has been expanded to include understanding the biology, epidemiology and management of bacterial diseases of onion. This has enabled a nationwide group of plant pathologists to work collaboratively to address this national problem. PIs Gugino, Beer and Hoepting are all active project members. |
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Outcomes In PA, pathogenicity tests were conducted with select isolates from each potential bacterial inoculum source including soil, transplants and weeds. Pathogenic isolates of Pectobacterium carotovorum subsp. carotovorum (Pcar), Pantoea agglomerans (Pagg), and Pseudomonas marginalis pv. marginalis (Pmar), were the most common bacterial pathogens in the PA onion cropping system and occurred in soil, transplant and weed tissues, in addition to being present as epiphytes on the surfaces of both onion transplants and common weeds. Pathogenicity tests of Pagg, Pcar, and Pmar isolates from the surfaces and tissue of weeds demonstrated a range in virulence exists among the weed-derived isolates of these bacterial species, ranging from non-pathogenic to highly aggressive. In addition, rep-PCR facilitated strain tracking of Pantoea ananatis isolates indicated matching strains isolates from surface-disinfested weed tissue collected at mid-season and a latently infected onion that had been in storage for four months. This suggests that weeds may play a role in onion bacterial diseases by supporting a wide diversity of strains of these bacteria. Pathogenicity testing also suggested that a higher proportion of pathogenic isolates were isolated as epiphytes from the surface of the common weed lambsquarters (Chenopodium album) than from redroot pigweed (Amaranthus retroflexus) and purslane (Portulaca olearacea), some of the most common weed species in Pennsylvania onion fields. In NY, Burkholderia cepacia was most commonly identified from symptomatic bulbs at harvest with 74% of samples testing positive, followed by Enterobacter cloacae (21%) and Pantoea ananatis (6%). Transplants and weeds were not surveyed for bacteria pathogenic to bacteria in NY. However, B. cepacia was identified in 83% of the soil samples collected in the NY survey. A novel technique was developed that has proven quite robust in identifying eight species of bacteria associated with onions (Bonasera et al. 2014). That technique has proven useful in identifying bacteria in New York during the later stages of this project. Several lines of evidence support the role of transplants as a source of inoculum inducing rot in PA onions. First, a relatively high frequency (22%) of detections of B. gladioli pv. alliicola in symptomatic onions at harvest coincided with a high frequency (64%) of B. gladioli pv. alliicola detected as endophytes from surface-sterilized transplants in 2012. Results from pathogenicity tests on Pcar, Pagg, and Pmar isolates from transplants indicated that over 52% of isolates, regardless of species, isolation location, or pathogenicity test, induced discoloration or maceration on onion. Combined, these results suggest research avenues for more directed disease management strategies for reducing the impact of environmental inoculum sources on bacterial rots of onion. Odds ratios were used to calculate the likelihood that select bacterial pathogens would be associated with symptomatic bulbs under particular management conditions. For example, the odds of detecting Pagg were increased in production systems using standard black plastic mulch to cover the onion bed compared to other plastic mulches. Also there were negative relationships between plant tissue carbon content and the detection of Pcar, and a negative overall influence of early-season levels of soil nitrate on some bacterial diseases that initiate in foliage. Continued mining of the extensive dataset collected may further elucidate these types of relationships between the bacterial pathogens and their environment that can be used in the future to help refine integrated management strategies. To determine the environmental and management factors associated with high bacterial disease incidence, independent variables were placed in a series of multiple linear regression models. Foliar nitrogen (N) and foliar carbon (C) were negatively related to total incidence of bacterial rots of onion from the PA datasets, while pre-/early season levels of soil nitrate (NO3) were positively related to total incidence of bacterial rots from PA and NY. Foliar nutrient levels implicate early-season fertility in managing bacterial rots of onion, while associations between soil NO3 and rot suggest ammonium (NH4) or organic N may be more effective N sources in managing bacterial rots. In addition, soil temperatures near the physiological onset of onion bulbing were positively related to total incidence of bacterial rot in PA in 2011 and in the combined NY dataset, which agree with previously published results. These results suggest greater complexity is necessary for N fertility recommendations: not only should growers be provided with a recommended field rate, but also the timing and type of N applied may play roles in bacterial disease development. In addition, if possible, growers should also take steps to lower soil temperatures, particularly near bulbing. The results of this research have been used to leverage additional funding to conduct replicated research trials designed to develop more targeted bacterial disease management tools and recommendations. Future research related to the source and application timing of nitrogen during the production season, evaluation of cultivar susceptibility, and pre-plant treatments to reduce bacterial populations associated with onion transplants will be conducted. In NY, further research efforts will focus on understanding when onion plants become infected with bacterial pathogens and reducing the inoculum in the soil. The ultimate goal of this project is to develop knowledge-based, sustainable and cost-effective management strategies for bacterial diseases of onion in the Northeast and Mid-Atlantic regions. Through funding from the NE-IPM Competitive Grants Program we have been able to greatly advance our understanding of the factors associated with onion bacterial diseases and growers are beginning to already augment their production practices based on the results and outreach efforts of this project. In addition, a collection of over 3500 bacterial isolates isolated from symptomatic onions and environmental samples in PA has been created and placed in long-term storage. This collection will serve as an important resource for future studies designed to elucidate the biology and epidemiology of the different onion bacterial pathogen populations. In PA, multiple bacterial species are frequently isolated from individual symptomatic bulbs and these isolates display a range of pathogenicity. Currently it is not well understood how these interactions may be contributing to disease losses or how they may be used to better manage disease. With funding received from the Penn State Department of Plant Pathology and Environmental Microbiology Jordan Endowment, a follow-up replicated field trial was conducted in 2013 to evaluate nitrogen source (calcium nitrate vs. ammonium sulfate) in combination with fertilizer application timing (pre-solstice or full-season). No differences in disease incidence or marketable yield were suggested from plots fertilized with different nitrogen forms, however, onions fertilized with ammonium sulfate had significantly higher sulfur content, a key component in onion pungency (P < 0.0001) and not desirable for sweet Spanish-type onions. Collectively, plots inoculated with Pantoea spp. (a causal agent of center rot) had significantly higher disease incidence than uninoculated plots (P = 0.009), and center rot incidence and marketable yield trended based on interactive effects between inoculation status and fertilization timing (P < 0.16). In other words, optimum timing of fertilizer application depended on the inoculum pressure in the field. In addition, to improve harvest timing recommendations, replicated trials were conducted in 2013 and 2014 in which uninoculated, adjacent-to-inoculated, and inoculated onion plants were rated for foliar disease severity and lodging, then harvested and evaluated for disease incidence and marketable yield; this occurred weekly for five weeks. Close correlations between foliar disease severity and bacterial disease incidence at harvest suggest that the proposed foliar disease severity scale (0 = healthy to 7 = ≥50% of leaves bleached and collapsed; Figure 2 - Appendix 1 Complete Report) can be used to effectively estimate disease incidence immediately prior to harvest. While the trials differed between years, average foliar disease severity at or nearing a rating of 3 (1/2 of leaf chlorotic or bleached) may be suggested as a threshold for growers to use while scouting their crops as they near harvest. Future work will encompass educating growers about scouting their fields using the foliar disease severity scale and refining the location and number of plants necessary to adequately estimate the severity of the entire field. With funding leveraged from the New York State Integrated Pest Management Program, a field trial was conducted to evaluate the effect on bacterial bulb decay and yield of plant and row spacing, and bulb area in large-scale production of direct seeded and transplanted onions. An on-farm small-plot research trial was conducted using transplanted onions (cv. Candy) and included 13 planting configurations. Plant size and maturity trends were generally opposite of what was found previously in studies with small-scale onion production. In this study, plant height, number of leaves per plant and maturity generally increased as plant population increased. Despite this, yield and bulb size distribution were similar to previous results. There was a general trend that total yield, boiler, small and medium sized bulb weight, and economic return increased as plant population increased. Within each row spacing (15-in., 10-in. and 7.5-in.), these parameters increased as the plant spacing decreased from 8-in. to 6-in. to 4-in. Although less dramatic, within each plant spacing (4-in., 6-in. and 8-in.), these parameters increased as row spacing decreased from 15-in. to 10-in. to 7.5-in. Generally, yield increased as plant population increased because there were more bulbs per area to contribute to yield. Our data suggests that plant spacing is a stronger predictor of yield and bulb size than row spacing or bulb area. Bacterial bulb decay 9 weeks after the onions were pulled from the field ranged from 0.8 to 3.6% incidence and no significant differences occurred among planting configurations. There were no significant correlations between incidence of bacterial bulb decay and plant size, maturity, yield and bulb size distribution. There was no relationship between bacterial bulb decay and plant population. Repeating this study is certainly warranted in order to generate robust data from which to make recommendations for planting configurations to manage bacterial diseases in large-scale onion production. PUBLICATIONS (REFEREED ARTICLES, PROCEEDINGS, TRADE JOURNALS, AND ABSTRACTS):
PRESENTATIONS:
Understanding potential sources of inoculum and the environmental and on-farm management factors that are contributing to onion yield losses associated with bacterial disease incidence has been the focus of Emily E. Pfeufer's Ph.D. research in the Department of Plant Pathology and Environmental Microbiology at Penn State. She successfully defended her dissertation research and graduated in August 2014. A research poster presentation given by Emily Pfeufer titled "Common weeds as potential sources of bacterial pathogen inoculum causing onion bulb rots in Pennsylvania" resulting from this research project was awarded 1st place in the Biological Sciences Division at the 2013 Penn State Gamma Sigma Delta Research Competition and 2nd place in the 2013 Penn State Graduate Exhibition Research Competition. She was also selected through a competitive process to present in the I.E. Melhus Graduate Student Symposium titled "What's in our toolbox to minimize the risk of plant disease?" at the 2013 Annual American Phytopathological Society Meeting in Austin, TX. This is one of the most prestigious awards that graduate students can receive from our national society. |
Report Appendices
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