Source: PENNSYLVANIA STATE UNIVERSITY submitted to
BIOLOGY AND MANAGEMENT OF IRIS YELLOW SPOT VIRUS (IYSV), OTHER DISEASES, AND THRIPS IN ONIONS (FROM W1008)
Sponsoring Institution
National Institute of Food and Agriculture
Project Status
TERMINATED
Funding Source
HATCH
Reporting Frequency
Annual
Accession No.
0231434
Grant No.
(N/A)
Project No.
PEN04508
Proposal No.
(N/A)
Multistate No.
W-2008
Program Code
(N/A)
Project Start Date
Oct 1, 2012
Project End Date
Sep 30, 2017
Grant Year
(N/A)
Project Director
Gugino, BE.
Recipient Organization
PENNSYLVANIA STATE UNIVERSITY
208 MUELLER LABORATORY
UNIVERSITY PARK,PA 16802
Performing Department
Plant Pathology & Environmental Microbiology
Non Technical Summary
Onion is an economically important crop in the U.S., generating over 900 million dollars annually in farm receipts from 2005 to 2010. U.S. onion production area ranges from 60,000 to 70,000 hectares annually, with over 80% of the summer production in the western states (NASS, 2010). Worldwide, 53 million metric tons of onion bulbs are harvested annually from nearly 3 million hectares. A significant portion of the U.S. and world supply of onion seed is produced in the western U.S., primarily in the Pacific Northwest (PNW). Onion thrips is the most serious pest of onion worldwide and it has become an even greater threat to onion as a vector of IYSV (Gent et al. 2004). In addition to feeding injury caused by thrips, Iris yellow spot virus (IYSV), which is vectored by onion thrips, has emerged as a devastating new disease of onion. In addition, more than 20 different bacterial and fungal pathogens cause onion losses under field and storage conditions. Each of these diseases can cause up to 50% crop loss, and stakeholders have requested IPM assistance from projects including the W1008. A total loss can be incurred if affected commercial onion loads are refused by buyers and have to be dumped (Schwartz and Mohan, 2008). The W2008 project is designed to develop and improve IPM approaches to managing pests that threaten the sustainability of the U.S. onion industry. In PA, efforts are being directed towards identifying potential sources of the pathogens (transplants, weeds, soil, etc.) for the bacteria most commonly associated with yield losses of onion. Management of bacterial diseases will ultimately require an IPM approach. In NY and PA, extensive surveys of commercial onion fields are also being used to gain a better understanding of the environmental and production conditions (e.g. nitrogen fertility) that favor bacterial disease development, which will facilitate the implementation of a more targeted IPM program. Techniques will be developed to consistently initiate bacterial infection in the laboratory, controlled environment chambers and field to facilitate the evaluation of potential control materials and strategies. We envision that such studies also would increase our understanding of the epidemiology of bacterial pathogens of onion for which only scant information is available. Research on the use of cultural practices to manage bacterial diseases including narrow plant spacing, alternative mulch types and reduced nitrogen fertility will continue. Biological and chemical materials will be investigated that have potential to control bacterial decay through reducing populations of pathogens or enhancing the resistance of onions. Schedules for their application will be developed and their use incorporated into an IPM program. Adoption of these practices to manage onion thrips, IYSV and bacterial pathogens by onion producers will increase the profitability and sustainability of the U.S. onion industry.
Animal Health Component
(N/A)
Research Effort Categories
Basic
(N/A)
Applied
(N/A)
Developmental
(N/A)
Classification

Knowledge Area (KA)Subject of Investigation (SOI)Field of Science (FOS)Percent
2121451116020%
2121451110030%
2161451116050%
Knowledge Area
212 - Pathogens and Nematodes Affecting Plants; 216 - Integrated Pest Management Systems;

Subject Of Investigation
1451 - Onion, garlic, leek, shallot;

Field Of Science
1160 - Pathology; 1100 - Bacteriology;
Goals / Objectives
The overall goal of the W2008 project is to develop and improve integrated pest management approaches to managing pests that threaten the sustainability of the U.S. onion industry. The multidisciplinary nature of W2008 will ensure that project personnel address potential interactions between strategies that are effective against one pathogen or pest that could negatively affect management of other pests of onion. The objectives being addressed by Penn State research include 3)investigation of the the biology, ecology and epidemiology of other pathogens to improve management strategies, and 4)facilitate interaction and information transfer between W2008 participants, the onion industry and other stakeholders. Outputs include the identification of new onion pathogens and determination of their importance and development of appropriate management strategies. In addition cultural management tactics with an emphasis on reduce nitrogen fertility will be developed for managing thrips, IYSV and bacterial diseases of onion and will contribute to the further development of a comprehensive biologically based and sustainable IPM program. The following project milestones were established by year: 2012) Hold the first W2008 meeting in conjunction with the NARC in New Mexico in December; 2013) conduct research to address the proposed objectives and hold W2008 meeting in conjunction with NOA meeting in December; 2014) continue conducting research to address proposed objectives and hold joint meeting between W2008, NARC and NOA; and 2017) Adoption of a comprehensive set of IPM practices by growers to address pests and pathogens.
Project Methods
Obj 3. Investigate the biology, ecology and epidemiology of other pathogens to improve management strategies. In PA efforts are being focused on bacterial diseases of onion. More specifically,efforts are being directed towards identifying potential sources of inoculum (transplants, weeds, soil, etc.) for the bacterial species most commonly associated with yield losses of onion. Similar to NY, these include P. agglomerans, P. ananatis, as well as P.carotovorum, P. marginalis and to a lesser extent, E. cloacae. In 2011, increased losses due to B. cepacia and B. gladioli pv. allicola were observed as well as post-harvest losses due to Botrytis alli. The development of a multiplex PCR (Mansfield and Gugino, 2010) for detection of the eight most common bacterial species associated with onion in PA facilitated the efficient processing of numerous samples during the season. Management of bacterial diseases will ultimately require an IPM approach. In NY and PA, extensive surveys of commercial onion fields are also being used to gain a better understanding of the environmental and production conditions (e.g. nitrogen fertility) that favor bacterial disease development, which will facilitate the implementation of a more targeted IPM program. Techniques will be developed to consistently initiate bacterial infection in the laboratory, controlled environment chambers and field to facilitate the evaluation of potential control materials and strategies. We envision that such studies also would increase our understanding of the epidemiology of bacterial pathogens of onion for which only scant information is available. Research on the use of cultural practices to manage bacterial diseases including narrow plant spacing, alternative mulch types and reduced nitrogen fertility will continue. Biological and chemical materials will be investigated that have potential to control bacterial decay through reducing populations of pathogens or enhancing the resistance of onions. Schedules for their application will be developed and their use incorporated into an IPM program. Obj 4. Facilitate interaction and information transfer between participants, the onion industry and other stakeholders. The proceedings from the W2008 meetings will be documented and distributed to relevant stakeholders. A concerted effort will be made to inform the onion community about www.alliumnet.com and upcoming W2008 meetings. A more integrated point of exchange between researchers, growers and other interested parties will be developed and expanded using the alliumnet web site while encouraging a more open dialogue of W2008 findings, questions and comments. We will support and initiate strategic plans that will identify priorities of the onion industry. Joint meetings of the W2008, NARC and NOA will maximize interactions among the W2008 researchers and NARC groups, onion growers and industry representatives. W2008 members will continue to share results and learn from colleagues involved with various research and extension projects funded in recent years by the SCRI related to thrips, IYSV, other pathogens and onion production and storage issues of relevance to the national onion industry.

Progress 10/01/12 to 09/30/17

Outputs
Target Audience:Current and future onion growers in Pennsylvania and in other onion growing regions benefited from the research conducted as part of this project. Smaller scale growers sell primarily sweet onions through local produce auctions and direct markets. These lucrative markets pay $1.00/lb or more for jumbo and colossal sized bulbs. Reducing losses associated with bacterial diseases of onion through education and the development of an integrated pest management program will aid in sustaining and building the fresh market onion industry in Pennsylvania. The general public will also benefit from having affordable high-quality locally-grown onions available. Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided?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 was 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 and graduated with her PhD in Plant Pathology in August 2014 and currently works at University of Kentucky as an Assistant Professor. The research described above which is designed to develop integrated and targeted research-based management strategies that will increase the number of tools available to growers and ultimately reduce harvest and post-harvest losses was the focus of Jennie D. Mazzone's M.S. research in the Department of Plant Pathology and Environmental Microbiology at Penn State. Jennie graduated with her M.S. in May 2017 and is current a diagnostician with Bartlett Tree Care. In addition Marlia Bosques, an undergraduate student from the University of Puerto Rico Mayaguez, participated in the 2016 Penn State Summer Research Opportunities Program which is designed to give exceptional under-represented minority students research experience in preparation for applying to graduate school in STEM-related fields. Her research complimented that being conducted by Jennie Mazzone. She is currently in graduate school at Ohio State. How have the results been disseminated to communities of interest?The research results have disseminated at numerous regional vegetable grower meetings/conferences including the Mid-Atlantic Fruit and Vegetable Convention in Hershey, PA in 2013, 2015, and 2016 as well as the 2015 Great Lakes Fruit, Vegetable and Farm Market Expo in Grand Rapids, MI, via Skype at the 2015 University of Wisconsin Extension and Wisconsin Vegetable Grower Conference and Farming for the Future, Pennsylvania Association for Sustainable Agriculture meeting in 2016. Proceeding articles have been written to accompany a number of these presentations. On a national level, results have been presented at the National and Northeast Regional American Phytopathological Society meetings, the annual W2008 Multi-State Regional Project meetings and bi-annual National Allium Research Conference. Internationally, results of this research were presented at the 6th National and 5th International Symposium on Pathogenic and Beneficial Bacteria of Plants held at the Hilton Guadalajara in Guadalajara, Mexico in Sep 2016. Locally, results were also disseminated throughout the season through one-on-one meetings with growers, at summer twilight meetings, Lancaster Onion Co-op meetings and through regular extension activities. What do you plan to do during the next reporting period to accomplish the goals? Nothing Reported

Impacts
What was accomplished under these goals? Reducing soil temperatures as a means of reducing onion center rot. In PA, the majority of onions grown are marketed through the PA Simply Sweet Onion Program, the state's only trademarked crop. In 2016, 94 growers produced program onions on 125 A which were valued at over $1.4M; over quadruple the production since the program's inception in 2004. Unfortunately, onion growers in PA and elsewhere are constantly challenged by yield losses due to bacterial diseases, even when actively trying to manage the disease. In some fields losses due to the bacterial disease center rot can approach 50% under favorable disease conditions. Through replicated field and commercial on-farm trials over several years, a strong positive correlation between increased soil temperature when the onions start bulbing in late June and increased losses due to bacterial disease has emerged. As a result, growers are shifting from producing onions on standard black plastic mulch to reflective silver which can significantly reduce soil temperatures and therefore losses due to center rot. In one on-farm trial, the marketable onion yield on silver mulch was over 3 times that of black plastic. The incorporation of silver plastic into the integrated pest management toolbox provides growers with another tactic that they can employ to reduce losses due to bacterial disease and provide consumers with fresh, high-quality locally sourced produce. In PA, research efforts focused on project objective 3: Investigate the biology, ecology and epidemiology of pathogens other than Iris yellow spot virus (IYSV) to improve management strategies. In 2011 and 2012, replicated on-farm survey plots were established on a total of 54 farms to identify potential sources of inoculum as well as environmental/management factors related to disease loss. The most important factors were identified through conducting a series of stepwise multiple linear regression models. Foliar N and foliar C were negatively related to total disease incidence, while pre-season levels of soil nitrate were positively related to total disease incidence. Foliar nutrient levels implicate early-season fertility in managing center rot, while associations between soil nitrate and bacterial disease suggest ammonium or organic nitrogen may be a more effective N source in managing disease. In addition, soil temperatures near the physiological onset of bulbing were positively related to disease incidence, which agrees with previously published results. Overall, 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, growers should also take steps to lower soil temperatures, particularly near bulbing through the use of alternative plastic mulches. Pathogenicity tests were conducted with select isolates from each potential inoculum source. Pathogenic isolates of Pectobacterium carotovorum subsp. carotovorum (Pcar), Pantoea agglomerans (Pagg), and Pseudomonas marginalis pv. marginalis (Pmar), were the most common bacterial pathogens in PA onion fields and occurred in soil, transplant and weed tissues, in addition to being present as epiphytes on 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 among the weed-derived isolates, ranging from non-pathogenic to highly aggressive. In addition, rep-PCR strain tracking of P. ananatis isolates indicated matching strains from surface-disinfested weed tissue collected at mid-season and a latently infected onion that had been in storage. 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 than from redroot pigweed and purslane. In addition, rep PCR has been used to link certain pathogen strains from transplants and weeds to onion bulbs at harvest. Combined, these results suggest research avenues for more directed management strategies to reduce the impact of environmental inoculum sources on bacterial rots of onion. From 2013 to 2017, field trials were conducted based on the production factors identified as being associated with increased disease and designed to further develop targeted research-based management strategies for center rot of onion. Previous research has indicated that applying the total crop nitrogen (N) required prior to onion bulbing increased total yield and may reduce disease losses. Four replicated field trials were conducted to evaluate the effect of N application timing and/or rate on onion bacterial disease incidence and marketable yield. The total N rates (NO3) included 160 lb/A, 105 lb/A, 50 lb/A and a water only (no fertilizer) control. These were applied either half-season weekly until bulbing or full-season weekly until harvest. Each plot within each N rate received the same total amount of N during the season. The trial was a split plot randomized complete block design with timing as the whole plot and rate as the sub-plot with four replications. The plots were further subdivided and select plants toothpick inoculated with a bacterial suspension containing a mix of three isolates of P. ananatis and P. agglomerans. Foliar N content, marketable yield, disease incidence and severity were evaluated. In 2016, inoculation proximity was highly significant in determining center rot incidence (P≤0.0001 for both trials) and there was a significant interaction between rate and timing of N application (P=0.0417; P=0.0376). In 2015, only N rate significantly affected disease incidence (P≤0.0001). A positive, quadratic relationship was found between foliar % N levels at bulbing and center rot incidence at harvest (P≤0.0001; R2=0.522) and when mid-season foliar % N levels and dry weight at bulbing was included and analyzed as a linear model, a positive relationship occurred (P≤0.0001; R2=0.611). In all three trials, marketable yield was not significantly different between the three N rates, excluding the control, and the reduced rate of 105 lb N/A had the highest numerical marketable yield. Identification of cultivars less susceptible to center rot will provide growers with another management tool for an integrated disease management program. Three trials were conducted over two years to evaluate the susceptibility of sweet Spanish-type onion cultivars to the commercial standard Candy. The trials were arranged in a randomized complete block design with four replications. Plots were subdivided and two alternate were toothpick inoculated with a bacterial suspension as previously described. Plants were rated weekly for foliar disease severity on a 0 to 7 scale. Horticultural parameters including longest leaf length, neck diameter and bulb diameter were recorded for five plants per inoculation status per plot 3 times. The plots were harvested and graded for marketable yield and disease incidence. The cultivars Blush, Red Sky, Crockett, Sedona, Spanish Medallion and BGS 280 had lower center rot incidence at harvest compared to the commercial standard Candy. In all three trials, the only cultivar that had both lower disease incidence and comparable marketable yield to Candy was Spanish Medallion. Cultivar Aruba consistently had the lowest yields and highest center rot incidence and foliar disease severity. There was a weak linear relationship between early-, mid- and late-season neck diameters and post-harvest center rot disease incidence (P = 0.0195, P ≤ 0.001, P = 0.0054 respectively; R2 = 0.3993). It had been hypothesized that post-harvest disease incidence would be higher for onions with larger neck diameters.

Publications

  • Type: Theses/Dissertations Status: Published Year Published: 2017 Citation: Mazzone, J.D. 2017. Responding to growers needs: Evaluation of management strategies for onion center rot, caused by Pantoea ananatis and Pantoea aggloerans. M.S. Thesis, The Pennsylvania State University, University Park, PA, May 2017.
  • Type: Journal Articles Status: Submitted Year Published: 2017 Citation: Pfeufer, E.E. and B.K. Gugino. Environmental and management factors associated with bacterial diseases of onion in Pennsylvania. Plant Disease xx.


Progress 10/01/15 to 09/30/16

Outputs
Target Audience:Current and future onion growers in Pennsylvania and in other onion growing regions benefited from the research conducted as part of this project during this past year. Smaller scale growers sell primarily sweet onions through local produce auctions and direct markets. These lucrative markets pay $1.00/lb or more for jumbo and colossal sized bulbs. Reducing losses associated with bacterial diseases of onion through education and the development of an integrated pest management program will aid in sustaining and building the fresh market onion industry in Pennsylvania. The general public will also benefit from having affordable high-quality locally-grown onions available. Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided?The research described above which is designed to develop integrated and targeted research-based management strategies that will increase the number of tools available to growers and ultimately reduce harvest and post-harvest losses is the focus of Jennie D. Mazzone's ongoing M.S. research in the Department of Plant Pathology and Environmental Microbiology at Penn State. In addition, Marlia Bosques an undergraduate student from the University of Puerto Rico. Mayaguez, participated in the Penn State Summer Research Opportunities Program which is designed to give exceptional under-represented minority students research experience in preparation for applying to graduate school in STEM-related fields. Her research complimented that being conducted by Jennie Mazzone. How have the results been disseminated to communities of interest?In PA, research results were disseminated at several regional vegetable grower meetings/conferences including the 2015 Great Lakes Fruit, Vegetable and Farm Market Expo in Grand Rapids, MI and the 2016 Mid-Atlantic Fruit and Vegetable Convention in Hershey, PA in Jan 2016. Internationally, results of this research were presented at the 6th National and 5th International Symposium on Pathogenic and Beneficial Bacteria of Plants held at the Hilton Guadalajara in Guadalajara, Mexico in Sep 2016. Locally, results were also disseminated throughout the season through one-on-one meetings with growers, at summer twilight meetings in Centre and Chester Co. and through regular extension activities. What do you plan to do during the next reporting period to accomplish the goals?Data analysis from the 2016 season field trials will continue and the results of these trials will be presented at winter grower meetings in 2017. Select treatments will be replicated in 2017 and select cultivars evaluated on commercial farms. This research was conducted with funding leveraged from a PA Department of Ag Specialty Crop Block Grant and the final report along with referred publications will be prepared.

Impacts
What was accomplished under these goals? In Pennsylvania, our research efforts continued to focus on project objective 3 which was to investigate the biology, ecology and epidemiology of pathogens other than Iris yellow spot virus (IYSV) to improve disease management strategies. Based on previous research several production factors were identified as being associated with increased disease and have become the focus of subsequent research designed to develop integrated and targeted research-based management strategies. To build upon the 2015 dataset, two additional replicated field trials were conducted in Centre and Lancaster Co. to evaluate the effect of nitrogen application timing and rate on onion bacterial disease incidence and marketable yield. The total nitrogen rates (applied as urea ammonium nitrate) included 160 lb/A, 105 lb/A, 50 lb/A and a water only (no fertilizer) control. These were applied either half-season weekly until bulbing (end of Jun) or full-season weekly until harvest (mid-Jul). ). Each plot within each nitrogen rate received the same total amount of nitrogen during the season. The trial was a split plot randomized complete block design with nitrogen application timing as the whole plot and application rate as the sub-plot with four replications. The plots were further subdivided and select plants toothpick inoculated with a bacterial suspension containing a mix of three isolates of Pantoea ananatis and P. agglomerans, the two causal pathogens of center rot. This method has been successfully used to establish varying levels of disease pressure (inoculation proximity). Foliar N content, marketable yield, disease incidence and severity were evaluated. In 2016, inoculation proximity was highly significant in determining center rot incidence (P≤0.0001 for both trials) and there was a significant interaction between rate and timing of N application (P=0.0417; P=0.0376). In 2015, only N rate significantly affected disease incidence (P≤0.0001). A positive, quadratic relationship was found between foliar % N levels at bulbing and center rot incidence at harvest (P≤0.0001; R2=0.522) and when mid-season foliar % N levels and dry weight at bulbing was included and analyzed as a linear model, a positive relationship occurred (P≤0.0001; R2=0.611). In all three trials, marketable yield was not significantly different between the three N rates, excluding the control, and the reduced rate of 117 kg N/ha had the highest numerical marketable yield. The results of this research will be used to further refine the optimum rate and timing of N application to reduce center rot losses and thus provide growers with higher precision disease management. Additional analysis of this complex dataset is ongoing. Host resistance is one of the most important tools available for disease management. Identification of less susceptible cultivars will provide growers with another management tool. Again to build upon the 2015 dataset, two replicated field trials were conducted in Centre and Lancaster Co. to evaluate the susceptibility of ten sweet Spanish-type onion cultivars to center rot. There were significant differences in both foliar disease severity over the course of the season and center rot disease incidence at harvest with some cultivars performing better than the commercial standard cv. Candy. There was a significant positive relationship between increasing neck diameter and center rot disease at harvest and preliminary data suggests that this relationship is even more pronounced in the post-harvest storage evaluation. We are continuing to evaluate the relationships between the horticultural characteristics evaluated and bacterial disease incidence and severity to gain a better understanding of the potential characteristics that may be associated with increased disease susceptibility (i.e. neck diameter, etc.) and post-harvest storage evaluations are currently underway.

Publications

  • Type: Conference Papers and Presentations Status: Published Year Published: 2015 Citation: Gugino, B.K., J.D. Mazzone and E.E. Pfeufer. 2015. Investigating bacterial rot management of onion. 2015 Great Lakes Fruit, Vegetable and Farm Market EXPO. http://www.glexpo.com/education-program/session-summaries/46-2015-session-handouts.
  • Type: Conference Papers and Presentations Status: Published Year Published: 2016 Citation: Mazzone, J.D. and B.K. Gugino. 2016. Evaluating select management strategies for bacterial diseases of onion: Host resistance and nitrogen fertility. 2016 Mid-Atlantic Fruit and Vegetable Convention Proceedings, Pennsylvania Vegetable Growers Association, Richfield, PA. pp 109-111.


Progress 10/01/14 to 09/30/15

Outputs
Target Audience:Current and future onion growers in Pennsylvania and in other onion growing regions benefited from the research conducted as part of this project during this past year. Smaller scale growers sell primarily sweet onions through local produce auctions and direct markets. These lucrative markets pay $1.00/lb or more for jumbo and colossal sized bulbs. Reducing losses associated with bacterial diseases of onion through education and the development of an integrated pest management program will aid in sustaining and building the fresh market onion industry in Pennsylvania. The general public will also benefit from having affordable high-quality locally-grown onions available. Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided?The research described above which is designed to develop integrated and targeted research-based management strategies that will increase the number of tools available to growers and ultimately reduce harvest and post-harvest losses is the focus of Jennie D. Mazzone's ongoing M.S. research in the Department of Plant Pathology and Environmental Microbiology at Penn State. How have the results been disseminated to communities of interest?In PA, research results were disseminated at several regional vegetable grower meetings/conferences including the 2015 Mid-Atlantic Fruit and Vegetable Convention in Hershey, PA in Jan 2015 and via Skype at the 2015 University of Wisconsin Extension and Wisconsin Vegetable Grower Conference. On a national level, results were presented at the 2014 National Allium Research Conference held in conjunction with the National Onion Association Meeting and the W2008 Multi-state regional project meeting in Scottsdale, AZ in December 2014. Results were also disseminated throughout the season through one-on-one with the growers collaborating and through regular extension activities. What do you plan to do during the next reporting period to accomplish the goals?Data analysis from the 2015 season field trials will continue and the results of these trials will be presented at winter grower meetings in 2016. All the trials conducted in 2015 will be repeated in one or more location in 2016 to build the data set across seasons and locations. This research is being conducted with funding leveraged from a PA Department of Ag Specialty Crop Block Grant.

Impacts
What was accomplished under these goals? In Pennsylvania, our research efforts are focused on project objective 3 which was to investigate the biology, ecology and epidemiology of pathogens other than Iris yellow spot virus (IYSV) to improve disease management strategies. Based on previous research several production factors were identified as being associated with increased disease and have become the focus of subsequent research designed to develop integrated and targeted research-based management strategies. Previous research has indicated that applying the total crop nitrogen required prior to onion bulbing increased total yield and may reduce disease losses. In 2015, one replicated field trial was conducted to evaluate the effect of nitrogen application timing and rate on onion bacterial disease incidence and marketable yield. The total nitrogen rates (applied as ammonium nitrate) included 160 lb/A, 105 lb/A, 50 lb/A and a water only (no fertilizer) control. These were applied either half-season weekly until bulbing (end of Jun) or full-season weekly until harvest (mid-Jul). Each plot within each nitrogen rate received the same total amount of nitrogen during the season. The trial was a split plot randomized complete block design with nitrogen application timing as the whole plot and application rate as the sub-plot with four replications. The field was plowed and a pre-plant fertilizer (N-P-K, 0-46-0, 223 lb/A) was applied and incorporated. Aside from the nitrogen applications, the field was planted and maintained following commercial production practices. At five points during the season composite plant tissue samples were collected from each plot for foliar nutrient assessment. On 24 Jun and again on 7 Jul, the 40 ft plots were subdivided and two alternate 20 ft rows were toothpick inoculated with a bacterial suspension containing a mix of three isolates of Pantoea ananatis and P. agglomerans, the two causal pathogens of center rot. This method has been successfully used to establish varying levels of disease pressure. The plots were scouted weekly for disease during Jul and rated using a 0 to 7 rating scale. In addition, horticultural parameters including longest leaf length, neck diameter and bulb diameter were recorded for five plants per inoculation status per plot 3 times. The plots were harvested and graded for marketable yield and disease incidence on 27 Jul. This complex data set is currently being analyzed to determine the effect of nitrogen rate and timing on bacterial disease incidence. Not surprisingly, preliminary data analysis indicates that marketable yields were most influenced by disease pressure (inoculated vs not inoculated), the nitrogen rate (more nitrogen increased yields) rather than the timing of application. Host resistance is one of the most important tools available for disease management. Identification of less susceptible cultivars will provide growers with another management tool. In 2015, one replicated field trial was conducted at Rock Springs to evaluate the susceptibility of ten cultivars to center rot. Ten sweet Spanish-type including Sedona, Great Western, Aruba, Ovation, BBGS 280 F1, BGS 300 F1 Blush, Red Sky, Spanish Medallion, Expression and Candy (commercial standard) were grown from seed and transplanted on 28 April into 20 ft long raised bed covered with black plastic and two rows of drip irrigation as is standard commercial practice. The trial was arranged in a randomized complete block design with four replications. As described for the nitrogen fertility trial, the 20 ft plots were subdivided and two alternate were toothpick inoculated with a bacterial suspension. On 2, 10, 16 and 21 Jul, ten plants per plot and inoculation status (uninoculated, inoculated, and adjacent to inoculated) were rated for foliar disease severity on a 0 to 7 scale. Horticultural parameters including longest leaf length, neck diameter and bulb diameter were recorded for five plants per inoculation status per plot 3 times. The plots were harvested and graded for marketable yield and disease incidence on 23 Jul. A subsample of 10 asymptomatic jumbo sized bulbs per cultivar were sent to Waters Agricultural Laboratory in Camilla, GA for determination of pungency and soluble sugars which ranged from 2.9 to 3.7 and 6.2 to 9.3, respectively. Significant differences in yield between the cultivars were observed with cv. Great Western and Expression being the highest yielding and Sedona, BGS 300F1 Blush and Aruba being the lowest yielding overall. Cultivar Aruba with the lowest yields also had the highest incidence of bacterial bulb rot at harvest as well as foliar disease symptoms during the season. We are continuing to evaluate the relationships between the horticultural characteristic evaluated and bacterial disease incidence and severity to gain a better understanding of the potential characteristics that may be associated with increased disease susceptibility (i.e. neck diameter, etc.).

Publications

  • Type: Conference Papers and Presentations Status: Published Year Published: 2015 Citation: Gugino, B.K. and E.E. Pfeufer. 2015. Bacterial disease management of onion. 2015 Mid-Atlantic Fruit and Vegetable Convention Proceedings, Pennsylvania Vegetable Growers Association, Richfield, PA. pp 79-81.


Progress 10/01/13 to 09/30/14

Outputs
Target Audience: Current and future onion growers in Pennsylvania and in other onion growing regions benefited from the research conducted as part of this project during this past year. Smaller scale growers sell primarily sweet onions through local produce auctions and direct markets. These lucrative markets pay $1.00/lb or more for jumbo and colossal sized bulbs. Reducing losses associated with bacterial diseases of onion through education and the development of an integrated pest management program will aid in sustaining and building the fresh market onion industry in Pennsylvania. The general public will also benefit from having affordable high-quality locally-grown onions available. Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided? 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 and graduated with her PhD in Plant Pathology in August 2014. How have the results been disseminated to communities of interest? In PA, research results were disseminated at several vegetable grower meetings/conferences including the Lancaster Onion Co-op meeting on 26 June. On a national level, results were presented at the 2014 American Phytopathological Society meeting in Minneapolis, MN in August 2014 and at the W2008 Multi-state regional project meeting in Denver, CO in December 2013. Results were also disseminated throughout the season through one-on-one with the growers collaborating and through regular extension activities. In addition, I participated in the Onion ipmPIPE and continued to disseminate the Diagnostic Pocket Series to interested growers and other stakeholders. What do you plan to do during the next reporting period to accomplish the goals? Data analysis from the 2014 season field trials will continue and the results of these trials will be presented at winter grower meetings in 2015. Based on these results as well as results from trials conducted in previous years and funding leveraged from a PA Department of Ag Specialty Crop Block Grant, field trials will be designed and conducted to evaluate the use of treatments to manage bacterial populations on onion transplants, the application timing and reduced nitrogen rates on marketable yield and bacterial disease incidence, cultivar susceptibility trials and further development of scouting guidelines and visual disease thresholds for timing harvest.

Impacts
What was accomplished under these goals? In Pennsylvania, our research efforts are focused on project objective 3 which was to investigate the biology, ecology and epidemiology of pathogens other than Iris yellow spot virus (IYSV) to improve disease management strategies. In 2011 and 2012, replicated on-farm survey plots were established on 28 and 26 farms, respectively to identify potential sources of inoculum as well as production factors related to harvest disease incidence. Continued analysis of the extensive dataset generated continues to be a source of new information. In PA, pathogenicity tests were conducted with select isolates from each potential inoculum source. 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 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 among the weed-derived isolates, ranging from non-pathogenic to highly aggressive. In addition, rep-PCR strain tracking of Pantoea ananatis isolates indicated matching strains from surface-disinfested weed tissue collected at mid-season and a latently infected onion that had been in storage. 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 than from redroot pigweed and purslane. Several lines of evidence support the role of transplants as a source of inoculum. 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. In addition, a rep-PCR method has been used to link certain pathogen strains from transplants and weeds to onion bulbs at harvest. 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. To determine the environmental and management factors associated with high bacterial disease incidence, independent variables were placed in a series of stepwise multiple linear regression models. Foliar nitrogen and foliar carbon were negatively related to total incidence of bacterial rots of onion from the PA datasets, while pre-season levels of soil nitrate 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 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, which agrees 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. To complement the trials conducted in 2013, during the 2014 growing season, one replicated field trial was conducted on our research farm and four sites were established in commercial fields to evaluate the relationship between the timing of harvest and bacterial disease losses at harvest. Plots were rated weekly for foliar disease severity using an ordinal 0 (healthy) to 7 (≥50% of leaves are symptomatic) scale. For the 2013 research trials, average disease severity was plotted against harvest disease incidence. Second order equations fitted to described the curve then the derivative of each yield curve used to determine optimum yield. For the 2014 on-farm plots, harvest disease incidence and marketable yield by average foliar disease severity ratings were best-fit using second-order polynomials. The combined results suggest a critical foliar disease severity threshold value of approximately 3.0. However, while foliar severity correlated well with harvest disease incidence, its correlation with marketable yield was only fair suggesting that additional independent factors are likely involved.

Publications

  • Type: Theses/Dissertations Status: Published Year Published: 2014 Citation: Pfeufer, E.E. 2014. Sources of inoculum, epidemiology, and integrated management of bacterial rots of onion (Allium cepa) with a focus on center rot, caused by Pantoea ananatis and Pantoea aggloerans. Ph.D. Dissertation, The Pennsylvania State University, University Park, PA, August 2014.
  • Type: Conference Papers and Presentations Status: Published Year Published: 2014 Citation: Gugino, B.K., M.A. Mansfield and E.E. Pfeufer. 2014. Bacterial rots of sweet onion in Pennsylvania: Tracking sources of infection and targeting critical management points. Phytopathology 104: S3.142.
  • Type: Conference Papers and Presentations Status: Published Year Published: 2014 Citation: E.E. Pfeufer and B.K. Gugino. 2014. Visual rating of bacterial disease severity as a threshold to time sweet onion harvest. Phytopathology 104: S3.92.


Progress 10/01/12 to 09/30/13

Outputs
Target Audience: Current and future onion growers in Pennsylvania and in other onion growing regions benefited from the research conducted as part of this project during this past year. Smaller scale growers sell primarily sweet onions through local produce auctions and direct markets. These lucrative markets pay $1.00/lb or more for jumbo and colossal sized bulbs. Reducing losses associated with bacterial diseases of onion through education and the development of an integrated pest management program will aid in sustaining and building the fresh market onion industry in Pennsylvania. The general public will also benefit from having affordable high-quality locally-grown onions available. Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided? 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. How have the results been disseminated to communities of interest? In Pennsylvania research results were disseminated at several vegetable grower meetings/conferences including the 2013 Mid-Atlantic Fruit and Vegetable Convention and Lancaster Onion Co-op meeting as well as during the 4th Annual Discovery Day at the Southeast Agricultural Research and Education Center which is geared towards the media and legislators who are interested in learning more about ag research that directly impacts the Commonwealth. On a national level, results were presented at the 2013 American Phytopathological Society meeting in Austin, TX in August 2013 and at the W2008 Multi-state regional project meeting in Las Cruces, NM in November 2012. Results were also disseminated throughout the season through one-on-one with the growers collaborating in the intensive field survey project. In addition, I participated in the Onion ipmPIPE and continued to disseminate the Diagnostic Pocket Series to interested growers and other stakeholders. What do you plan to do during the next reporting period to accomplish the goals? Data analysis from the 2013 season field trials will continue and the results of these trials will be presented at winter grower meetings in 2014. Based on these results as well as results from trials conducted in previous years, field trials will be designed and conducted to better understand the biology and epidemiology of bacterial diseases on onion to further develop integrated disease management strategies that reduce both in-field and post-harvest storage losses.

Impacts
What was accomplished under these goals? In Pennsylvania, our research efforts are focused on project objective 3. In 2011 and 2012, replicated on-farm survey plots were established on 28 and 26 farms, respectively to identify potential sources of inoculum as well as production factors related to harvest disease incidence. The extensive dataset generated continues to be a source of new information. 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 field trial to evaluate nitrogen source (calcium nitrate vs ammonium sulfate) and application timing (half season vs full season) was conducted. Preliminary results indicate that neither nitrogen source or timing affected bacterial disease incidence however, marketable yield was further reduced in inoculated plots where fertility was applied throughout the season versus pre-bulbing only. A rep-PCR method was also developed to facilitate our ability to track bacterial strains within the same bacterial species. This will enable us link bacterial isolates collected from the surface of transplants, weeds, and soil to those isolated from symptomatic onion bulb tissue at harvest or after storage. Management strategies to reduce these potential sources of bacterial inoculum can then be developed. 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.

Publications

  • Type: Conference Papers and Presentations Status: Published Year Published: 2013 Citation: 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.
  • Type: Other Status: Published Year Published: 2013 Citation: 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.