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 agglomerans

Open Access
Author:
Pfeufer, Emily E.
Graduate Program:
Plant Pathology
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 04, 2014
Committee Members:
  • Beth Krueger Gugino, Dissertation Advisor/Co-Advisor
  • Beth Krueger Gugino, Committee Chair/Co-Chair
  • Gary William Moorman, Committee Member
  • Maria Del Mar Jimenez Gasco, Committee Member
  • Paul Anthony Backman, Committee Member
  • Shelby Jay Fleischer, Committee Member
Keywords:
  • Onion
  • bacterial plant pathogens
  • vegetable diseases
  • integrated disease management
Abstract:
Commercial acreage devoted to onion production has increased exponentially in Pennsylvania (PA) over the past fifteen years for several reasons, including the development of the PA Simply Sweet® marketing program, the establishment of grower cooperatives, and renewed consumer interest in fresh, local produce. Even with increasing acreage of onions in PA, consumer demands for the crop are not met by current production. Bacterial rots of onion are the most significant diseases reducing harvest and storage yields of the crop, in some instances diminishing marketable yields by 60%. Growers manage bacterial rots of onion through combinations of chemical and cultural practices, including copper fungicides, plastic mulch, and drip irrigation; however, yields remain variable between seasons and farms. These producers are interested in alternative practices, including plant defense-inducing treatments, carefully planned applications of fertilizer during drip irrigation (fertigation), and targeted insect management for more consistent control of bacterial diseases, however, data is lacking on the effectiveness of these practices in PA. The ultimate goal of the following research is to contribute additional integrated management tools to the existing grower knowledge base to increase the profitability of growing onions in PA. As a relatively new pathosystem in PA, basic and applied research was conducted in order to better understand the impact of bacterial rots on marketable yield of onion in the state. The principal bacterial pathogens were identified as Pectobacterium carotovorum subsp. carotovorum, Pantoea agglomerans, and Pseudomonas marginalis pv. marginalis. Sources of bacterial inoculum, including soil, transplants, and weeds, were elucidated in addition to investigation of ecological interactions between these species, their hosts, and the cropping system. Aspects of the production system are suggested to affect plant disease in both pathogen- and disease-specific ways, such as the association of black plastic mulch with increased detection of P. agglomerans, and early-season soil nitrate resulting in decreased detections of leaf pathogens. On-farm management factors as observed in PA and New York indicate that higher incidences of bacterial rots of onion are associated with low foliar nitrogen and high soil temperatures near the physiological onset of bulbing. Replicated field trials in which plant defense-inducing and growth-promoting compounds were compared for their efficacy in managing center rot of onion (Pantoea ananatis and P. agglomerans) indicated moderate disease incidence among all treatments, including the copper-based grower standard treatment, which was only effective at low levels of inoculum pressure. Comparisons of the source and timing of nitrogen fertilizer application were completed in a replicated field study, and one year of data suggests an association between late-season fertilizer application and higher incidence of center rot. In addition, data were generated in an effort to understand the role of thrips in the epidemiology of center rot disease. Taken together, these datasets have increased the overall knowledge about the bacterial rot – onion pathosystem in Pennsylvania, elucidated management practices that hold promise for future replicated study, and improved management of bacterial rots of onion, particularly through dissemination of research results to growers during extension presentations.