Plant Science & Landscape Architecture Theses and Dissertations

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Bacterial communities of the specialty crop phyllosphere: response to biological soil amendment use, rainfall, and insect visitation
(2016) Allard, Sarah Michelle; Micallef, Shirley A; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Microorganisms in the plant rhizosphere, the zone under the influence of roots, and phyllosphere, the aboveground plant habitat, exert a strong influence on plant growth, health, and protection. Tomatoes and cucumbers are important players in produce safety, and the microbial life on their surfaces may contribute to their fitness as hosts for foodborne pathogens such as Salmonella enterica and Listeria monocytogenes. External factors such as agricultural inputs and environmental conditions likely also play a major role. However, the relative contributions of the various factors at play concerning the plant surface microbiome remain obscure, although this knowledge could be applied to crop protection from plant and human pathogens. Recent advances in genomic technology have made investigations into the diversity and structure of microbial communities possible in many systems and at multiple scales. Using Illumina sequencing to profile particular regions of the 16S rRNA gene, this study investigates the influences of climate and crop management practices on the field-grown tomato and cucumber microbiome. The first research chapter (Chapter 3) involved application of 4 different soil amendments to a tomato field and profiling of harvest-time phyllosphere and rhizosphere microbial communities. Factors such as water activity, soil texture, and field location influenced microbial community structure more than soil amendment use, indicating that field conditions may exert more influence on the tomato microbiome than certain agricultural inputs. In Chapter 4, the impact of rain on tomato and cucumber-associated microbial community structures was evaluated. Shifts in bacterial community composition and structure were recorded immediately following rain events, an effect which was partially reversed after 4 days and was strongest on cucumber fruit surfaces. Chapter 5 focused on the contribution of insect visitors to the tomato microbiota, finding that insects introduced diverse bacterial taxa to the blossom and green tomato fruit microbiome. This study advances our understanding of the factors that influence the microbiomes of tomato and cucumber. Farms are complex environments, and untangling the interactions between farming practices, the environment, and microbial diversity will help us develop a comprehensive understanding of how microbial life, including foodborne pathogens, may be influenced by agricultural conditions.
Bt GENETICS EFFECT ON CORN HYBRID PERFORMANCE: A COMPARISON OF TWO NEAR ISOLINE CORN HYBRIDS
(2018) Thorne, Louis; Kratochvil, Robert J; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Most corn (Zea mays L.) hybrids planted in the U.S. are the result of genetic modification that gives them a Bt gene or genes obtained from the bacterium, Bacillus thuringiensis (Berliner) (Bt), that express insecticidal proteins and enables these hybrids to be resistant to several insects. European corn borer (ECB) (Ostrinia nubilalis, Hübner) is the main Lepidopteran pest targeted by the Bt corn technology. All Bt events used in current corn hybrids provide 100% control of ECB. This has led to widespread use of Bt hybrids and has resulted in a drastic decrease in the ECB population. This raises the question whether it is still economically feasible to plant Bt hybrids that have higher seed costs in environments where the ECB pest level is low. The objectives of this study were: 1) compare the yield and agronomic performance of a pair of corn near-isoline hybrids with and without the ¬Bt traits; and 2) evaluate the agronomic and economic optimums for yield and nitrogen (N) rate for each near-isoline hybrids. A two-year study at three University of Maryland research farms in 2013-2014 examined each hybrid type for stalk damage due to ECB, yield performance, the optimum N rate for maximizing yield, and the economic returns the two hybrids provided. This study found minimal ECB stalk damage and no consistent agronomic or economic yield difference between the Bt and non-Bt hybrids. Neither hybrid type was determined to have a consistent nitrogen use efficiency (NUE) advantage. The results of this study indicate that producers should not have concerns over hybrid type choice, now that there is significant regional suppression of ECB below economic levels.
CUCURBIT DISEASE MANAGEMENT WITH REDUCED CHLOROTHALONIL AND IMPROVED UNDERSTANDING OF PSEUDOPERONOSPORA CUBENSIS POPULATION DYNAMICS
(2020) Jones, Jake Gardner; Everts, Kathryne L; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Research has linked chlorothalonil exposure to declines in pollinator health due to an increased likelihood of Nosema ceranae infection, altered gut microbial community, and a reduction in colony fitness and survival of honey bees (Apis mellifera). Therefore, a reduction in use of chlorothalonil, a large component of cucurbit disease management, may be needed. Without chlorothalonil, a widely used, broad-spectrum fungicide, the fungal and oomycete pathogens in cucurbit cropping systems can more quickly evolve resistance to targeted fungicides due to a limited number of efficacious modes of action and frequent sprays. Pseudoperonospora cubensis, the causal agent of cucurbit downy mildew, for example, has a short life cycle, experiences repeated applications of fungicides, and has a wide host range making it a high risk for fungicide resistance development. Our research focused on the development of an alternative fungicide spray program in melons to reduce the use of chlorothalonil, identifying the fungicide insensitivities of local P. cubensis populations and determining the efficacy of fungicides used to manage cucurbit downy mildew, and investigating the clade-host relationship and formation of oospores in regional P. cubensis samples. Efficacy on two important diseases in melon, powdery mildew and gummy stem blight, can be largely maintained without chlorothalonil but anthracnose control was not adequate without the inclusion of chlorothalonil. Currently, there are a number of highly effective targeted fungicides available to growers for management of cucurbit downy mildew including oxathiapiprolin, zoxamide + chlorothalonil, chlorothalonil, and cyazofamid. Our research shows evidence of P. cubensis clade-host associations, with clade 1 preferentially infecting acorn and summer squash (Cucurbita pepo), butternut squash (Cucurbita moschata), and watermelon (Citrullus lanatus), while clade 2 preferentially infects cucumber (Cucumis sativus). Melons (Cucumis melo) and pumpkin (Cucurbita maxima) are hosts to both clade 1 and clade 2 P. cubensis. Using these findings, producers can choose the fungicide that most appropriately targets the more virulent clade 2 or less virulent clade 1 infections.
EFFECT OF COMPOST ON THE MICROCLOVER ESTABLISHMENT AND USE OF COMPOST AND MICROCLOVER TO REDUCE LAWN NUTRIENT RUNOFF
(2016) Xiao, Xiayun; Carroll, Mark; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
High volume compost incorporation can reduce runoff from compacted soils but its use also associated with elevated N and P concentrations in runoff making it difficult to assess if this practice will reduce nutrient loading of surface waters. Additionally, little is known about how this practice will effect leguminous species establishment in lawns as means to reduce long term fertilizer use. When 5 cm of compost was incorporated into soil a reduction in runoff of 40 and 59% was needed for N and P losses from a tall fescue + microclover lawn to be equivalent to a non-compost amended soil supporting a well fertilized tall fescue lawn. Use of compost as a soil amendment resulted in quicker lawn establishment and darker color, when compared to non-amended soil receiving a mineral fertilizer. Biosolid composts containing high amounts ammonium severely reduce the establishment of clover in tall fescue + micrclover seed mixture.
Effects of a Simulated Dicamba Misapplication on Non-tolerant Soybeans (Glycine max)
(2015) Morris, Matthew; Ritter, Ronald L; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Approval is pending for the registration of dicamba tolerant (DT) soybeans [Glycine max (L.) Merr.]. The use of dicamba on DT soybeans and other DT crops will increase. Risks associated with dicamba applications include off-target movement to sensitive crops. The objective of this study was to evaluate misapplication of dicamba on non-DT soybeans. Greenhouse and field studies examined a rate titration (0.004 to 0.5 lb ai a-1) of dicamba on non-DT soybeans (V3 stage - three trifoliates). Field studies also examined dicamba application to various growth stages (PRE- preemergence to R5- early pod fill) of non-DT soybeans. Results from the greenhouse and field studies showed that as the rate of dicamba increased, the level of injury to vegetative and yield components also increased. Soybean growth stage at time of application influenced the amount of injury. Less injury was observed when dicamba was applied at the PRE growth stage.
EVALUATION OF COMPOST TOPDRESSING, COMPOST TEA AND CULTIVATION ON TALL FESCUE QUALITY, SOIL PHYSICAL PROPERTIES AND SOIL MICROBIAL ACTIVITY
(2015) Chen, Siqi; Carroll, Mark J.; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Compost topdressing, compost tea, and hollow tine cultivation are common cultural practices employed in organic lawn care programs. Restrictions on the amount of bagged fertilizer nitrogen and phosphorus applied to turf have raised questions about the need to place similar restrictions on compost turfgrass applications. In a three-year study the effect of reduced and common practitioner use rates of compost topdressing, the use of compost tea and of hollow tine cultivation on soil physical and biological properties and turfgrass quality were evaluated. Cultivation, monthly compost tea application and compost topdressing applied at rates consistent with annual bagged fertilizer nitrogen restrictions had little effect on soil organic matter, microbial activity, bulk density and infiltration. The use of a synthetic fertilizer resulted in higher turf quality than the use of compost on most evaluation dates. Nutrient fertilizer restrictions if applied to compost will likely result in a decline in turf quality.
Genetic Control of Flowering Time in a Soft Red Winter Wheat Doubled Haploid Population
(2015) Miller, Daniela Michelle; Costa, Jose M; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Flowering time in wheat is regulated mainly by response to seasonal environmental cues and controlled by the photoperiod and vernalization pathways. Allelic diversity in genes controlling these pathways is used by breeders to adapt wheat for optimal yield in a broad range of environments. This study characterized genetic loci influencing heading date in a soft red winter wheat doubled haploid population. Two photoperiod insensitivity alleles, Ppd-A1a and Ppd-D1a, were found to have major effects in eight field locations. The Ppd-A1 locus explained up to 16.8% of variation in heading date, whereas the Ppd-D1 locus explained up to 39.7%. In reduced vernalization greenhouse experiments, a QTL in the same region as the VRN-A1 gene explained up to 42.4% of variation in heading date, suggesting that the population differed in this region. Assays for previously-described allelic diversity in the VRN-A1 gene, however, did not detect any polymorphism between parents of the population.
Getting Legume Cover Crops to Work in Mid-Atlantic Crop Rotations
(2020) Peterson, Cara; Tully, Katherine L; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
In the mid-Atlantic United States, legume cover crop adoption is limited by the shortened establishment window after double-crop soybean (Glycine max (L.) Merr.) harvest. Interseeding legume cover crops into wide-row (76 cm) double-crop soybean presents an opportunity to supplement inorganic nitrogen (N) fertilizer in the subsequent corn (Zea mays L.) crop. We conducted field trials in Maryland and Delaware in which mixtures of cereal rye (Secale cereale L.) + hairy vetch (Vicia villosa Roth), crimson clover (Trifolium incarnatum L.), red clover (Trifolium pratense L.), or winter pea (Pisum sativum var. arvense (L.) Poir) were interseeded into double-crop soybean. We then examined the N contributions of the cover crop mixtures in combination with sidedress applications of inorganic N fertilizer on corn yields in Maryland in 2018 and 2019. This research demonstrated that interseeding cover crops into double-crop soybean is a potential strategy for increasing regional adoption of legume cover crops.
MANAGEMENT OPTIONS FOR FARMERS FACING SALTWATER INTRUSION ON THE EASTERN SHORE OF THE CHESAPEAKE BAY
(2023) Schulenburg, Alison Nicole; Tully, Kate; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Rising sea levels, storms, and perigean spring tides push saltwater into coastal agricultural fields. This phenomenon, known as saltwater intrusion, alters nutrient cycling and damages crop yields. As sea levels continue to rise, saltwater intrusion will only worsen, with devastating consequences to agroecosystems along the coast of the Chesapeake Bay. Researchers and farmers alike are looking for solutions to adapt to and mitigate the effects of saltwater intrusion. Landowners may respond by altering their management practices. Farmers may 1) adapt by planting a salt-tolerant crop, 2) attempt to remediate soils with trap crops, 3) restore native marsh grasses, or 4) abandon fields altogether. My project investigates the survival of different crops and plant treatments under saltwater-intruded conditions and the potential for these plants to survive and to remove excess nutrients (e.g. sodium and phosphorus) from the soil, with the overall goal to benefit both the farming community and water quality in the Chesapeake Bay. Results from this study will help inform new management practices to increase soil health and maintain crop yields. Finally, the goal of this work is to guide local best management practices and potential easement opportunities for landowners facing saltwater intrusion, and ultimately determine optimal strategies for climate resilience.
MANAGING WATER, NITROGEN, AND ALLELOPATHY WITH A CEREAL RYE COVER CROP
(2018) Otte, Briana; Tully, Katherine L; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
A cereal rye (Secale cereale) cover crop is a multi-functional tool in a no-till corn agroecosystem. The objectives of this study were to (1) quantify soil phenolic acid concentration under cereal rye shoots and roots, and how tillage impacts their release (2) evaluate the effects of cereal rye termination date on soil water, nitrogen, and corn performance compared to no cover crop. Soil phenolic acids have known allelopathic effects, inhibiting some weed seed germination or growth. Results suggest that cereal rye roots release more phenolic acids into the soil than cereal rye shoots, a novel finding. Results also suggest that corn grain yield following a late-terminated cereal rye cover crop is mediated by precipitation pattern and N release from cereal rye residues. During years of above average summer precipitation a late-terminated cereal rye cover crop does not affect corn grain yields and decreases residual inorganic soil N in the agroecosystem.
MECHANISMS OF DISEASE SUPPRESSION BY A HAIRY VETCH (VICIA VILLOSA) COVER CROP ON FUSARIUM WILT OF WATERMELON AND THE EFFICACY OF THE BIOCONTROL ACTINOVATE.
(2013) Himmelstein, Jennifer Carol; Everts, Kathryne; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The cover crop Vicia villosa suppresses Fusarium wilt of watermelon but the mechanisms of disease suppression are unknown. Possible mechanisms were examined in field, greenhouse, and in vitro experiments. The effects of cover crop treatments (V. villosa, Trifolium incarnatum, Secale cereale, Brassica juncea) and the biocontrol treatment Actinovate (Streptomyces lydicus WYEC 108) on Fusarium wilt of watermelon and its causal pathogen, Fusarium oxysporum f. sp. niveum (FON) were evaluated. In four of five field experiments there were significant elevations in soil microbial respiration. Arbuscular mycorrhizal colonization of watermelon roots following cover crop amendments of V. villosa and T. incarnatum, were significantly higher compared to bare ground. The elevation in respiration was significantly positively correlated with disease suppression of Fusarium wilt induced by V. villosa and T. incarnatum (both cover crops reduced Fusarium wilt as much as 21%). In greenhouse experiments using infested field soil, Fusarium wilt suppression was observed in pots amended with V. villosa and T. incarnatum. However, there was an increase in Fusarium wilt of watermelon in pots that were amended with V. villosa and T. incarnatum which were also inoculated with FON when compared to plants in nonamended, inoculated pots. These leguminous cover crops may have served as a nutrient source for the pathogen. In addition, in vitro growth experiments showed that media amended with V. villosa leachate significantly stimulated the in vitro growth rates of FON and Trichoderma harzianum compared to nonamended plates. It was hypothesized that V. villosa stimulation of nonpathogenic F. oxysporum spp., which provides cross protection against FON, may have contributed to the wilt suppression. Cover crop leachate amendments did not significantly influence colony forming units of S. lydicus. In both field and greenhouse trials Actinovate applications either had little or no effect on Fusarium wilt of watermelon. However, S. lydicus significantly inhibited in vitro growth of FON. These studies demonstrate that both general and specific disease suppression play a role in V. villosa suppression of Fusarium wilt of watermelon and that T. incarnatum is a viable alternative biocontrol.
MODELLING DECOMPOSITION AND NITROGEN RELEASE FROM SURAFCE COVER CROP RESIDUES IN NO-TILL SYSTEMS IN THE MID-ATLANTIC AND SOUTHEASTERN US
(2020) Thapa, Resham; Tully, Katherine L.; Mirsky, Steven B.; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
In the mid-Atlantic and Southeastern US regions, cover crops (CCs) are planted during the winter fallow periods or between cash crops to provide living roots and to cover soil for extended time periods. Cover crops can provide a suite of agroecosystem services to cropping systems including soil and water conservation, weed suppression, and nitrogen (N) cycling. After CCs are terminated, the rate of residue decomposition determines both N availability and the longevity of residue cover in conservation tillage (reduced- and no-till) systems. Accurate predictions of plant-available N from decomposing CCs are needed to improve N fertilizer recommendations in order to reduce environmental losses of N while meeting cash crop N needs. The objective of this work is to improve our understanding of the factors controlling CC residue decomposition in conservation tillage systems at varying temporal (diurnal to seasonal) and spatial (laboratory to regional) scales. At a diurnal scale, the moisture (θg)/water potential (ψresidue) and temperature in the surface CC residue layers fluctuated more dramatically and dynamically than the underlying soils. Decomposition of surface CC residues also showed distinct diurnal patterns that were closely related to diurnal variations in residue θg or ψresidue. In a controlled microcosm experiment, the effect of residue location on C and N mineralization during repeated dry-wet cycles were also primarily explained by differences in residue water dynamics than by differences in soil-residue contact between the surface and incorporated residues. At a regional scale, the combination of residue quality and climatic variables explained the majority of the variations in residue decomposition rates, i.e. k-values. I found faster decomposition of surface CC residues in humid environments and in site-years with more frequent rain events. The k-values decreased with increasing biomass, C:N, residue holo-cellulose concentrations, and lignin:N, but increased with increasing residue carbohydrate concentrations. Mathematical equations were developed and integrated into the existing CERES-N sub-model to adjust k-values based on residue environment. Once such models are well-calibrated and well-validated, they will be used to make evidence-based management recommendations to farmers. Thus, this research helps to optimize provisioning of agroecosystem services in CC-based conservation tillage crop production systems.
MODIFYING GREEN ROOF SUBSTRATE FOR NUTRIENT RETENTION IN URBAN FARMING SYSTEMS
(2020) Howard, Ian Nathaniel; Lea-Cox, John D; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Interest in urban agriculture is steadily increasing in the Mid-Atlantic region. The conversion of extensive green roofs to food production is particularly appealing due to space availability. The modification of a relatively unfertile shale-based substrate for increased water and nutrient availability was investigated, adding mushroom and yard-waste composts, but potentially contributing to nutrient runoff from rainfall and irrigation events. Alumina and biochar were therefore tested as substrate amendments to determine their effect nutrient availability and retention. Fifteen substrate mixes were screened by column leaching tests, and four were further studied over nine-months, with crop and leachate studies. Basil, lettuce and peppers were grown and harvested in succession in replicated 50-liter tubs, with leachate collection systems. Biochar did not reduce nitrogen or phosphorus leaching and did not have an effect on plant growth. Alumina significantly reduced the amount of phosphorus leached from substrates with little to no effect on plant growth.
MOWING TO GROWING: TRANSFORMING A MUNICIPAL GOLF COURSE TO URBAN AGRICULTURE IN BALTIMORE CITY
(2016) Allen, Nathan; Kweon, Dr. Byoung-Suk; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
This thesis demonstrates how landscape architects can transform underused golf course facilities located within cities for urban agriculture (UA). In the last decade more than 1000 golf courses have closed in the United States. Municipal golf courses represent some of the largest pieces of open space in cities and because of their inherent infrastructure they can provide the ideal location to support large-scale UA. In Southwest Baltimore large food deserts are a serious health concern and represent a lack of access to healthy food options for residents. Carroll Urban Agriculture Park is a design response resulting from a detailed analysis of the existing Carroll Park Golf Course and the surrounding community of Southwest Baltimore. The design will create an urban farm in a park-like setting to provide readily accessible healthy food options and various educational opportunities, and to support current and future urban agriculture related businesses in Baltimore.
Nitrogen cycling by grass-brassica mixtures in the Mid-Atlantic
(2019) Gaimaro, Joshua Ruben; Tully, Kate; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Mixtures of cover crop species may be more effective than monocultures at internal nutrient cycling due to their ability to occupy different niches. Our study investigates nitrogen (N) cycling of radish (Raphanus sativus L.) and rye (Secale cereal L.) in monocultures and mixtures compared to a no cover crop control. The study was established on fine-textured soils near Laurel, MD where we estimated N leaching losses, quantified mineral soil N (to 60 cm), and cover crop biomass N for two years. Forage radish suppressed estimated N leaching in the fall, while cereal rye suppressed estimated N leaching in the spring. In this study, growing radish in a mixture with rye decreased the risk of N leaching losses and enhanced N cycling due to the difference in timing of N uptake and release. Our research indicates that grass-brassica mixtures are a flexible management tool for mitigating N leaching in the Mid-Atlantic.
Nitrogen Management in Corn: Influences of Urea Ammonium Nitrate (UAN) Applications With and Without Nitrogen Stabilizer Products.
(2013) Watkins, Patrick Howard; Kratochvil, Robert J; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Corn (Zea mays, L.) is a major crop produced in the nutrient sensitive Mid-Atlantic region. Nitrogen use efficiency (NUE) for corn is considered sub-optimal and farmers in the region use a number of best management practices (BMPs) to improve corn NUE. Two sidedress application methods (surface banding and sub-surface injection) and four commercially available nitrogen stabilizer products (`Agrotain', `Agrotain Plus', `Instinct', `Nutrisphere-N') were investigated during 2009-2011 over three N fertilizer rates at nine total locations. Headspace ammonia accumulation (post-sidedress) was indexed to the surface applied UAN treatment and resulted in application method and stabilizer products having a significant effect for headspace ammonia accumulations. Post-harvest inorganic soil nitrogen was not affected by application method or stabilizer products. Yield was not significantly affected by application method or stabilizer products but was affected by N rate. Total plant N concentration was not significantly affected by application method or stabilizer products.
SPRAY STRATEGIES AND SELECTION FOR FUNGICIDE RESISTANCE: FENHEXAMID RESISTANCE IN BOTRYTIS CINEREA AS A CASE STUDY
(2023) Boushell, Stephen Carl; Hu, Mengjun; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Fungicide resistance is a limiting factor in sustainable crop production. Despite the wide adoption of general resistance management strategies by growers, the recent rate of resistance development in important fungal pathogens is concerning. In this study, Botrytis cinerea and the high-risk fungicide fenhexamid were used to determine the effects of fungicide dose, tank mixture, and application timing on resistance selection across varied frequencies of resistance via both detached fruit assays and greenhouse trials. The results showed that application of doses lower than the fungicide label dose, mixture with the low-risk fungicide captan, and application post-infection seem to be the most effective management strategies in our experimental settings. In addition, even a small resistant B. cinerea population can lead to a dramatic reduction of disease control efficacy. Our findings were largely consistent with the recent modeling studies which favored the use of the lowest possible fungicide dose for improved resistance management.
TEMPORAL DYNAMICS, ANTIMICROBIAL RESISTANCE AND PHYLOGENETIC RELATIONSHIPS OF BACTERIAL TAXA IN IRRIGATION WATER SOURCES AND RELEVANCE TO FOOD SAFETY
(2021) Solaiman, Sultana; Micallef, Shirley SAM; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
As climate change continues to stress freshwater resources, we have a pressing need to identify alternative (nontraditional) sources of microbially safe water for irrigation of fresh produce. Unfortunately, open water sources are often contaminated with many known human pathogens such as E. coli, Salmonella and Listeria and unknown/understudied pathogens such as Aeromonas that are associated with foodborne outbreaks. To facilitate the adoption of microbiologically safe irrigation water sources, a comprehensive study on the prevalence and virulence potential of human pathogens and their transferability to fruit and fresh produce vegetables was conducted. The effect of irrigation water types on crop surface microbial community structure, presence of virulence factors and antimicrobial resistance were investigated to evaluate the potential of transfer of pathogenic and antimicrobial resistant bacteria in humans. Initially, the prevalence of indicator bacteria was determined using culture methods and then microbiological water quality profiles (MWQP) was created to identify water sources that complied with the U.S Food Safety Modernization Act water standards. Next, using culture and molecular methods, investigation of the antimicrobial resistance profile of one known foodborne pathogen Escherichia coli retrieved over a two-year period was done. E. coli resistance against widely prescribed antibiotics, extended spectrum β-lactams, was determined phenotypically and genetically. The diversity, distribution and potential for pathogenesis of one understudied pathogen, Aeromonas, prevalent in a variety of typical or potential irrigation water sources and collected over a one-year period was investigated. The study revealed spatial and temporal patterns in species richness, evenness, virulence gene carriage and attachment behaviors on both biotic and abiotic surfaces, of this bacterial taxon. Finally, the effect of using highly treated reclaimed water and pond water on lettuce surface microbiomes was investigated. The study provided an integrated assessment of the shifts in microbial community that result from using different irrigation water sources for irrigation of lettuce. Understanding the ecology of lettuce associated microbiota can be useful to infer risks of transfer and establishment potential of possible pathogenic strains from irrigation water sources to minimally processed raw consumed fresh produce crops.

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