Browsing by Subject "Plant Pathology"

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Association Mapping for Net Blotch Resistance in Barley and a Study of Barley/Cereal Yellow Dwarf Virus in Minnesota
(2017-08) Adhikari, Anil
Genome wide association studies (GWAS) were conducted to identify net blotch (net form: caused by Pyrenophora teres f. teres) resistance loci in Ethiopia and Eritrea (EEBC) barley landraces and the elite germplasm of the Barley Coordinated Agricultural Project (Barley CAP). Two quantitative trait loci (chromosomes 5H and 6H) were identified in the EEBC. Resistance QTL were found in chromosomes 4H and 6H in the two-row panel, chromosomes 3H and 6H in the six-row panel and in chromosomes 3H, 4H, 6H and at an unmapped location in the combined Barley CAP germplasm. The distribution of Barley yellow dwarf (BYD), caused by Barley/cereal yellow dwarf virus (B/CYDV) in the Luteovirus family, in Minnesota was studied using 243 cereals and grass samples collected from 2013 to 2015. Reverse transcription polymerase chain reaction using virus strain specific primers revealed that BYDV-PAV was the most prevalent strain of B/CYDV.
Colonization of soybean (Glycine max) by the pathogen Phialophora gregata colonization of soybean (Glycine max) by the pathogen Phialophora gregata and endophytic fungi.
(2010-04) Impullitti, Ann E.
Fungal pathogens, beneficial symbionts, and endophytes colonize plants and reduce, enhance, or have cryptic effects on plant productivity. My dissertation focused on colonization of soybean by the fungal pathogen Phialophora gregata and characterization of the endophytic fungal population within soybean. Soybean was used as a model plant because it is economically important, its colonization by P. gregata has well-defined latent and pathogenic phases, and endophytes within the stem were not characterized. The first goal of this research was to investigate infection and plant responses during latent and pathogenic phases of disease development. Susceptible and resistant cultivars were infected with type A of P. gregata that causes stem and leaf necrosis or type B that causes only stem necrosis. Latent infection occurs after plants are colonized by a pathogen and remain asymptomatic. The number of vessels, leaf area, stomatal conductance, and yield of photosystem II (PSII) of infected and noninfected plants were determined. During latent infection, differences in the number of vessels was observed between susceptible and resistant plants, P. gregata was rarely observed in stems, and leaf area of susceptible plants was reduced by infection with type A. During pathogenic infection by type A, the resistant cultivar had fewer than 10% of vessels colonized and 20 to 25% more vessels than uninfected plants, while more than 70% of vessels were colonized in the susceptible cultivar and 50% fewer vessels were present compared to uninfected plants. During pathogenic infection by type B, more than 10% of vessels were colonized and no differences in vessel numbers were observed compared to the uninfected resistant plants. Type A did not reduce the leaf area of the resistant cultivars, but the leaf area of the susceptible cultivar was reduced by 80%. Type B reduced the leaf area of susceptible and resistant plants by 30%. Stomatal conductance was reduced 80% by types A and B in susceptible plants and by 40% in resistant plants. No differences in yield of PSII iii were observed. Qualitative differences in colonization were observed during pathogenic infection using GFP and RFP-tagged isolates. Type B-RFP was observed in the primary xylem, while type A-GFP was observed outside of the primary xylem in the resistant cultivar. Whereas in the susceptible cultivar, PgAGFP was in the primary xylem and PgB-RFP was limited to the interfascicular region. In summary, latent infection reduced the photosynthetic area of infected plants, but did not significantly modify their vascular structure and may lead to reduction of photosynthetic efficiency and increased susceptibility to biotic and abiotic stress. During pathogenic infection, resistant cultivars produced more vessels, restricted or excluded P. gregata from the vascular system, and reduced stomatal conductance and photosynthetic area. These responses may compensate for reduced vessel function and allow water movement. The second goal was to determine the diversity of fungal endophytes in soybean stems. Stems from field-grown plants that were either treated or not treated with glyphosate were surface-disinfested, and fungal endophytes were assessed using culture-dependent (CD) and –independent (CI) methods. For the CD method, stem segments were dissected into an outer stem composed of the epidermal and vascular tissues and an inner stem composed of the pith tissues. Cultured fungi were grouped based on colony morphology and identified based on rDNA ITS sequences. For the CI method, DNA was extracted from stems and the ITS-region was amplified using fungal-specific primers, cloned, and sequenced for identification. More isolates were obtained from the outer than inner stems, and from the base of the stems compared to the apex. The most frequently isolated genera were Cladosporium (32%), Phomopsis/Diaporthe (15%), Alternaria (14%), Fusarium (11%), and Phoma (8%), The CD method detected more endophytic diversity (H’ = 2.35) than the CI method (H’ = 0.76). The most prevalent genus identified using the CI method was Cladosporium (83%). Soybean genotype influenced the diversity of endophytes more than glyphosate treatment. This research also suggests soybean harbors an endophytic fungal population much less diverse than plants in the tropics and in polycultures.
Etiology of crown decline and dieback in bitternut hickory
(2011-09) Park, Ji-Hyun
Bitternut hickory is the most abundant and uniformly distributed hickory species in the north central and northeastern USA. Higher rates of hickory decline or mortality than expected have been recently reported from multiple states in the regions. Although hickory decline historically had been attributed to outbreaks of hickory bark beetle during a drought period, it has been recently described as a “decline disease” for which major determinants have not been identified. In 2007 and 2008, a regional survey was conducted by the U.S. Forest Service to elucidate the major determinants of the recent episodes. The survey found that declining hickories exhibited three types of crown symptoms (e.g. rapid crown decline, top dieback, and slow crown decline) and several types of fungal-associated stem damage (e.g. cankers and galls). However, to date, the potential involvement of fungal agents in hickory decline has not been thoroughly examined. This dissertation presents results of investigations to elucidate fungal agents associated with hickory decline and their relationships with declining trees. Associations of putative pathogenic fungi with major stem damage were determined by collecting samples from geographically dispersed regions and isolating and identifying fungal species by standard methods. Ceratocystis smalleyi and Fusarium solani were two key fungal associates. Their ability to cause cankers on maturing bitternut hickory was confirmed by fulfilling Koch’s postulates under field conditions. Since C. smalleyi appeared to be the predominant fungal pathogen associated with trees exhibiting rapid crown decline, its role in development of the symptom was examined using physiological, anatomical, and histological approaches. A general trend was observed of reduced sap flow velocity in diseased compared to control trees. Of selected xylem properties, the extent of vessel occlusion by tyloses in the outer annual rings was inversely and significantly correlated with sap flow velocity. C. smalleyi was found to induce a set of general defense responses in xylem tissues upon infection but its xylem colonization was locally restricted compared to that of true vascular wilt pathogens. Etiology of rapid crown decline is described using the disease triangle concept and its comparisons with other related patho-systems are discussed.
Factors affecting Fusarium head blight development and trichothecene accumulation in fusarium-infected wheat heads.
(2010-06) Gautam, Pravin
Fusarium head blight (FHB), primarily caused by Fusarium graminearum Schwabe, is an economically important disease as it results in yield loss and quality losses of infected grain and the accumulation of mycotoxins produced by the invading fungus. Environmental factors, host genetics, and isolate aggressiveness impact FHB development and subsequently trichothecene production and accumulation. Though it is well established that moisture around anthesis promotes FHB development and trichothecene accumulation, the role of moisture, either in the form of rainfall or mist-irrigation during the period from anthesis to harvest has been largely overlooked. A three year field experiment was conducted in 2006, 2007 and 2008 to examine the influence of environmental factors, especially moisture, host resistance, and pathogen variation with respect to mycotoxin production capacity and pathogen aggressiveness, on infection, FHB development and mycotoxin production and accumulation in planta. In mature harvested grain FHB severity, visually scabby kernel (VSK) and mycotoxin concentration were significantly higher in Wheaton (FHB susceptible) than in the other two cultivars examined, Alsen and 2375. Although FHB severities were not significantly different in plots receiving different durations of mist-irrigation, VSK were significantly lower in the treatments receiving the least amount of mist-irrigation (14 DAI) than for treatments receiving mist-irrigation for longer periods, suggesting that extended periods of moisture promote disease development. DON concentration in harvested grain was, however, significantly lower in the treatment receiving the longest duration of mist-irrigation than those treatments receiving less water. In the whole head samples, which were collected 0, 7, 11, 14, 21, 28 and 41 days after inoculation, DON and other trichothecenes either declined with increased durations of mist-irrigation or remained low while water was being applied by the misting system. However, trichothecene accumulation was observed to increase after the cessation of mist-irrigation, with increases being most pronounced for the treatments with shorter mist-irrigation periods. The largest reduction in DON observed as a result of extended mist-irrigation periods was seen in the susceptible cultivar Wheaton. The influence of host resistance and pathogen variation on infection, FHB infection, disease development and mycotoxin accumulation in planta was examined in the series of greenhouse experiments utilizing point and spray inoculations. The levels of FHB severity and mycotoxins were higher in spray inoculated experiment than point inoculation in all cultivars examined. Wheaton (FHB susceptible) had the highest FHB severity and levels of mycotoxins. Alsen (moderately resistant to FHB) had significantly lower FHB severities, DON, 15-ADON, 3-ADON and NIV than either 2375 or Wheaton. Though there were no significant differences in initial infection among cultivars examined, Alsen had reduced spread of FHB symptoms from initial infection presumably due to type II resistance. DON production did not peak in all treatments, but where evident, the peak was earlier in 2375 (11 dai) than Alsen and Wheaton (21 or 14 dai). Multiple peaks and declines in DON levels were also evident. The performance of isolates was highly variable, though generally isolates Butte86Ada-11 and B63A were the most aggressive isolates and 49-3 and B45A were the least. The impact of free moisture, such as that from irrigation systems or rainfall, on mycotoxin accumulation was evaluated in greenhouse experiments. Despite the similar levels of FHB severity observed, the levels of mycotoxins were significantly less in the plants that received a single six hour wetting treatment compared to the respective control. The loss of DON and other mycotoxins was evident in all cultivars examined. Further, DON and 15-ADON were detected in run-off water. The results of these studies suggest that the availability of free moisture such as from mist-irrigation or rainfall may increase FHB severity and VSK, although DON and other trichothecene concentrations may be concomitantly reduced. Leaching appears to contribute to reductions in DON following wetting events.
Fungi in Antarctica: a circumpolar study of biodiversity in soils and historic structures.
(2010-07) Arenz, Brett Evan
Antarctica is the most remote and isolated continent on Earth and is generally thought to have low biodiversity due to environmental extremes. These relatively simple ecosystems are important to study because they can be used to improve understanding of more complex systems world-wide that are difficult to analyze directly. The isolation of the continent, extreme environmental conditions and the lack of functional redundancy in ecosystem processes make it particularly vulnerable to human disturbance and require improved understanding. The results of this research support the hypothesis that fungal abundance and distribution are generally linked to the presence of primary producers and their effect on carbon and nitrogen quantities in the soil. Experiments introducing sterile plant-derived nutrient sources (wood and cellulose) to soils resulted in increased fungal abundance up to three to four orders of magnitude greater than background soil levels. This suggests that the extremes of the Antarctic environment (low moisture, high salinity, cold temperatures) are primarily affecting fungi by limiting the distribution of flora and direct effects on the fungi are relatively less important as these indigenous soil fungi appear well adapted to Antarctic environment. A survey of fungal diversity near historic sites and areas where materials were introduced to the Antarctic Peninsula reveals a very similar composition to those affecting historic sites on Ross Island. The fungi found in greatest abundance were species of Geomyces and Cadophora. These two genera also formed a large percentage of the fungal colonization of buried nutrient substrates. The frequent reports of these fungi from many areas in Antarctica and the large diversity of species found indicates they are well adapted to their environment and suggests they are indigenous to Antarctica. The dominance of these fungi on human-introduced material indicates direct human influences may be of more benefit to generalist indigenous decomposer fungi which are pre-adapted to the environmental extremes rather than human-introduced fungi which may be better adapted to utilizing these substrates but not well adapted to the Antarctic environment. It also supports the hypothesis of indigenous Antarctic fungi being primarily limited by nutrient availability.
Fusarium and Phytophthora Species associated with root rot of soybean (Glycine max)
(2011-01) Bienapfl, John Christopher
Root diseases of soybean cause substantial yield reduction in the United States. Fusarium and Phytophthora represent groups of fungal pathogens commonly associated with root rot of soybean. Little is known regarding their distribution, etiology, and how they may interact in causing root rot on soybean. Additionally, diagnostic tools that allow for rapid and accurate detection of these pathogens are essential for disease management, but need to be developed and validated. Furthermore, fungicidal compounds that potentially affect root infection by these fungal pathogens are being studied to minimize yield losses due to root diseases of soybean and improve crop productivity.
The host range of Fusarium virguliforme on rotational crops and common plant species and its survival and growth on crop residue.
(2010-03) Kolander, Tammy Mae
Sudden death syndrome (SDS), caused by Fusarium virguliforme (Fv), is an important soybean (Glycine max) disease. Crop rotation is not an effective management strategy, suggesting that the pathogen may survive long periods in the soil or may infect or be sustained on weeds or crops other than soybean. Minimal research has been conducted to understand Fv survival or its host range and ability to grow on different plants. The objectives of this thesis research were to determine the host range of Fv, the best methods for extracting Fv DNA from crop residue and macrocondia, and to determine how long Fv DNA can be detected on crop residue after burial in a field. Fifteen plant species were inoculated in a greenhouse to determine the host range of Fv. In at least one experiment, soybean, alfalfa (Medicago sativa), pinto bean and navy bean (Phaseolus vulgaris), white clover (Trifolium repens), red clover (T. pretense), pea (Pisum sativum), and Canadian milk vetch (Astragalus canadensis) developed foliar and/or root symptoms. In at least one experiment, corn (Zea mays), wheat (Triticum aestivum), ryegrass (Lolium perenne), pigweed (Amaranthus retroflexus), sugar beet (Beta vulgaris), lambsquarters (Chenopodium album), and canola (Brassica napus) appeared to be asymptomatic hosts for Fv. Thus, multiple plant species may be negatively affected by Fv and/or promote its survival and growth. Three commercial DNA extraction kits were compared to determine which would yield the greatest purity and quantity of Fv DNA from crop residue and macroconidia. The FastDNA® kit was generally most effective for extracting Fv DNA from crop residue and the MO BIO PowerSoil™ kit was superior for extracting Fv DNA from macroconidia. This knowledge was used to determine if Fv DNA can be detected on soybean, corn, alfalfa and wheat residue over time after placement in three crop fields and whether different inoculation methods influence the duration of detection. Soybean and corn tissue that was infected while growing retained detectable amounts of Fv DNA for at least 8 months after burial in the field. Dead tissues inoculated with Fv macroconidia typically did not retain quantifiable amounts of Fv DNA after burial. Results from this study suggest that the inoculation method is important for survival and detection of Fv DNA, and that Fv DNA remains detectable on crop residue from fall into the following summer.
Interactions between plants and antagonistic streptomycetes.
(2011-06) Bakker, Matthew Gene
This work explores a variety of mechanisms through which plants impact associated streptomycete communities. Streptomycete antibiotic-mediated antagonistic activity is a particular focus. Plant host species and plant richness are addressed as independent and interacting factors driving impacts on associated soil microbial communities. The possibility of modification to antagonistic phenotypes through chemical signaling between plants and streptomycetes is explored.
Interactions of Fusarium virguliforme with Other Common Soybean Root Pathogens and the Soybean Aphid
(2020-08) Geske, Alissa
Many diseases reduce soybean (Glycine max) yield every year, with root diseases often having the greatest impacts on yield in the Midwestern U.S. One important root pathogen of soybean is Fusarium virguliforme (Fv), the causal agent of sudden death syndrome (SDS). Even though soybean plants displaying SDS symptoms are commonly attacked by multiple fungal pathogens and insects, it is unknown how interactions between Fv and other attackers may alter SDS disease development and soybean growth. In our first study, we examined potential interactions between Fv and other common soybean root pathogens including F. acuminatum, F. oxysporum, F. solani, and Clonostachys rosea. Interactions were assessed in vitro and in soybean plants to evaluate their effect on SDS disease development and soybean growth. Results from the in vitro studies suggest that the growth of Fv was not affected by any of these fungi, but the growth of F. acuminatum and F. oxysporum isolates was reduced in the presence of Fv. Further results suggest that coinfection of soybean with Fv and C. rosea and Fv and F. solani can reduce SDS foliar disease severity and that coinfection of soybean with Fv and F. oxysporum and Fv and F. acuminatum can increase SDS foliar severity compared to the Fv-only treatment under growth chamber conditions. Coinfections of soybean with Fv and all eight fungal test isolates individually did not increase or decrease levels of root rot or plant biomass compared to infection by Fv alone. In our second study, we examined potential interactions between Fv and the soybean aphid, Aphis glycines, in greenhouse, growth chamber, and field experiments to evaluate their effect on SDS disease development, aphid growth, and soybean growth. Overall, the results suggest that Fv and soybean aphids have minimal to no interaction with one another while co-occurring on soybean when SDS develops to low levels, as was the case in these studies. There was no clear effect of soybean aphid herbivory on SDS foliar or root disease development in these studies nor did we detect a clear effect of combined soybean aphid herbivory and Fv infection on soybean growth. To our knowledge this is the first investigation into potential interactions between Fv and other common root pathogens and Fv and the soybean aphid.
Non-Conventional Methods of Soil-borne Fungal Disease Management in Soybean and Pea
(2009-04-08) Testen, Anna L.
As demand increases for organic and sustainable methods of agricultural production, interest also increases in non-conventional methods of disease management. Non-conventional methods for managing fungal disease, such as biological control and biorational pesticides, are becoming more important as chemical inputs are scrutinized and fungicide resistance develops in pathogen populations. In this study, non-conventional methods of managing soybean sudden death syndrome, a fungal disease caused by Fusarium virguliforme, and Aphanomyces root rot of peas caused by A. euteiches, were compared to conventional (chemical) methods of control. A biological control agent Bacillus pumilus and rapeseed meal were compared to two chemical fungicides in greenhouse assays to determine which treatments could reduce disease severity and maintain plant biomass when plants were subjected to conditions suitable for disease development. B. pumilus was also tested for antagonism in vitro against these pathogens, and all fungi showed reduced growth in the presence of B. pumilus. The results of this study showed that conventional methods of fungal disease control were more effective than non-conventional methods in maintaining plant biomass under conditions conducive to disease. However, the non-conventional methods reduced disease severity and may possibly have value in organic production systems. In the United States, Minnesota is the third largest producer of soybeans and the largest producer of organic soybeans and a major producer of peas, so any treatment that reduces the impact of soil-borne pathogens has the potential to make a large economic impact.
The Parasitism of Colletotrichum Lindemuthianum (Sac. and Mag.) Bri. and Cav.
(1922-05) Leach, J. G.
Relationship between pathogens of Eucalyptus and native Myrtaceae in Uruguay
(2008-08) Perez Rodriguez, Carlos Alberto
Eucalyptus (Myrtaceae) is one of the most important hardwood crops in the world, planted primarily for pulp and timber production. In Uruguay, the area planted to Eucalyptus has tripled in the last 10 years. The explosive increase in the area planted has been associated with increases in disease problems. Despite this, very few investigations have been carried out to study Eucalyptus pathogens and knowledge regarding the etiology, biology and epidemiology of these diseases is limited. Eucalypts are exotics species in Uruguay and pathogens affecting their production could be exotics too. However, it has been demonstrated that different species of native trees could be host to some pathogens affecting eucalypts. Many species belonging to the Myrtaceae have been reported as potential hosts of Eucalyptus pathogens. Since Myrtaceae are dominant species in Uruguayan natural forests, the aim of this study was to determine the relationship between pathogens occurring on Eucalyptus and those occurring on native Myrtaceae. Between 2005 and 2008 several surveys were made to examine fungal infections on both Eucalyptus and native forest trees located geographically close to Eucalyptus plantations. Fungal identification was based on morphological characteristics and confirmed with DNA sequence comparisons. Puccinia psidii, Quambalaria eucalypti , and several species residing in the Botryosphaeriaceae and Mycosphaerellaceae were found occurring in both hosts countrywide. Interestingly, results suggest that most likely host jumps are occurring from native trees to Eucalyptus plantations (eg. P. psidii ) and vice versa (eg. Q. eucalypti and N. eucalyptorum ). These results raise concern about the host speciation of these pathogens and illustrate the danger of moving crop plants between countries, together with fungi that are poorly understood. The negative impact of host jumping events in plant pathology has been well documented and many examples have been cited in the literature. Biotic exchanges are expected to increase as the planted area and age of plantation increase. This study provides a better understanding of the biology and ecology of these pathogens in Uruguay and will assist breeding programs in attempts to obtain disease resistant Eucalyptus plantations. The results also establish new concerns for the threat of these pathogens to native trees.
Remote sensing for detection of Rhizoctonia crown and root rot in sugar beet and the impact of the disease on chlorophyll content.
(2010-05) Reynolds, Gregory James
Abstract summary not available
Studies on Flag Smut of Wheat
(1922-05) Noble, Robert J.
Studies on the Parasitism of Helminthosporium Sativum P. K. & B.
(1922-05) Christensen, Jonas J.
A study of Fusarium graminearum virulence factors.
(2011-05) Menke, Jon R.
The plant pathogen F. graminearum (Gibberella zeae) presents a two-fold threat to farmers and consumers. Not only does this filamentous fungus cause the disease Fusarium head blight (FHB) that results in significant yield loss in infected grains, it also taints these grains with potent mycotoxins harmful to humans, animals, and plants alike. Equally alarming is the evidence that grain can appear to be physically sound while still being significantly contaminated with trichothecene mycotoxins. Tri12 encodes a predicted major facilitator superfamily transporter protein suggested to play a role in the export of trichothecene mycotoxins produced by the Fusarium species. However, the role of Tri12p in toxin sensitivity and plant pathogenicity of Fusarium graminearum was previously unknown. In this study, the correct intron positions for Tri12 in F. graminearum (FgTri12) were established using cDNA sequencing, EST data, and comparative genomics. Reverse genetics was used to establish that FgTri12 plays a role in self-protection and influences toxin production and virulence of the fungus in planta. To identify the subcellular location of FgTri12p during toxin production in culture, FgTri12p was tagged with eGFP. FgTri12p::eGFP was localized in small motile vesicles, the plasma membrane, and the lumen of vacuoles within fungal cells. Treatment of cells with latrunculin A resulted in the absence of motile vesicles labeled with FgTri12p::eGFP, suggesting their formation relies upon actin polymerization. To determine if FgTri12p co-localizes with enzymes involved in trichothecene metabolism, its cellular fate was compared with FgTri1p::eGFP, a fluorescently tagged oxygenase catalyzing a key intermediate step in trichothecene biosynthesis. While FgTri1p::eGFP initially localizes to small motile vesicles and later accumulates in the vacuole, during the period of initial trichothecene biosynthesis it is targeted to the periphery of intermediate sized vesicles, presumed to be the site of toxin synthesis. These results indicate FgTri12 plays a role in self-protection and influences toxin production and virulence of the fungus in planta. The interactions between F. graminearum and its hosts – wheat, rice, or barley – differ in disease severity and the levels of trichothecenes that accumulate in response to infection. The transcriptome of the fungus in rice and wheat was examined in order to identify genes expressed in planta. The hypothesis that fungal genes expressed in planta, but not during growth in culture, could include those that determine the plant infection phenotype was tested by reverse genetics: Four genes expressed exclusively in planta were deleted and two of these were determined to significantly alter disease phenotype. FGSG_03539, also called Tri9, is a previously uncharacterized gene in the major trichothecene biosynthetic gene cluster. A mutant with a tri9 deletion has attenuated virulence and lower trichothecene levels in wheat compared to wild type or a mutant strain complemented with the intact Tri9 gene. FGSG_11164 encodes a predicted trypsin protease and deletion of this gene also results in a small but significant reduction in pathogenicity toward wheat. The results demonstrate that a reverse genetic approach using in planta gene expression data may supplement forward genetic screens for identifying genes encoding virulence factors.
Use of the systemic fungicide propiconazole for oak wilt management: an assessment of uncharacterized host - pathogen - fungicide interactions.
(2009-04) Blaedow, Ryan Arthur
Propiconazole is a systemic fungicide widely used for the control of oak wilt, however, the long-term efficacy of this fungicide has not been well established and treatment effectiveness may be below levels that justify its use in many situations. To date, it is not known if propiconazole applications prevent root graft transmission of Ceratocystis fagacearum nor if they can completely eradicate this pathogen from the root system of an infected tree. Significant translocation of propiconazole into the roots from the point of injection has not been demonstrated and fungitoxic concentrations of the fungicide have not been determined in vivo. Furthermore, symptom development is not induced by pathogen colonization of the root system; oaks respond to infection only when C. fagacearum spreads above ground in the vascular system. Therefore, it is impossible to determine the extent of pathogen distribution in the root systems of trees near disease centers. Treatment failure may result from poorly informed management decisions that do not take into account pathogen distribution or the capabilities of the systemic fungicide being utilized. This dissertation presents the results of three investigations conducted to examine use of propiconazole to control root graft transmission of C. fagacearum in red oaks (Section Lobatae). The distribution of propiconazole in the root system of treated trees was examined using gas chromatography-mass spectrometry. While substantial movement of the fungicide into the root system did occur following injection, the inability of propiconazole to prevent infection or eradicate the pathogen from root-inoculated oaks was demonstrated. The spatial and temporal spread of C. fagacearum in the root system of oaks in or near disease centers, and the role of self- and inter-tree root grafts in pathogen spread was examined. Distribution of C. fagacearum in the root systems of wilted and wilting trees was sporadic, and the prevalence and importance of self-grafts was noted. Finally, the effects of propiconazole on the growth and anatomy of treated oaks was investigated. Plant growth regulating properties of the fungicide suggest that fungitoxicity alone may not be responsible for propiconazoleinduced disease protection. The implications of these previously unknown aspects of the host-pathogen-fungicide interactions in propiconazole treated trees, and suggestions for improved treatment efficacy and oak wilt management are discussed.
Utilization of Hordeum vulgare subsp. spontaneum for improvement of disease resistance in cultivated barley.
(2009-06) Alsop, Benjamin P.
As a result of extensive inbreeding within small populations, cultivated barley (Hordeum vulgare subsp. vulgare) has come to have a narrow genetic base, especially with regard to disease resistance. Wild barley (Hordeum vulgare subsp. spontaneum) is known to be a diverse source of novel alleles for introgression of disease resistance into cultivated barley. Two wild barley accessions collected in Israel, Damon 11-11 and Shechem 12-32, were found to carry resistance to several economically important diseases. Two doubled haploid mapping populations were created from crosses between these two accessions and the two-rowed malting quality standard cultivar Harrington. A new set of wild barley-derived Diversity Arrays Technology (DArT) markers were developed and combined with the previously developed cultivated-derived barley markers to create a ‘comprehensive’ barley DArT array capable of more robust genotyping of both wild and cultivated barley germplasm. This array was utilized to genotype the Damon/Harrington (D/H) and Shechem/Harrington (S/H) populations and to create genetic linkage maps of these two populations as well as a synthetic map of the barley genome containing 3,542 markers. The D/H and S/H populations were phenotyped for resistance to seven economically important diseases, including powdery mildew, stem rust, stripe rust, leaf rust, net blotch, spot blotch, and Septoria speckled leaf blotch. Additionally, several aspects of heading date, inflorescence and leaf characteristics, plant height, and yield were assessed and all traits were subjected to quantitative trait loci (QTL) analyses. Significant QTL were detected for all traits. Identification of markertrait associations for loci controlling both disease resistance and morphological and agronomical traits should help limit linkage drag by allowing for simultaneous selection for and against desirable and undesirable alleles, respectively. These marker-trait associations will be utilized for implementing marker-assisted selection to introgress and maintain favorable alleles into elite breeding germplasm..

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