Neil Gudmestad - Thesis Committee

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    Parasitic Fitness of SDHI-Sensitive and -Resistant Isolates of Alternaria Solani
    (North Dakota State University, 2017) Bauske, Mitchell James
    Early blight of potato (Solanum tuberosum L.) is caused by Alternaria solani Sorauer. The single-site mode of action chemistries of the succinate dehydrogenase inhibitors (SDHIs) and quinone outside inhibitors (QoIs) have been widely used for early blight control but resistance has developed rapidly to a number of fungicide chemistries. QoI resistance in A. solani has been attributed to the F129L mutation, or the substitution of phenylalanine with leucine at position 129. Resistance to SDHI fungicides in A. solani is conferred by five known point mutations on three AsSdh genes. Over 1,300 A. solani isolates collected from 11 states in 2013 through 2015 were characterized for the presence of mutations associated with QoI and SDHI resistance through real-time, SDH multiplex, and mismatch amplification analysis (MAMA) polymerase chain reaction (PCR). Approximately 95% of isolates collected from 2013 to 2015 possessed the F129L mutation. Additionally, 95% of the A. solani population was determined to be SDHI-resistant, with the most prevalent mutation being on the AsSdhC gene. All A. solani isolates collected that were characterized as possessing the D123E mutation, or the substitution of aspartic acid for glutamic acid at position 123 in the AsSdhD gene, were evaluated for boscalid and fluopyram sensitivity in vitro. Furthermore, 15 isolates characterized as being SDHI-sensitive or -resistant, including five D123E-mutant isolates, were evaluated in vivo for percentage disease control to boscalid and fluopyram. Sensitivity of D123E-mutant isolates to fluopyram ranged from 0.8 to 3 ug/ml, and were found to be sensitive or only slightly higher than those of baseline isolates, ranging from 0.1 to 0.6 ug/ml. Disease control of all five D123E isolates evaluated in vivo was reduced significantly with the application of fluopyram compared to SDHI-sensitive isolates. Fitness was compared among 120 SDHI-sensitive and -resistant A. solani isolates using the parameters of spore germination and mycelial growth in vitro and aggressiveness in vivo. Spore germination and mycelial growth between SDHI-sensitive and -resistant isolates was not significantly different. However, D123E-mutant isolates were significantly more aggressive in in vivo assays compared to other SDHI-resistant and SDHI-sensitive isolates. These results illustrate the importance of implementing fungicide resistance management strategies.
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    Spatial and Temporal Sensitivity of Alternaria Species Associated with Potato Foliar Diseases to Demethylation Inhibiting and Anilino-Pyrimidine Fungicides
    (North Dakota State University, 2015) Fonseka Gunawardena, Arthur Dimitri Lakshan
    Early blight and brown spot, caused by Alternaria solani and Alternaria alternata, respectively, are important foliar diseases of potato, affecting both tuber yield and quality. Most of the commercial cultivars lack resistance; therefore, application of foliar fungicides remains a primary management strategy. Correlation coefficients comparing EC50 values for conidial germination and mycelial growth of A. solani and A. alternata in response to boscalid and fluopyram, respectively, were low, indicating that the association between these two in vitro assays was very weak. Baseline sensitivities of Alternaria spp. to difenoconazole, metconazole, and pyrimethanil using mycelial growth assays demonstrated high intrinsic activity against the two pathogens. Six out of 245 A. solani isolates exhibited reduced-sensitivity to pyrimethanil in in vitro assays and reduced-sensitive isolates were not controlled except at 100 μg/ml in greenhouse efficacy tests. The DMI chemistries and pyrimethanil remain valuable options for fungicide rotation programs in areas of high disease pressure.
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    Developing Management Tools for Sunflower Rust (Puccinia Helianthi)
    (North Dakota State University, 2013) Friskop, Andrew John
    Sunflower rust, caused by Puccinia helianthi (Schwein), is an economically important disease in North Dakota. Since 2008, sunflower rust incidence and severity have increased. Therefore, a four year study was initiated to examine the management strategies for the disease. A total of 19 fungicide efficacy and timing trials were conducted from 2008-2011. Results indicated that DMI and QoI chemistries can be used effectively to manage the disease. Timing trials indicated that disease control was highest when fungicide applications were made at R5.0-R5.8. Results indicate a fungicide application is warranted when rust severities at approximately 1% are found on the upper-four leaves at R5. To observe phenotype variability in the pathogen, a P. helianthi survey was completed in 2011 and 2012. Single-pustule isolates were obtained and virulence phenotypes were evaluated on a set of nine differentials. Race characterization was assigned based on virulence phenotypes. In 2011, the most commonly detected races were 300 and 304, while the most virulent was 776. In 2012, races 304 and 324 were the most commonly detected and the most virulent was 777. To identify new sources of rust resistance, the core-set of Helianthus annuus germplasm was obtained from the USDA-North Central Regional Plant Introduction Station. The accession lines were screened both in the greenhouse and the field. The accessions were screened in the greenhouse individually to races 300, 304, 336, 337, and 777. Lines were screened in the field to a mixture of P. helianthi isolates coding to 300, 304, 336, and 337. The majority of lines were susceptible in both the field and greenhouse. At both field locations, PI 431538, PI 432512, and PI 650362 had year-end severities under 1%. Similarly, PI lines 432512 and 650362 had resistant infection types across all five races. The results of these studies provide information and tools that are being used currently to manage rust and will contribute to management in the future.
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    Biology and Development of Two Wilt Fungi of Potato: Verticilliam Dahliae and Colletotrichum Coccodes
    (North Dakota State University, 2012) Pasche, Julie Sherman
    The demand for high quality tubers for the frozen processing industry has exerted increased pressure on producers to control diseases that can compromise tuber quality, including, C. coccodes and V. dahliae. Infection of potato plants by C. coccodes, the causal agent of black dot, can result in foliar necrosis, plant wilting, yield loss, tuber vascular discoloration and skin blemishes. Black dot of potato can originate from foliar-, seed- of soil-borne infections and, all plant tissues can be affected. The development of colonization in above- and below-ground plant tissues, from each inoculation/infestation source, was evaluated at two sites across two growing seasons. Colonization of potato tissue was detected at the first sampling date, even prior to plant emergence. While disease resulted from natural inoculum only, as inoculation/infestation sources increased, so did host colonization. Disease development also was greater in treatments with more than one site of inoculation/infestation. Overall, it was determined that stem tissue was colonized at greater frequency than root or stolon tissue. Infection of potato plants with V. dahliae, the causal agent of Verticillium wilt, occurs mainly through root contact with infested soil and can result in premature senescence as well as losses from decreased tuber yield and quality. Control of this pathogen is difficult and expensive and, therefore, efforts recently have increased towards the development of resistance. Resistance to V. dahliae in potato is thought to be multigenic, and therefore, quantification of the host:pathogen interaction is required to accurately define the level of resistance in a particular cultivar. Unfortunately, current methods used for quantification of V. dahliae in potato stems are time and labor intensive. A real-time duplex quantitative PCR assay was developed to simplify pathogen quantification to help breeding programs and researchers identify resistance in germplasm and cultivars. QPCR assays were validated using plant material from greenhouse and field trials, demonstrating specificity for V. dahliae, as well as sensitivity and accuracy when compared to traditional plating assays. Results from greenhouse and field evaluations also indicated that resistance is present among the eight russet-skinned cultivars evaluated in these studies.
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    Movement and Accumulation of Candidatus Liberibacter Solanacearum in Potato Plants
    (North Dakota State University, 2012) Rodriguez, Juan Jose
    A new disease affecting potatoes was first detected in Mexico in 1993. Affected plants had aerial symptoms similar to those caused by potato purple top and psyllid yellows, but tubers had internal brown discoloration when sliced and dark stripes and streaks when processed to produce potato chips. The disease has been found in many potato production areas in Guatemala, Mexico, Honduras, New Zealand and the United States. The disease, termed Zebra Chip (ZC), has been associated with the presence of heavy infestations of the potato-tomato psyllid (Bactericera cockerelli). In 2009, a research group in New Zealand discovered that a new disease in tomato and pepper plants was caused by Candidatus Liberibacter solanacearum (Lso) and subsequently this same bacterium was associated with ZC in potato samples from Texas. The objectives of this study were: to assess the accumulation of Lso in various potato organs, to determine the effect of plant age on detection of Lso, symptom development and plant death, and (iii) to determine the effect of phosphorous acid on the development of ZC. Results from these studies showed significant differences in Lso populations between above and below ground tissues of the potato plant, with Lso populations in stolons and tubers being three to four times higher than those of leaf tissue and over seventy times greater than in stems. Time for detection of Lso by PCR in potato leaves of different ages at the time of inoculation ranged from 21 to 26 days after inoculation, symptoms development took 23 to 36 days. Plant death, took 24 to 47 days in plants of different age groups at the time of inoculation. In plants 15 weeks old at the time of inoculation, Lso was detected after 14 days in one plant out of 18; in plants 16 weeks old at the time of inoculation, Lso was detected after seven days in two plants out of 18. Phosphorous acid applications had no effect on the populations of Lso in potato tubers, onset of symptoms or plant death. All tubers showed ZC symptoms, making them unacceptable for the market.
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    Studies on the Development and Management of Powdery Scab and Root Gall Formation Caused by Spongospora Subterranea on Potato (Solanum Tuberosum L.)
    (North Dakota State University, 2015) Bittara-Molina, Francisco Gabriel
    The biotroph protozoan Spongospora subterranea causes root gall formation and powdery scab on potato. Symptoms on tubers affect directly the quality and marketability of the harvested product while infection in roots are associated with yield reductions. Moreover, S. subterranea is the vector of the Potato mop-top virus. The management of the disease is difficult due to the limited number of current control options and requires the integration of control measures among which host resistance represents the most economically and long-term approach. This dissertation focuses on the evaluation of management strategies for the control of powdery scab and root gall formation. In the first study, a total of 43 potato cultivars and 80 advanced clones representing a range of skin types were assessed for their response to powdery scab and root gall formation in five field experiments. High levels of resistance was observed among genotypes against the development of disease on tubers and roots which accounted for 32.5% of the evaluated population. Resistance to powdery scab and root gall formation was shown to follow a continuum from very susceptible to very resistant. In the second study, the effect of chloropicrin soil fumigation on the soil populations of S. subterranea and the concomitant development of disease in roots and tubers was investigated in seven field trials. Results indicated a reduction of pathogen soil inoculum, which in most cases was accompanied by an increase in yield and disease on tubers and roots. These results were confirmed in controlled condition experiments in which the amount of pathogen DNA detected in roots increased with the fumigant rate. In the third study, reciprocal grafts between ‘Shepody’ (susceptible to powdery scab and root gall formation) and ‘Dakota Trailblazer’ (resistant) were prepared in order to assess the effect of grafting on the translocation of susceptibility factors affecting the infection and development of root galls. The amount of pathogen DNA increased, as did the number of root galls, on graft combinations involving ‘Shepody’. The results presented in this dissertation highlight the importance of cultivar selection in the management of root gall formation and powdery scab on tubers.
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    Studies on the Management of Potato Mop-Top Virus-Induced Tuber Necrosis
    (North Dakota State University, 2015) Domfeh, Owusu
    Tuber necrosis caused by the Potato Mop-top Virus (PMTV) has become one of the most important tuber necrosis diseases in the United States. PMTV is transmitted by the powdery scab pathogen (Spongospora subterranea f.sp subterranea (Sss)) and no effective methods of control are currently available for these pathogens. Potato cultivars have been reported to exhibit natural variability in their reaction to PMTV infection, making cultivar selection a viable management option. This dissertation focuses on finding short to long term strategies for managing PMTV tuber necrosis. In the first study, potato cultivars were assessed for sensitivity to PMTV-induced tuber necrosis in three separate field trials. Results of tuber assessments demonstrated that sensitivity to PMTV-induced tuber necrosis among cultivars follows a continuum of tolerant to sensitive. In the second study, advanced breeding selections of potato were evaluated for sensitivity to PMTV-induced tuber necrosis. The results revealed high variability in PMTV-induced tuber necrosis incidence and severity among selections and identified 17 of them to be tolerant, nine – moderately tolerant, eight - moderately sensitive, while six were found to be sensitive. Results of the field trials show that russet-skinned cultivars and selections are less sensitive to PMTV tuber necrosis than red-, yellow- and white-skinned types. In the third study, a growth chamber experiment was conducted to investigate the potential of using moisture regime to manage PMTV tuber necrosis. The results showed significant differences in PMTV tuber necrosis and powdery scab infection among moisture regimes. Maintaining soil moisture level at 90% field capacity throughout the season resulted in significantly higher incidence and severity of PMTV tuber necrosis and powdery scab infection than keeping soil at 60% field capacity. The results also show that the potato plant may be susceptible to PMTV-induced tuber necrosis and powdery scab infection throughout the season. The results of these investigations offer potato growers the option to plant less sensitive cultivars in areas where PMTV and powdery scab exist. This information can be utilized in future breeding efforts to obtain resistant potato cultivars. Useful information on using soil moisture regime as a management strategy for PMTV tuber necrosis has been provided.
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    An Application of Association Rule Mining to Unit Test Selection
    (North Dakota State University, 2013) Gunderson, Karl Nils
    Appropriate selection of unit tests during the software development process is vital when many unit tests exist. The developer may be unfamiliar with some tests and non-obvious relationships between application code and test code may exist. Poor test selection may lead to defects. This is especially true when the application is large and many developers are involved. By the application of association rule mining to the unit test selection process and by comparison with extant selection techniques, we will provide a quantitative analysis of the benefits of heuristic and its limit to development where process patterns are stable.