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Phoma black stem (PBS), caused by Phoma macdonaldii Boerema (teleomorph Leptosphaeria lindquistii Frezzi), is the most common stem disease of sunflower (Helianthus annuus L.) in the northern Great Plains region of the United States. However, the impact of PBS on sunflower yield in the United States is unclear, and a near complete absence of information on the impact of fungicides on disease management exists. The objectives of this study were to determine the impact of PBS on sunflower yield, the efficacy of available fungicides, the optimal fungicide application timing, and the economic viability of fungicides as a management tool. Fungicide timing efficacy was evaluated by applying single and/or sequential applications of pyraclostrobin fungicide at three sunflower growth stages in 10 field trials between 2017 and 2019. Efficacy of 10 fungicides from the Fungicide Resistance Action Committee (FRAC) groups 3, 7, and 11 were evaluated in four field trials between 2018 and 2019. The impact of treatments on PBS were evaluated by determination of incidence, severity, maximum lesion height, disease severity index (DSI), and harvested yield. Nine of the 10 fungicides evaluated and all fungicide timings that included an early bud application resulted in disease reductions when compared with the nontreated controls. The DSI was negatively correlated to sunflower yield in high-yield environments (P = 0.0004; R2 = 0.3425) but not in low- or moderate-yield environments. Although FRAC 7 fungicides were generally most efficacious, the sufficient efficacy and lower cost of FRAC 11 fungicides make them more economically viable in high-yielding environments at current market conditions.

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Root-lesion nematode (RLN; Pratylenchus neglectus) is a migratory endoparasite and a major soilborne pathogen that affects wheat (Triticum spp.) production worldwide. Genetic resistance is one of the most economical and effective ways to manage P. neglectus in wheat. This study evaluated 37 local cultivars and germplasm lines in seven greenhouse experiments, including 26 hexaploid wheat, six durum wheat, two synthetic hexaploid wheat, one emmer wheat, and two triticale for P. neglectus resistance from 2016 to 2020. North Dakota field soils infested with two RLN populations (350 to 1,125 nematodes per kilogram of soil) were used for resistance screening under controlled greenhouse conditions. The final nematode population density for each cultivar and line was counted under the microscope to categorize the resistance ranking of these entries as resistant, moderately resistant, moderately susceptible, and susceptible. Out of the 37 cultivars and lines, one was classified as resistant (Brennan); 18 were moderately resistant (Divide, Carpio, Prosper, Advance, Alkabo, SY Soren, Barlow, Bolles, Select, Faller, Briggs, WB Mayville, SY Ingmar, W7984, PI 626573, Ben, Grandin, and Villax St. Jose); 11 were moderately susceptible; and seven were susceptible to P. neglectus. The resistant to moderately resistant lines identified in this study could be used in breeding programs after the resistance genes or loci are further elucidated. This research provides valuable information about P. neglectus resistance among wheat and triticale cultivars used in the Upper Midwest region of the United States.

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In the semiarid regions of North Dakota and Montana, low annual precipitation favors production of high-quality durum wheat (Triticum turgidum subsp. durum). However, conducive weather conditions for disease epidemics have occurred more frequently in recent years. Modification of planting date can reduce disease risk by decreasing the timeframe in which a susceptible crop overlaps with conducive disease conditions. The effect of planting date on fungal leaf spotting diseases (leaf spot), ergot, Fusarium head blight (FHB), and yield of durum was evaluated in 11 experiments across four sites in eastern Montana and western North Dakota. Six durum cultivars with differing levels of susceptibility to leaf spot and FHB were planted at three planting dates from 2017 to 2019. Early planting maximized yield and influenced ergot incidence. Although there was no effect of planting date, reduced susceptibility to leaf spot and FHB was associated with a reduction in leaf spotting disease severity and deoxynivalenol, respectively, in the harvested grain. Growers in the semiarid regions of these states should prioritize the selection of disease-resistant cultivars to help manage sporadic disease outbreaks and continue to plant early to maximize yield.

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Goss's bacterial wilt and leaf blight (Goss's wilt) of corn is the most important corn disease in North Dakota (ND), and yield loss due to the disease has not been reliably quantified in northern corn growing regions. To help quantify the amount of yield loss caused by Goss's wilt, a total of six field experiments were conducted from 2015 to 2017. Experiments were designed in a randomized complete block with a split plot arrangement. Hybrids served as main plots and Clavibacter nebraskensis inoculation timings as subplots. Three hybrids were used and classified as susceptible, moderately susceptible, and resistant. Inoculation timings included a noninoculated control, six to 10 leaf collars (V6 to V10), reproductive silk stage (R1), or a sequential combination of V6 to V10 and R1. A high level of disease (greater than 50% on susceptible hybrid) occurred in three experiments, a low level of disease (less than 5% on susceptible hybrid) in one experiment, and no disease was reported in two experiments. A combined analysis of the high disease experiments indicated yield losses of 34 to 41% on the susceptible hybrid when C. nebraskensis inoculation occurred at V6 to V10. Yield losses of 22 to 25% occurred on the moderately susceptible hybrid when C. nebraskensis inoculation occurred at V6 to V10, and statistical differences in yield loss were not found among inoculations timings on the resistant hybrid. Correlation analyses suggest that for every 1% increase in R1 disease severity on the susceptible hybrid, yield was reduced by 117 kg/ha (1.9 bu/acre). The current study further demonstrates the importance of hybrid resistance and provides updated yield loss information on Goss's wilt in a northern corn growing region.

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Downy mildew is a yield-limiting disease of sunflower, caused by the pathogen Plasmopara halstedii. Zoospore infection of root tissue shortly after planting results in systemic infection, causing postemergence damping off or severe stunting and head sterility. Although fungicide-applied seed treatments can be an effective management tool, the pathogen is resistant to phenylamide fungicides in many growing regions, and other available fungicides have limited efficacy. Oxathiapiprolin, the first member of the piperidinyl thiazole isoxazoline fungicides, was evaluated for efficacy on downy mildew in field trials conducted from 2011 to 2015 in North Dakota. Throughout the course of the study, the rate range was narrowed from active ingredient (a.i.) at 0.45 to 116.0 µg a.i. seed-1 to an optimal effective rate of 9.37 to 18.75 µg a.i. seed-1. Within that optimal range, the downy mildew incidence of sunflower planted with oxathiapiprolin-treated seed was significantly lower than the incidence in the nontreated sunflower in all 11 trials with disease pressure. Additionally, downy mildew incidence of sunflower planted with oxathiapiprolin-treated seed was significantly lower than sunflower planted with competitive commercially available fungicide-treated seed in 10 of those 11 trials. The use of oxathiapiprolin by sunflower growers is likely to reduce disease incidence and subsequent yield loss to downy mildew.

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Sunflower rust is an important yield-limiting disease in sunflower production in the Great Plains of the United States. Rust severity and incidence have increased between 2002 and 2011, and genetic resistance is limited in most commercial hybrids, particularly the high-value confectionary market type. Although fungicides are available for rust management in the United States, management recommendations are insufficient. Specifically, efficacy and timing data are very limited for fungicides in FRAC groups 7 and 11. Seventeen fungicide efficacy and timing trials were conducted between 2008 and 2011 in North Dakota. Timings evaluated across the four years included single or multiple applications at growth stages (GS): GS V8-V12 (late vegetative), GS R1 (terminal bud formation), GS R3-4 (elongation of bud), GS R5 (flowering), and GS R6 (completion of flowering). With few exceptions, fungicide applications of DMIs and QoIs controlled disease greater than SDHI fungicides. Fungicide applications made at R5, either singly or in combination, consistently resulted in greater disease control. A negative correlation (r = -0.7756) between disease control and yield was observed, resulting in a yield reduction of 6.6% for every 1% increase in disease severity.

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Puccinia helianthi, causal agent of sunflower rust, is a macrocyclic and autoecious pathogen. Widespread sexual reproduction of P. helianthi was documented in North Dakota and Nebraska for the first time in 2008 and has since frequently occurred. Concurrently, an increase in sunflower rust incidence, severity, and subsequent yield loss on sunflower has occurred since 2008. Rust can be managed with resistance genes but determination of virulence phenotypes is important for effective gene deployment and hybrid selection. However, the only P. helianthi virulence data available in the United States was generated prior to 2009 and consisted of aggregate virulence phenotypes determined on bulk field collections. The objective of this study was to determine the phenotypic diversity of P. helianthi in the United States. P. helianthi collections were made from cultivated, volunteer, and wild Helianthus spp. at 104 locations across seven U.S. states and one Canadian province in 2011 and 2012. Virulence phenotypes of 238 single-pustule isolates were determined on the internationally accepted differential set. In total, 29 races were identified, with races 300 and 304 occurring most frequently in 2011 and races 304 and 324 occurring most frequently in 2012. Differences in race prevalence occurred between survey years and across geography but were similar among host types. Four isolates virulent to all genes in the differential set (race 777) were identified. The resistance genes found in differential lines HA-R3 (R4b), MC29 (R2 and R10), and HA-R2 (R5) conferred resistance to 96.6, 83.6, and 78.6% of the isolates tested, respectively.