Cercospora leaf spot (CLS) is a destructive foliar pathogen impacting sugarbeet (Beta vulgaris L.) production worldwide (Weiland & Koch, 2004). The causal fungus, Cercospora beticola Sacc., reduces root yield and recoverable sucrose while increasing sugar impurity concentrations resulting in revenue losses up to 40% (Shane & Teng, 1992; Lamey et al., 1996). Current management strategies rely heavily on fungicide application, host plant resistance, and tillage for inoculum reduction (Khan & Smith, 2005; Miller et al., 1994). Reduced fungicide efficacy and increased resistance to current control mechanisms have been attributed to the high genetic variability, prolific sporulation, and polycyclic life cycle of C. beticola (Rosenzweig et al., 2015; Shrestha et al., 2020). Alternative control measures including foliar applied micronutrients have shown efficacy for maize ear rot, coffee rust, and cucumber powdery mildew (Farahat et al., 2018; Pérez et al., 2020; M. Reuveni et al., 1997; R. Reuveni & M. Reuveni, 1998). New CLS management strategies may need to integrate a balanced plant nutrition program, fungicide rotation, resistant germplasm, and cultural practices to enhance sugarbeet plant health and grower profitability.
Primary C. beticola inoculum in sugarbeet is distributed through wind dispersal of asexual conidia from plant residue or long distance transfer by sugarbeet seed (Spanner et al., 2021; Weiland & Koch, 2004). After conidiation, water-splash, wind, and insects aid in spore transfer to leaf surfaces where hyphae elongate and infect via stomates (Weiland & Koch, 2004). Optimal conditions for CLS development are relative humidity > 60%, prolonged leaf wetness, and air temperatures > 16 °C (61 °F) (Shane & Teng, 1992; Tedford et al., 2018). Following hyphal establishment, toxins are produced within leaf tissue and necrotize cells in close proximity (Rathaiah, 1977; Steinkamp et al., 1979). Symptoms include grey-tan circular lesions with distinct borders that coalesce forming large necrotic areas and death of older leaves (Rangel et al., 2020). Protecting newly emerged leaves is vital for suppressing CLS disease progression and reducing plant stress. Cercospora leaf spot is characterized as a polycyclic disease. C. beticola produces the phytotoxins cercosporin and beticolin both known to debilitate cells and enhance fungal growth throughout the growing season (Weiland & Koch, 2004; Windels et al., 1998). Multiple application timings of foliar protection agents are often necessary to mitigate quality reductions through harvest. Conidia persistence and spore dispersal require a combination of contact and systemic protection via fungicides. While disease control is heavily reliant on fungicide rotation, questions persist regarding the use of foliar boron to aid in CLS leaf surface protection.
A knowledge gap exists regarding the use of B-containing compounds and the potential to aid in CLS management. While sugarbeets have become less responsive to B applications over time in Michigan, B-containing products have been reported to contain fungistatic properties with studies in Egypt observing reduced in vitro growth of C. beticola and decreased in-field CLS severity when utilizing sodium tetraborate and boric acid applications (El-Fawy, 2016; Warncke et al., 2009). Researchers suggested reductions in mycelial growth were related to cell membrane disruption of the pathogen leading to cytoplasmic leakage and death. Additionally, B may stimulate reactive oxygen species accumulation in fungal spores leading to mitochondrial damage (Qin et al., 2010; Shi et al., 2010). Fungistatic properties combined with the role of B in plant defense warrant further CLS management studies.
In addition to root yield and quality, foliar B may affect plant metabolism including cell wall and membrane structure, ion, hormone, and metabolite transfer (Brdar-Jokanović, 2020; Brown & Shelp, 1997; Camacho-cristóbal et al., 2008). Micronutrients such as B function as cofactors or activators of enzyme systems which are pivotal to disease resistance and the production of defense barriers (Datnoff et al., 2007). Key roles of B in cell wall structure and plasma membrane integrity are directly impacted by C. beticola colonization and necrotrophic disruption. Boron deficiencies may decrease root yield, sugar quality, and root quality by inducing ‘heart rot’ symptoms and subsequent ‘dry rot’ within the root (Armin & Asgharipour, 2012; Cox, 1940). Previously, B applications were utilized for preventative management of ‘heart rot’ disease which increased frequency of B application. The demand for B in sugarbeet as an essential nutrient is greater than other field crops. Sufficient leaf tissue concentrations range from 26-80 ppm with observed deficiency symptoms at < 20 ppm (Voth et al., 1979; Robertson & Lucas, 1981; Christenson et al., 1991). Current Michigan soil test B recommendations suggest < 0.7 ppm as deficient and > 1.0 ppm as sufficient with marginal likelihood for deficiency between these values (Warncke et al., 2009).
Boron fertilizer application practices have evolved over time. Previously, bulk fertilizers (i.e., urea, monoammonium phosphate, muriate of potash) contained B impurities which decreased need for supplemental B application (Nelson, 1965). As fertilizer processing and manufacturing improved to produce more highly concentrated fertilizers, the indirect B inclusion within bulk fertilizers no longer occurred. In high pH soils (>7.5) the borate anion (HBO4–) prevails and is subject to leaching. Some field crops grown in rotation with sugarbeet in Michigan (i.e., winter wheat (Triticum aestivum L.), dry beans (Phaseolus vulgaris L.), and soybeans (Glycine max L.)) are sensitive to excess soil B which may limit B application and accumulation within these crop rotations. Additionally, sugarbeet varietal response to supplemental B has decreased in modern varieties (i.e., 2000 and later) on fine-textured Michigan soils (Voth et al., 1979; Christenson et al., 1991; Warncke et al., 2009). The role of improved plant genetics on changes in nutrient demand have not been determined.
Lack of previous response to B application, limited B accumulation in the soil profile, changes in the soil microenvironment (i.e., warmer soil temperatures longer into autumn), and fungicide efficacy may all contribute to increased CLS occurrence. Sources of B and application timing may impact CLS reductions and sugarbeet response to B utilization. Approximately 96% of B uptake occurs as uncharged boric acid molecules with little uptake from borate anions (Bolaños et al., 2004). Application timing is reportedly most effective at 80-100 days after planting with sodium tetraborate and boric acid contributing to increases in yield and quality (Armin & Asgharipour, 2012; Gobarah & Mekki, 2005; Mekdad et al., 2015). Integrating foliar B to improve sugarbeet fertility and reduce CLS may result in synergistic improvements to sugarbeet quality, plant defense mechanisms, and reduced C. beticola growth and sporulation.
The objectives of the current study were to 1) evaluate in-field applications of sodium tetraborate with and without a standard fungicide program on CLS growth and development, and 2) evaluate in vitro growth of C. beticola isolates in response to a concentration gradient of sodium tetraborate and boric acid. The working hypothesis was application of B-containing compounds would reduce in-field CLS severity and incidence and decrease in vitro C. beticola growth.
Materials and Methods
Field trials were established in the 2020 and 2021 growing seasons at the Saginaw Valley Research and Extension center near Richville, MI (43°23’57.3”N, 83°41’49.7”W) on a Tappan-Londo loam (fine-loamy, mixed, active, calcareous, mesic Typic Epiaquoll). Located in northeastern Michigan, the site was non-irrigated, tile-drained, and representative of sugarbeet production across the state. Fields were previously cropped to corn (Zea mays L.) and autumn plowed followed by spring field cultivation (0-10 cm depth). Pre-plant soil characteristics (0-20 cm) were 6.2-7.2 pH (1:1 soil/water), 22-28 g kg-1 soil organic matter (loss-on-ignition), 22-24 mg kg-1 P (Olsen sodium bicarbonate extraction), and 138-178 mg kg-1 K (ammonium acetate method) (Table 1). Prior to planting, soil samples (0-30 cm) for nitrate-N (NO3-N) analysis were air-dried and ground to pass through a 2 mm sieve resulting in pre-plant concentrations of 5.5 and 6.3 mg NO3-N kg-1 soil (nitrate electrode method) in 2020 and 2021, respectively (Gelderman and Beegle, 2015). Monthly precipitation and temperature data were collected and recorded throughout the growing season from Enviro-weather (http://mawn.geo.msu.edu) Michigan State University, East Lansing, MI) (Table 2).
Experimental Procedures
Trials were planted on 7 April 2020 and 5 April 2021 to variety ‘Crystal G932NT’ (ACH Seeds, Inc., Eden Prarie, MN) with a John Deere planter (Deere & Company, Moline, IL). Trials were replanted 7 May 2021 due to a freezing event 23-24 April 2021. Plots measured 3.05 m in width by 10.7 m in length with 76-cm row spacing. Trial consisted of eight treatments arranged as a randomized complete-block design with four replications. Treatments consisted of 1) a non-treated check containing no fungicide or boron, 2) grower standard fungicide program (GS), 3-5) three rates of sodium tetraborate (low, medium, and high) in combination with a grower standard fungicide program (GS+ FBL, GS+FBM, GS+FBH), and 6-8) three rates of sodium tetraborate (low, medium, and high) individually excluding fungicide (FBL, FBM, FBH) (Tables 3 and 4). A CO2-powered backpack sprayer with four TJ 8002XR nozzles (XR TeeJet® Flat Fan Spary Tips; TeeJet® Technologies, 220 Glendale Heights, IL) (76-cm spacing) at 140 L ha-1 was utilized for application every 10-14 days starting 6 July and 28 June in 2020 and 2021, respectively. Fungicides were applied 6, 16, 27 July, 11, 24 August, and 4, 14 September in 2020. Fungicides were applied 28 June, 12, 26, July, 5, 16, 25 August, 9, and 27 September in 2021. All treatments received 101 kg N ha-1 as pre-plant urea. Sidedress 67 kg N ha-1 injected to a 12.7-cm depth halfway between the rows as 28% UAN was applied at the 4-6 leaf stage on 9 June 2020 and 1 June 2021.
Inoculation of C. beticola (1×103 conidia mL-1) was applied at 140 L ha-1 using a tractor mounted sprayer after 20:00 on 9 and 23 July 2020 and 12 July 2021. Inoculum suspensions were prepared from rehydrated desiccated symptomatic leaves, naturally and artificially infested with a mixture of local isolates, collected the previous season (Ruppel & Gaskill, 1971). A precipitation event reduced inoculation efficacy in 2020 resulting in an additional inoculation. Bi-weekly disease ratings began 9 and 26 July and continued to 6 October and 27 September in 2020 and 2021, respectively. Plots were assigned a severity rating using the KWS scale based on infected leaf area: 1=0.1% (1-5 spots/leaf), 2=0.35% (6-12 spots/leaf), 3=0.75% (13-25 spots/leaf), 4=1.5% (26-50 spots/leaf), 5=2.5% (51-75 spots/leaf), 6=3%, 7=6%, 8=12% 9=25%, 10=50% (Kleinwanzlebener Saatzucht, 1970). Incidence (DI, 0-100%) and severity (DS) ratings were utilized to calculate disease index (DX): DI x DS/10 and quantify differences in CLS development among treatments. Disease incidence was recorded to represent the frequency of new lesion activity and severity ratings were used to calculate area under the disease progress curve for disease severity (AUDPC) (Madden et al., 2017).
Plant emergence was counted 20-30 days after planting and prior to harvest to confirm population (data not shown). Fractional green canopy coverage (FGCC) utilizing the software Canopeo and normalized difference vegetation index (NDVI) were collected every 10-14 days coinciding with fungicide application (Patrignani and Ochsner, 2015). The uppermost fully developed and extended leaf and petiole were collected from 25 plants plot-1 at the 12-14 leaf growth stage in 2020. Additional tissue samples were collected at 6-8 leaf, 12-14 leaf, and 18-20 leaf in 2021 to monitor B uptake throughout the growing season. Plant tissue samples were dried at 60°C, mechanically ground to pass through a 1-mm mesh screen and analyzed for total N using a micro-Kjeldahl digestion method and colorimetric analysis with a Lachat rapid flow injector autoanalyzer (Nelson and Sommers, 1973; Bremner, 1996). Beets from the center two rows of each plot were harvested on 14 October 2020 and 20 October 2021 with a mechanical plot harvester and weighed. Root subsamples were collected (10-12 roots plot-1) analyzed for sucrose concentration, extraction percentage, and recoverable sucrose at the Michigan Sugar Co. (MSC) Laboratory (Bay City, MI).
Expected economic net return was calculated using both root yield and recoverable sucrose (kg Mg-1) in addition to MSC’s average payment standard (2020-2021) (Michigan Sugar Company, Bay City, MI). Expected net return was based on US$48.58 Mg-1 and US$24.25 Mg-1 (fresh weight) for sugarbeets in 2020 and 2021, respectively which was later adjusted based on a ratio of observed recoverable sucrose (kg Mg-1) to average MSC recoverable sucrose (kg Mg-1) value. Michigan Sugar Company payment standards were calculated using adjustment factors based on harvest date to determine amount of sugar delivered (kg ha-1). Adjustment factors used were 1.00 and 1.04 for root yield and recoverable sucrose (kg ha-1) and then multiplied by US$0.16 kg-1 and US$0.10 kg-1 to equal total payment ha-1 in 2020 and 2021, respectively. Variable costs including trucking (US$4.13 Mg-1) were subtracted from expected net return across years.
Data were analyzed in SAS v. 9.4 (SAS, Cary, NC) using the GLIMMIX procedure (SAS Institute, 2012). Year and treatment were considered fixed effects and replication as random. The UNIVARIATE procedure in SAS was used to examine the normality of residuals (P ≤ 0.05). Squared and absolute values of residuals were examined with Levene’s Test to confirm homogeneity of variances (P ≤ 0.05). Least square means were separated using the LINES option when ANOVA indicated significance (P ≤ 0.10).
Experimental Procedures for Determining In Vitro Sensitivity of C. beticola
Sensitivity of C. beticola isolates to B was evaluated using a mycelial growth on solid media assay. Trials were arranged in a randomized complete block design with four replications and repeated twice for each isolate and concentration. Treatments consisted of a concentration gradient of 0, 1, 10, 50, 100, 500, and 1000 μg ml-1 sodium tetraborate, boric acid, and thiophanate-methyl (FRAC Group 1). Thiophanate-methyl was selected as a positive control due to higher EC50 value to achieve closest comparison to B compounds. Technical-grade thiophanate-methyl (Millipore Sigma, Burlington, MA) was dissolved in methanol to prepare a stock solution of 7,500 μg ml-1. Technical grade boric acid (Fisher Scientific, Waltham, MA) and sodium tetraborate (20 Mule Team, Borax) were weighed and added to molten media to achieve desired concentration. The test medium was prepared by mixing potato dextrose agar (PDA) 39 g L-1 for 15 minutes, autoclaving at 121°C for 30 minutes, and cooling to 60°C prior to adding appropriate dry boron product or thiophanate-methyl stock quantities to achieve desired concentrations. Media was mixed for 10 minutes (until homogenous) once products were added and maintained at 60 °C for plate transfer. Agar plates were prepared by transferring 20 mL of amended-agar solution to 100 mm x 20 mm Petri dishes for constant depths. Nonamended control plates consisted only of PDA.
Cercospora beticola isolates ‘Blum 1-2’ and ‘Range A’ were obtained from the United States Department of Agriculture- ARS Sugar Beet Research Unit (SBRU) fungal collection. ‘Blum 1-2’ was obtained from symptomatic sugarbeet leaf lesions in Saginaw County, MI in 2017. ‘Range A’ was collected from a symptomatic sugarbeet leaf in Ingham County, MI in 2008. Single spore transfer protocols were utilized to obtain pure cultures with fungal ball storage at -20 ℃. Isolates were cultured on clarified V8 (CV8) agar medium to produced inoculum and incubated at room temperature (21-24°C) for 30 days. Five-mm agar discs were excised from the actively growing margin of the colony. Agar disks were inverted, and a single disk was placed in the center of each amended PDA plate and incubated at room temperature for 21 days. Cercospora beticola radial growth diameter was collected every seven days. Diameters were corrected for the 5-mm agar disk. Diameters after 21 days were used to calculate growth relative to the control for each replicate and EC50 values were generated using R version 4.1.2 (R Core Team, 2022) with the three-parameter log-logistic (LL.3) function in package ‘drc’ (Ritz et al., 2015). Mean EC50 values were obtained from each experimental repetition. Means were further analyzed in a generalized linear mixed model (GLIMMIX) ANOVA in SAS v. 9.4 (SAS, Cary, NC). Isolate and compound were considered fixed effects while experimental repetition was considered a random effect.
Results and Discussion
Environmental Conditions
April through September growing season precipitation was 12.5% and 8.3% below the 30-yr mean in 2020 and 2021, respectively (Table 2). Cool April soil temperatures combined with deficit June 2020 precipitation (i.e., 55% below the 30-yr mean) slowed plant emergence and development. In 2021, April and May rainfall was 75% and 65% below the 30-yr mean, respectively, resulting in delayed emergence. June precipitation, however, was 50% above the 30-yr mean, contributing to favorable conditions for disease. Except for April 2020, monthly growing season air temperatures were near or above the 30-yr mean. Above average April 2021 soil temperatures resulted in a 5 April planting date but a frost on 21 April resulted in replanting the field trial on 5 May. The 2021 replant resulted in minimal impact on sugarbeet emergence and early season growth.
Effect of Sodium Tetraborate and Boric Acid on In Vitro Growth of Cercospora beticola
Relative radial growth of C. beticola grown in vitro decreased with inclusion of sodium tetraborate and boric acid (Table 5). Across both isolates, radial growth decreased 14-19% with sodium tetraborate as compared to the control. Boric acid reduced mean radial growth 5-10% in Range A and Blum 1-2. Thiophanate-methyl (i.e., positive control for both isolates) demonstrated 8 and 86% growth reductions in Range A and Blum 1-2, respectively. A polymerase chain reaction – restriction fragment length polymorphism (PCR-RFLP) analysis of the β-tubulin gene (Rosenzweig et al., 2015) confirmed the E198A point mutation conferring benzimidazole resistance in the Range A isolate resulting in minimal effectiveness by thiophanate-methyl as compared to Blum 1-2. Benzimidazole resistance was previously identified in Michigan C. beticola isolates in the 1990s further contributing to current CLS control challenges (Rosenzweig et al., 2015).
Estimated EC50 values (i.e., value of half maximal concentration) for control of C. beticola were significantly lower with sodium tetraborate ranging from 772 and 876 mg kg-1 for Blum 1-2 and Range A, respectively, indicating greater effectiveness than boric acid (Table 6). Estimated EC50 values for boric acid exceeded 1,000 mg kg-1 for Range A and Blum 1-2. Response of C. beticola may be impacted by pH as the pH of the boric acid and sodium tetraborate solutions were 5.1 and 9.3, respectively, indicating that growth of C. beticola decreased at greater pH. Iamandei et al. (2013) evaluated the influence of pH on in vitro development of C. beticola colonies observing a wide pH spectrum in which vegetative mass and condia growth began at a pH of 3.0 with optimal values falling between 4.0 and 7.0. Vegetative C. beticola growth declined as pH increased but continued to produce numerous conidia.
Labeled rates of current B-containing products indicate that EC50 values could be achieved with in-field applications. However, labeled rates of boric acid and 100% sodium tetraborate (i.e., borax) products may range from 2,000-9,000 mg kg-1 B or 200-900 mg kg-1 B, respectively, for a single in-field application with variations in product use rate, active ingredient, and B concentrations strongly influencing concentration ranges. Current B-containing products are formulated to correct B nutrient deficiencies across a wide range of crops. With minimal B concentrations needed to correct soil and plant tissue nutrient deficiencies, current products may not be sufficient for disease management due to the higher required concentrations. Utilizing greater B concentrations for CLS control could result in secondary impacts including B toxicity to ensuing highly sensitive crop rotations.
Environmental fate of foliar B is a determining factor in both disease and nutritional response. Plant response is species-specific and highly dependent on method of application, soil characteristics, temperature, and humidity leading to discrepancies in environmental fate and plant B utilization (Brdar-Jokanović, 2020). Soil pH and trace element interaction are known to affect B availability and ion reactions in soil (Ibekwe et al., 2010). In arid and semiarid irrigated areas, high soil B concentrations are often associated with high salt concentrations and can be a limiting factor to plant growth (Ayars et al., 1993; Grieve & Poss, 2008; Shouse et al., 2006). El-Fawy (2016) reported significant CLS reductions and increased root yield and recoverable sucrose with B application in El-Behera Governorate, Egypt. Yield reductions up to 60% have been attributed to salinity levels in similar regions of Egypt as compared to soils with reduced salinity levels (Ahmed Bakry et al., 2014). While C. beticola response was attributed to application of foliar B, soil salinity levels of this region may have increased plant B response for improved sugarbeet quality thus ultimately improving response to CLS.
Effect of Foliar Boron and Fungicide on Root Yield, Quality, and Expected Net Return
Increased air temperatures combined with adequate precipitation resulted in root yields ranging between 44.8-89.3 Mg ha-1 (20-40 T A-1) in 2021 as compared to 40.0-60.9 Mg ha-1 (17.9-27.2 T A-1) in 2020. Replanting did not reduce yield in 2021. Across site years, application of foliar B did not increase root yield when compared to the GS treatment (Table 7, 8). In 2020, the FBH treatment reduced root yield while FBL and FBM yielded similar to GS. In 2021, B treatments with a GS fungicide program reduced root yield > 22.2 Mg ha-1 ( > 9.9 T A-1)across treatments. In saline soils, Gobarah & Mekki (2005) observed application of up to 3.7 kg B ha-1 (3.3 lb B A-1) applied as sodium borate increased root length, diameter, and yield with the highest recoverable sucrose at rates of 4.9 kg ha-1 (4.4 lb B A-1). In Michigan, current soil test B recommendations suggest < 0.7 mg kg-1 as deficient and > 1.0 mg kg-1 as sufficient with marginal deficiency conditions in-between these values (Warncke et al., 2009). Current B recommendations indicate 1.1 kg B ha-1 (1.0 lb B A-1) may be beneficial with 2.2 kg B ha-1 (2.0 lb B A-1) in coarse-textured soils (Vitosh et al., 2006). Soil B concentrations of 1.2 and 0.8 mg kg-1 in 2020 and 2021, respectively, indicate sufficiency (Table 1) and therefore less probability of a positive impact from foliar B applications on root yield across site years.
Similar to root yield, sugarbeet quality parameters indicated lack of response to foliar B. The addition of foliar B did not improve recoverable sucrose in 2020 with FBH individually decreasing recoverable sucrose per hectare (Table 7). In 2021, recoverable sucrose per hectare decreased > 48-57% with treatments excluding the GS program (Table 8). Plant response to foliar B is dependent upon soil physical and chemical properties (i.e., nutrient solubility, solution pH, surface tension, retention, and molecular structure of B source), environmental conditions, and leaf characteristics which may help determine the efficacy, uptake, and usage of foliar nutrient solutions (Fernandez & Brown, 2013; Fernandez & Eichert, 2009). Application of sodium borate and boric acid have been linked to increases in recoverable sucrose and improved juice purity by decreasing Na and K uptake (Abdallah & Mekdad, 2015; Armin & Asgharipour, 2012; Dordas et al., 2007). Tissue nutrient concentration of B remained sufficient (i.e., 32-46 ppm) throughout the growing season for all treatments (data not shown) indicating foliar B was not limiting and not likely to affect sugarbeet yield and quality. Root yield and recoverable sucrose results indicate foliar B applications did not provide or enhance protection from CLS.
Root yield and quality responses to foliar B were also reflected in economic return across site years. Expected net return was similar or reduced as compared to the grower standard fungicide program when including foliar B individually or when combined with a standard fungicide program (Table 9). Profitability was similar between the GS, GS+FBL, and GS+FBH treatments with reductions > $973.00 ha-1 ($394 A-1) when removing fungicide applications altogether in 2021. Increases in recoverable sucrose and root yield did not translate to profitability when considering both sugar volume and quality parameters (Table 9).
Effect of Foliar Boron and Fungicide on CLS Development
In 2020, no significant disease index (DX) differences were detected between foliar B rates for the entire growing season (Table 10). All treatments containing fungicide reduced DX values with no impact of foliar B. Similar results were recorded through 9 Sept. 2021. However, a final DX rating on 27 Sept. 2021 demonstrated reduced DX with FBM as compared to FBL. Disease development was delayed in 2020 with initial symptoms beginning 20 Aug. Despite inoculation, CLS did not develop until later in the season resulting in a smaller window for treatment effectiveness. Absence of disease during the first half of the growing season allowed increased canopy development thus reducing risk for production losses in treatments excluding fungicide. Decreased June precipitation and sporadic rainfall events in July likely extended the symptomless biotrophic phase of C. beticola colonization in 2020 (Table 2). Lesion development occurs as the fungus transitions to a necrotrophic phase (Ebert et al., 2021). Without adequate moisture, relative humidity below 90-95%, and overnight temperatures remaining < 16 °C, sporulation was reduced between June – August 2020 resulting in delayed infection. June 2021 precipitation was frequent with rainfall events taking place on 19 of 30 days resulting in improved conditions for early-season infection (initial symptoms 20 Jul) followed by near to above normal precipitation and above normal air temperatures for the remainder of the growing season.
Significant differences in fractional green canopy coverage (FGCC) occurred across both study years (Table 11). The grower standard fungicide program maintained greater canopy coverage and helped determine treatment differences when included within any treatment whereas foliar B applications individually resulted in significantly reduced canopy coverage across both years. No differences in normalized difference vegetation index (NDVI) occurred throughout 2020 but did occur in 2021 with fungicide application again producing greener plants with more biomass and lower CLS occurrence (Table 12). Area under the disease progress curve (AUDPC) values indicated reduced CLS control with treatments excluding fungicide in 2020 (Table 13). In 2021, AUDPC of FBL was greater than the grower standard fungicide program while FBM and FBH did not differ.
Due to coalescing lesions and loss of older leaves, CLS can be difficult to precisely measure (Steddom et al., 2007). Vegetative indices are largely impacted by percentage of photosynthetically active tissue resulting in difficulty monitoring treatment differences in canopy reflectance as affected by foliar B. Rating variability and physiological response of sugarbeet to C. beticola induce challenges to quantify differences among treatments without severe levels of infection. Early CLS pressure in 2021 had a greater impact on measurable response.
Effect of Foliar Boron and Fungicide on Tissue B Concentration
Across site years, tissue B concentrations remained > 32 ppm at the 14-16 leaf stage indicating sufficiency. In 2021, an additional sample timing was included to evaluate foliar B uptake throughout the growing season. Late season tissue samples demonstrated greater B tissue concentrations (43-46 mg kg-1) when including fungicide as compared to no fungicide (38-40 mg kg-1). Despite statistical differences, B tissue concentrations were above critical thresholds for all treatments. Sugarbeet has one of the larger B requirements among field crops with reported sufficient leaf tissue concentrations ranging between 26-80 mg kg-1 and observed deficiency symptoms at < 20 mg kg-1 (Voth et al., 1979; Robertson and Lucas, 1981; Christenson et al., 1991). Sufficient (i.e., > 0.7 mg kg-1) B soil test levels, above critical tissue B concentrations (i.e., > 20 mg kg-1), and application of B in the form of sodium tetraborate reduced the likelihood of plant response by means of foliar uptake.
Physical and chemical leaf surface conditions are fundamental to parasitic microorganism development that initiate at the leaf boundary and may also affect the efficiency and persistence of foliar applied pesticides (Oertli et al., 1977). Potential buffering of leaf surface pH may impact effectiveness of foliar B on CLS control. Hutchinson et al. (1986) examined neutralizing abilities of sugarbeet, radish (Raphanus sativus L.), sunflower (Helianthus L.), and wormwood (Artemisia tilesii L) to acid rain ranging in pH from 2.4-4.7. Radish, sunflower, and wormwood significantly increased pH in all droplets while sugarbeet resulted in little to no change. The mechanism behind acid rain neutralization may be facilitated by leaching and exchange of base cations (e.g., Ca2+, K+, Mg+ and Na+ for H+) on leaf surfaces induced by cell membrane and cuticle damage (Tukey, 1980). Lack of acidic droplet neutralization by sugarbeet was attributed to absence of leaf injury as compared to other species examined (Hutchinson et al., 1986). The sodium tetraborate product used in the current study has a pH range between 6-7 reducing the direct impact on leaf surface pH. However, a combination of cuticle injury due to necrotic lesions of CLS may influence sugarbeet leaf ion exchange resulting in neutralization of alternative B-containing compounds (i.e., boric acid).
In addition to chemically altering the foliar microenvironment, B-containing compounds may prevent or reduce parasitic spore germination by synthesis of substances such as phytoalexins (Oertli et al., 1977). Researchers suggest foliar application of B, Mn, and Cu result in exchange of Ca2+ cations from cell walls and interact with salicylic acid (involved with phytoalexin response) to activate resistance mechanisms in the host plant (M. Reuveni et al., 1997; R. Reuveni & M. Reuveni, 1998). While foliar B may support natural plant resistance, application is unlikely to overcome rapid development of CLS. Intact cuticles of sugarbeet, slow rates of ion exchange, low susceptibility of inorganic ion leaching, and limited sodium borate uptake indicate that foliar B application may not be an effective strategy for CLS management (Bolaños et al., 2004; Tukey & Tukey, 1962). Lack of disease response, reductions in root yield, and decreased quality suggest foliar B failed to provide disease suppression in current field environments.
Toxin Role in C. beticola Development and Pathogenicity
The pathogenicity of C. beticola is driven by cercosporin, a photoactivated polyketide toxin that acts as a cell membrane sensitizer and producer of singlet oxygen (Daub & Briggs, 1983; Mitchell et al., 2002). Peroxidation of membrane lipids leads to membrane breakdown, cell death, and leakage of nutrients into leaf intercellular spaces allowing for fungal growth and sporulation (Daub & Briggs, 1983). In addition to cercosporin, beticolins are non-host-specific phytotoxins of C. beticola that induce loss of electrolytes, amino acids, and betacyanin via ion channel formation and permeabilization of host cell membranes (Goudet et al., 2000; Macrì & Vianello, 1979). Physiological parameters including pH, nutrient conditions, temperature, and C:N ratios all influence toxin production (Daub & Ehrenshaft, 2000). Toxin production in culture is highly variable among and within species. Cercospora beticola isolates are capable of producing cercosporin, beticolin, or both (Daub & Chung, 2007). While cercosporin and beticolin aid in host pathogenicity of C. beticola, auto resistance (AR) is essential for self-protection (Rangel et al., 2020). Cercospora AR is facilitated by toxin export and reductive detoxification of the cercosporin molecule (Daub et al., 1992; Leisman & Daub, 1992; Sollod et al., 1992). Cercosporin derivatives absorb less light and generate significantly less singlet oxygen (1O2) when stably methylated and acetylated compared to wild‐type cercosporin (Leisman & Daub, 1992). Herrero et al. (2007) conducted cercosporin toxicity assays to evaluate isolate strain AR sensitivity to pH and discovered the crg1-null strain of C. nicotianae to cercosporin was strongly impacted by pH. In the presence of cercosporin on media at pH levels < 6, observations included almost complete lack of growth in the presence of cercosporin suggesting certain isolates lack detection of acidic environments and inability to adjust intracellular pH creating cercosporin susceptibility (Herrero et al., 2007). Environmental conditions including changes in pH and ion concentration may influence methylation and acetylation of cercosporin or reduction in isolate AR resulting in altered pathogenicity.
Cercosporin and beticolin levels were not quantified in the current study. However, notable differences in isolate color were consistent with varying concentrations of sodium tetraborate and boric acid (Figs. 1, 2). Cercosporin is characterized by red pigments that turn green in alkaline conditions, while beticolins are yellow in color and turn orange with pH increase (Goodwin & Dunkle, 2010; Goudet et al., 1998). Changes in isolate color suggest that cercosporin and beticolin production may be influenced by presence of B-containing compounds and altered growth media pH. You et al. (2008) reported changing pH values to 4.0 –7.0 reduced cercosporin and isolate radial growth compared to non-buffered medium. However, addition of citrate or phosphate buffers caused cercosporin reduction regardless of the pH values indicating solution buffer directly impacts cercosporin production. Further examination of metal ions (Zn2+, Fe3+, Co2+, Mn2+, Cu2+, and Mg2+) slightly enhanced or had no effect on cercosporin production unlike high quantities of Na+ or K+ which inhibited cercosporin production (You et al., 2008). The role of cercosporin and beticolin in cell membrane disruption, nutrient leakage, and alteration of ion concentration suggests that leaf surface microenvironment directly impacts cercosporin and beticolin production in sugarbeet. Visual differences in isolate color indicate change in toxin production and suggest altered pathogenicity of C. beticola and potential for enhanced host defense by means of ion exchange.
Conclusions
Foliar B applications did not reduce CLS in field environments across site years. Grower standard fungicide treatment increased root yield, recoverable sucrose, and canopy coverage with minimal differences detected among foliar B rates. Plant health indicators such as NDVI, fractional green canopy coverage (FGCC), and DX did not support improvement in CLS protection with foliar B. Radial growth of C. beticola decreased with increasing concentrations of B in vitro. Sodium tetraborate more effectively inhibited growth than boric acid. Differences in growth response and estimated EC50 values could be attributed to secondary physiological effects based on increasing pH. Boron-compounds were not as effective as thiophanate-methyl with regard to mycelial growth reduction. Previous findings of reduced CLS with B application in sugarbeet may be due to increased plant health and nutritional improvement rather than improved disease resistance. Evaluation of soil test levels, sugarbeet varietal characteristics, and environmental and disease conditions are necessary to make appropriate B recommendations. Reduced control options, increased CLS resistance, and increased sugarbeet B requirements enhance the need for further evaluation of alternative CLS control measures. In-field evaluation of various B timings, increased B concentrations, and addition of B-containing compounds may contribute to future CLS control.
Acknowledgements
The authors would like to thank the USDA National Institute of Food and Agriculture, Michigan Sugar Company, Michigan State University College of Agriculture and Natural Resources, Michigan State University AgBioResearch, and MSU-Project GREEEN for partial funding and support of this research. In addition, the authors would like to thank Andrew Chomas, undergraduate research assistants, graduate research assistants, and research farm staff for their support and assistance.
LIST OF TABLES
Table 1. Soil physical and chemical properties including mean NO3-N (0 – 30 cm), P (0 – 20 cm), and K soil test (0 – 20 cm) nutrient concentrations obtained prior to sugarbeet planting, Richville, MI, 2020-2021.
Year |
Soil |
NO3–N |
Soil test† | ||||
description | P | K | B | pH | OM | ||
mg kg–1 | g kg–1 | ||||||
2020 | Tappan-Londo Loam | 5.5 | 24 | 138 | 1.2 | 7.2 | 22 |
2021 | Tappan-Londo Loam | 6.3 | 22 | 178 | 0.8 | 6.2 | 28 |
†P phosphorus (Olsen sodium bicarbonate extraction); K potassium (ammonium acetate extractable K).
Table 2. Mean monthly and 30-yr precipitation† and temperature for the sugarbeet growing season, Richville, MI, 2020 – 2021.
Year | Apr. | May | Jun. | Jul. | Aug. | Sept. | Total |
cm (in | |||||||
2020 | 5.3 (2.1) | 9.5 (3.7) | 3.4 (1.3) | 8.2 (3.2) | 8.6 (3.4) | 7.1 (2.8) | 42.1 (16.6) |
2021 | 1.8 (0.7) | 3.0 (1.2) | 11.4 (4.5) | 7.3 (2.9) | 7.8 (3.1) | 12.8 (5.0) | 44.1 (17.4) |
30-yr‡ avg. | 7.3 (2.9) | 8.6 (3.4) | 7.6 (3.0) | 6.6 (2.6) | 8.1(3.2) | 9.9 (3.9) | 48.1 (18.9) |
°C (°F | |||||||
2020 | 6.2 (43) | 13.8 (57) | 20.6 (69) | 23.7 (75) | 21.4 (71) | 15.8 (60) | — |
2021 | 9.3 (49) | 14.1 (57) | 21.8 (71) | 21.3 (70) | 22.8 (73) | 17.6 (64) | — |
30-yr avg. | 7.4 (45) | 13.2 (56) | 18.7 (66) | 20.9 (70) | 19.7 (68) | 15.8 (60) | — |
†Precipitation and air temperature data were collected from Michigan State University Enviro-weather (https://mawn.geo.msu.edu). ‡30-yr means were obtained from the National Oceanic and Atmospheric Administration (https://www.ncdc.noaa.gov/cdo-web/datatools/normals).
Table 3. Treatment design and application timings for sugarbeet field trial evaluating boron applications with and without standard fungicide program for control of Cercospora leaf spot, Richville, MI, 2020.
Treatment | Product Rate† and Timing‡ |
Grower Standard Fungicide | Manzate Max (3.7 L)[1.6 qt] ABCDEF + Inspire XT (0.5 L)[7 fl oz] ADF + Super Tin (0.6 L)[8 fl oz] BE + Priaxor (0.6 L)[8 fl oz], Topsin (1.5 L)[20 fl oz] C + Badge (2.3 L)[2 pt] G |
Foliar Boron – Low No Fungicide | SprayBor (112 g)[0.1 lb] ABCDEFG |
Foliar Boron – Medium No Fungicide | SprayBor (280 g)[0.25 lb] ABCDEFG |
Foliar Boron – High No Fungicide | SprayBor (560 g)[0.5 lb] ABCDEFG |
Grower Standard + Foliar Boron Low | SprayBor (112 g)[0.1 lb] ABCDEFG +Manzate Max (3.7 L) [1.6 qt] ABCDEF + Inspire XT (0.5 L)[7 fl oz] ADF + Super Tin (0.6 L)[8 fl oz] BE + Priaxor (0.6 L)[8 fl oz], Topsin (1.5 L)[20 fl oz] C + Badge (2.3 L)[2 pt] G |
Grower Standard + Foliar Boron Medium | SprayBor (280 g)[0.25 lb] ABCDEFG +Manzate Max (3.7 L) [1.6 qt] ABCDEF + Inspire XT (0.5 L)[7 fl oz] ADF
+ Super Tin (0.6 L)[8 fl oz] BE + Priaxor (0.6 L)[8 fl oz], Topsin (1.5 L)[20 fl oz] C + Badge (2.3 L)[2 pt] G |
Grower Standard + Foliar Boron High | SprayBor (560 g)[0.5 lb] ABCDEFG +Manzate Max (3.7 L) [1.6 qt] ABCDEF + Inspire XT (0.5 L)[7 fl oz] ADF
+ Super Tin (0.6 L)[8 fl oz] BE + Priaxor (0.6 L)[8 fl oz], Topsin (1.5 L)[20 fl oz] C + Badge (2.3 L)[2 pt] G |
Check | No Fungicide, No Foliar Boron |
†All rates, unless otherwise specified, are listed as a measure of product per hectare followed by product per acre. ‡Application letters code for the following dates: A=6 Jul, B=16 Jul, C=27 Jul, D=11 Aug, E=24 Aug, F=4 Sept, G=14 Sept.
Table 4. Treatment design and application timings for sugarbeet field trial evaluating boron applications with and without standard fungicide program for control of Cercospora leaf spot, Richville, MI, 2021.
Treatment | Product Rate† and Timing‡ |
Grower Standard Fungicide | Manzate Max (3.7 L)[1.6 qt] ABCDEFG + Inspire XT (0.5 L)[7 fl oz] BEG + Super Tin (0.6 L)[8 fl oz] CF + Priaxor (0.6 L)[8 fl oz], Topsin (1.5 L)[20 fl oz] D + Badge (2.3 L)[2 pt] H |
Foliar Boron – Low No Fungicide | SprayBor (112 g)[0.1 lb] ABCDEFGH |
Foliar Boron – Medium No Fungicide | SprayBor (280 g)[0.25 lb] ABCDEFGH |
Foliar Boron – High No Fungicide | SprayBor (560 g)[0.5 lb] ABCDEFGH |
Grower Standard + Foliar Boron Low | SprayBor (112 g)[0.1 lb] ABCDEFGH +Manzate Max (3.7 L)[1.6 qt] ABCDEFG + Inspire XT (0.5 L)[7 fl oz] BEG
+ Super Tin (0.6 L)[8 fl oz] CF + Priaxor (0.6 L)[8 fl oz], Topsin (1.5 L)[20 fl oz] D + Badge (2.3 L)[2 pt] H |
Grower Standard + Foliar Boron Medium | SprayBor (280 g)[0.25 lb] ABCDEFGH +Manzate Max (3.7 L)[1.6 qt] ABCDEF + Inspire XT (0.5 L)[7 fl oz] BEG + Super Tin (0.6 L)[8 fl oz] CF + Priaxor (0.6 L)[8 fl oz], Topsin (1.5 L)[20 fl oz] D + Badge (2.3 L)[2 pt] H |
Grower Standard + Foliar Boron High | SprayBor (560 g)[0.5 lb] ABCDEFGH +Manzate Max (3.7 L)[1.6 qt] ABCDEF + Inspire XT (0.5 L)[7 fl oz] BEG
+ Super Tin (0.6 L)[8 fl oz] CF + Priaxor (0.6 L)[8 fl oz], Topsin (1.5 L)[20 fl oz] D + Badge (2.3 L)[2 pt] H |
Check | No Fungicide, No Foliar Boron |
†All rates, unless otherwise specified, are listed as a measure of product per hectare followed by product per acre. ‡ Application letters code for the following dates: A=28 Jun, B=12 Jul, C=26 Jul, D=5 Aug, E=16 Aug, F=25 Aug, G=9 Sept, H=27 Sept.
Table 5. Relative radial growth for Blum 1-2 and Range A as affected by isolate and compound.
Isolate | Compound | Relative Growth |
Blum 1-2 | Boric Acid | 0.90 ab |
Blum 1-2 | Sodium Tetraborate | 0.86 bc |
Blum 1-2 | Thiophanate-Methyl | 0.14 d |
Range A | Boric Acid | 0.95 a |
Range A | Sodium Tetraborate | 0.81 c |
Range A | Thiophanate-Methyl | 0.92 ab |
P > F | < 0.01 |
†Means followed by the same lowercase letter are not significantly different at (α=0.1). ‡ Relative growth (21 days) calculated as compared to control.
Table 6. Estimated EC50 values for Blum 1-2 and Range A as affected by compound.
Isolate | Compound | EC † Estimate mg kg-1
50 |
Blum 1-2 | Boric Acid | >1000 |
Blum 1-2 | Sodium Tetraborate | 772 |
Blum 1-2 | Thiophanate-Methyl | 0.35 |
Range A | Boric Acid | >1000 |
Range A | Sodium Tetraborate | 876 |
Range A | Thiophanate-Methyl | >1000 |
†Value of half maximal effective concentration i.e., 50% growth reduction as compared to control.
Table 7. Sugarbeet root yield, recoverable sucrose (kg ha-1 and kg Mg-1), sucrose concentration, and extraction in response to fungicide and foliar boron, Richville, MI, 2020.
Treatment | Root Yield | Recoverable Sucrose | Sucrose | Extraction | |
-Mg ha-1–
(T A-1) |
-kg ha–1–
(lb A–1) |
-kg Mg-1–
(lb T–1) |
–%– | –%– | |
Grower Standard (GS) Fungicide | 55.2 abc† (24.6) | 7389 ab
(6591) |
134 a
(268) |
17.9 a | 95.9 |
Foliar Boron – Low (FBL), No Fungicide | 59.5 ab (26.5) | 7561 ab
(6745) |
127 b
(254) |
17.1 b | 95.5 |
Foliar Boron – Medium (FBM), No Fungicide | 46.6 cd
(20.8) |
5900 bc
(5263) |
126 b
(252) |
16.9 b | 95.6 |
Foliar Boron – High (FBH), No Fungicide | 40.0 d
(17.9) |
5107 c
(4556) |
126 b
(252) |
17.0 b | 95.7 |
Grower Standard + FBL | 52.5 abc
(23.4) |
7109 ab
(6342) |
135 a
(270) |
18.0 a | 95.9 |
Grower Standard + FBM | 55.0 abc
(24.5) |
7361 ab
(6567) |
133 a
(266) |
17.7 a | 95.8 |
Grower Standard + FBH | 60.9 a
(27.2) |
8172 a
(7290) |
134 a
(268) |
17.9 a | 95.8 |
Check – No Fungicide, No Boron | 47.2 bcd
(21.1) |
5878 bc
(5244) |
124 b
(248) |
16.7 b | 95.7 |
P > F | = 0.09 | <0.01 | = 0.06 | < 0.01 | NS |
†Means in the same column following by the same lowercase letter are not significantly different at P ≤ 0.10.
Table 8. Sugarbeet root yield, recoverable sucrose (kg ha-1 and kg Mg-1), sucrose concentration, and extraction in response to fungicide and foliar boron, Richville, MI, 2021.
Treatment | Root Yield | Recoverable Sucrose | Sucrose | Extraction | |
-Mg ha-1–
(T A-1) |
-kg ha-1–
(lb A–1) |
-kg Mg-1–
(lb T–1) |
–%– | –%– | |
Grower Standard (GS) Fungicide | 89.3 a†
(39.8) |
10759 a
(9598) |
121 a
(241) |
16.4 a | 94.9 |
Foliar Boron – Low (FBL), No Fungicide | 54.3 c
(24.2) |
5577 c
(4975) |
103 b
(205) |
14.2 b | 94.5 |
Foliar Boron – Medium (FBM), No Fungicide | 44.8 d
(20.0) |
4571 c
(4078) |
101 b
(202) |
14.0 b | 94.2 |
Foliar Boron – High (FBH), No Fungicide | 52.2 cd
(23.3) |
5331 c
(4756) |
102 b
(204) |
14.2 b | 94.7 |
Grower Standard + FBL | 82.7 ab
(36.9) |
10045 ab
(8961) |
122 a
(243) |
16.4 a | 94.5 |
Grower Standard + FBM | 77.4 b
(34.5) |
8962 b
(7995) |
116 a
(232) |
15.8 a | 94.7 |
Grower Standard + FBH | 76.5 b
(34.1) |
9797 ab
(8740) |
121 a
(241) |
16.3 a | 95.0 |
Check – No Fungicide, No Boron | 54.0 cd
(24.1) |
5526 c
(4930) |
103 b
(205) |
14.2 b | 94.5 |
P > F | <0.01 | <0.01 | <0.01 | < 0.01 | NS |
†Means in the same column following by the same lowercase letter are not significantly different at P ≤ 0.10.
Table 9. Sugarbeet expected net return and expected net return minus trucking costs as affected by foliar boron and fun- gicide combinations, Richville, MI, 2020-21.
Treatment | Expected Net Return ‡ | Expected Net Return Minus Trucking Costs | ||
US$ ha-1 ( US$ A–1 | ||||
2020 | 2021 | 2020 | 2021 | |
Grower Standard (GS) Fungicide | 2929 ab†
(1186) |
2481 a
(1005) |
2701 ab
(1094) |
2112 a
(855) |
Foliar Boron – Low (FBL), No Fungicide | 2999 ab
(1215) |
1286 c
(520) |
2753 ab
(1115) |
1061 c
(430) |
Foliar Boron – Medium (FBM), No Fungicide | 2338 bc
(947) |
1054 c
(427) |
2146 bc
(869) |
868 c
(352) |
Foliar Boron – High (FBH), No Fungicide | 2024 c
(820) |
1229 c
(498) |
1859 c
(753) |
1013 c
(410) |
Grower Standard + FBL | 2818 ab
(1141) |
2316 ab
(938) |
2601 ab
(1053) |
1975 ab
(800) |
Grower Standard + FBM | 2917 ab
(1181) |
2066 b
(837) |
2690 ab
(1089) |
1746 b
(707) |
Grower Standard + FBH | 3196 a
(1294) |
2259 ab
(915) |
2945 a
(1193) |
1923 ab
(779) |
Check – No Fungicide, No Boron | 2330 bc
(944) |
1274 c
(516) |
2134 bc
(864) |
1051c (426) |
P > F | = 0.08 | < 0.01 | = 0.08 | < 0.01 |
†Means in the same column following by the same lowercase letter are not significantly different at P ≤ 0.10. ‡Expected net returns based upon MSC payment adjustment with volume and quality incentives and trucking costs of $US$4.13 Mg-1 or $US$3.75 T-1 .
Table 10. Sugarbeet final disease index (DX, %) ratings Richville, MI 2020-21.
Treatment | 2020 | 2021 | ||
Sept. 14‡ | Oct. 6 | Sept. 9 | Sept. 27 | |
Grower Standard (GS) Fungicide | 0.88 b† | 1.8 b | 17.3 b | 41.5 c |
Foliar Boron – Low (FBL), No Fungicide | 73.5 a | 90.3 a | 89.0 a | 73.8 a |
Foliar Boron – Medium (FBM), No Fungicide | 70.4 a | 85.3 a | 87.5 a | 61.3 b |
Foliar Boron – High (FBH), No Fungicide | 71.5 a | 83.5 a | 88.8 a | 71.3 ab |
Grower Standard + FBL | 2.1 b | 4.0 b | 21.3 b | 30.0 cd |
Grower Standard + FBM | 1.0 b | 2.5 b | 20.3 b | 31.3 cd |
Grower Standard + FBH | 1.3 b | 2.1 b | 12.5 b | 28.0 d |
Check – No Fungicide, No Boron | 77.5 a | 87.5 a | 90.0 a | 80.0 a |
P > F | <0.01 | <0.01 | <0.01 | <0.01 |
†Means followed by the same lowercase letter are not significantly different within rating date at (α=0.1). ‡ Disease index calculated from disease incidence and severity ratings recorded every 10-14 days post infection.
Table 11. Sugarbeet fractional green canopy coverage (FGCC) as affected by fungicide and foliar boron Richville, MI 2020-21.
Treatment | 2020 | 2021 | ||
Sept. 14 | Oct. 6 | Sept. 9 | Sept. 27 | |
canopy | ||||
Grower Standard (GS) Fungicide | 75.4 a | 77.3 a | 87.0 a | 87.0 a |
Foliar Boron – Low (FBL), No Fungicide | 49.7 b | 37.4 c | 35.1 c | 29.0 c |
Foliar Boron – Medium (FBM), No Fungicide | 54.5 b | 39.5 c | 33.7 c | 30.0 c |
Foliar Boron – High (FBH), No Fungicide | 48.7 b | 37.0 c | 35.6 c | 32.0 c |
Grower Standard + FBL | 70.9 a | 67.5 b | 82.6 ab | 82.0 ab |
Grower Standard + FBM | 72.2 a | 68.5 ab | 80.0 b | 79.0 b |
Grower Standard + FBH | 70.9 a | 71.9 ab | 82.6 ab | 79.0 b |
Check – No Fungicide, No Boron | 55.3 b | 38.7 c | 34.6 c | 33.0 c |
P > F < 0.01 < 0.01 <0.01 <0.01 |
†Means followed by the same lowercase letter are not significantly different at (α=0.1).
Table 12. Sugarbeet normalized difference vegetation index (NDVI) as affected by fungicide and foliar boron Richville, MI 2020-21.
Treatment | 2020 | 2021 | ||
Sept. 14 | Oct. 6 | Sept. 9 | Sept. 27 | |
Grower Standard (GS) Fungicide | 0.80 | 0.74 | 0.85 a | 0.89 a |
Foliar Boron – Low (FBL), No Fungicide | 0.73 | 0.63 | 0.62 b | 0.74 b |
Foliar Boron – Medium (FBM), No Fungicide | 0.82 | 0.72 | 0.61 b | 0.77 b |
Foliar Boron – High (FBH), No Fungicide | 0.71 | 0.61 | 0.56 b | 0.74 b |
Grower Standard + FBL | 0.83 | 0.72 | 0.81 a | 0.86 a |
Grower Standard + FBM | 0.82 | 0.77 | 0.82 a | 0.86 a |
Grower Standard + FBH | 0.76 | 0.68 | 0.81 a | 0.87 a |
Check – No Fungicide, No Boron | 0.80 | 0.68 | 0.57 b | 0.73 b |
P > F | NS | NS | <0.01 | <0.01 |
†Means followed by the same lowercase letter are not significantly different at (α=0.1).
Table 13. Area under the disease progress curve (AUDPC) as affected by fungicide and foliar boron Richville, MI 2020-21.
Treatment | 2020 | 2021 |
Grower Standard (GS) Fungicide | 62.4 c | 285.1 bc |
Foliar Boron – Low (FBL), No Fungicide | 356.8 b | 371.6 a |
Foliar Boron – Medium (FBM), No Fungicide | 550.0 a | 339.0 ab |
Foliar Boron – High (FBH), No Fungicide | 337.8 b | 343.4 ab |
Grower Standard + FBL | 57.9 c | 201.3 d |
Grower Standard + FBM | 37.9 c | 280.4 bc |
Grower Standard + FBH | 41.4 c | 223.1 cd |
Check – No Fungicide, No Boron | 355.6 b | 337.4 a |
P > F | < 0.01 | < 0.01 |
†Means followed by the same lowercase letter are not significantly different at (α=0.1).
LIST OF FIGURES
Figure 1. Day 21 radial growth of C. beticola isolate ‘Blum 1-2.’
†Sodium tetraborate (1A), boric acid (1B), thiophanate-methyl (1C) concentrations displayed left to right (0, 1, 10, 50, 100, 300, 500 ppm).
Figure 2. Day 21 radial growth of C. beticola isolate ‘Range A.’
†Sodium tetraborate (2A), boric acid (2B), thiophanate-methyl (2C) concentrations displayed left to right (0, 1, 10, 50, 100, 300, 500 ppm).
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