Management of Soybean Cyst Nematode and Sudden Death Syndrome with Nematode-Protectant Seed Treatments Across Multiple Environments in Soybean.

As soybean (Glycine max) production continues to expand in the United States and Canada, so do pathogens and pests that directly threaten soybean yield potential and economic returns for farmers. One such pathogen is the soybean cyst nematode (SCN; Heterodera glycines). SCN has traditionally been managed using SCN-resistant cultivars and rotation with nonhost crops, but the interaction of SCN with sudden death syndrome (SDS; caused by Fusarium virguliforme) in the field makes management more difficult. Nematode-protectant seed treatments have become options for SCN and SDS management. The objectives of this study were to evaluate nematode-protectant seed treatments for their effects on (i) early and full season SCN reproduction, (ii) foliar symptoms and root-rot caused by SDS, and (iii) soybean yield across environments accounting for the above factors. Using a standard protocol, field trials were implemented in 13 states and one Canadian province from 2019 to 2021 constituting 51 site-years. Six nematode-protectant seed treatment products were compared with a fungicide + insecticide base treatment and a nontreated check. Initial (at soybean planting) and final (at soybean harvest) SCN egg populations were enumerated, and SCN females were extracted from roots and counted at 30 to 35 days postplanting. Foliar disease index (FDX) and root rot caused by the SDS pathogen were evaluated, and yield data were collected for each plot. No seed treatment offered significant nematode control versus the nontreated check for in-season and full-season nematode response, no matter the initial SCN population or FDX level. Of all treatments, ILEVO (fluopyram) and Saltro (pydiflumetofen) provided more consistent increases in yield over the nontreated check in a broader range of SCN environments, even when FDX level was high.

First Report of Charcoal Rot Caused by Macrophomina phaseolina on Hemp (Cannabis sativa) in Missouri.

Charcoal rot, caused by Macrophomina phaseolina, is abundantly present in the soil and has been reported as pathogenic to both soybean and corn, as well as numerous other hosts, including hemp grown for fiber, grain, and cannabinoids (Casano et al. 2018; Su et al. 2001). Hemp (Cannabis sativa) production in Missouri was a relatively new addition to the 2021 growing season. Charcoal rot was reported in Reynolds, Knox, and Boone counties in Missouri from commercial and experimental fields. One of the fields in question experienced heavy disease pressure and had an uneven stand loss, but the total loss was estimated at approximately 60% of the field and was attributed to charcoal rot. Charcoal rot signs and symptoms, microsclerotia on the lower stem and root tissue, wilting and stem discoloration, were observed on a majority of the hemp plants received at the University of Missouri Plant Diagnostic Clinic in July and late Fall of 2021, including samples from Bradford Research Farm in Boone County and Greenley Research Center in Knox County. Root and crown tissue from the hemp plants from the Greenley Research Center were cultured onto acidified potato dextrose agar (APDA). Macrophomina phaseolina and other fungi grew from the plated tissue after about three days of incubation at room temperature. Macrophomina phaseolina was confirmed based on the presence of melanized hyphae and microsclerotia (Siddique et al. 2021). The microsclerotia were black, round to ovoid shaped and ranged from about 34-87 µm (average 64 µm) in length and 32-134 µm (average 65 µm) in width (n = 44). A single-hyphae isolation from a putative M. phaseolina isolate was conducted to obtain a pure culture. The M. phaseolina culture from the Greenley Research Center was used to complete Koch's postulates of charcoal rot on four hemp cultivars. Sterilized toothpicks were added to pure cultures of M. phaseolina on APDA and incubated at room temperature for one week to allow for colonization and for use in greenhouse inoculation. Four hemp cultivars (Katani, Grandi, CFX-2, and CRS-1) were grown in a sterilized silt loam for three weeks in a greenhouse. About four plants per cultivar were grown for inoculation and one plant per cultivar was used as a control. The plants were inoculated with the M. phaseolina colonized toothpicks that were gently rubbed onto stem tissue and subsequently inserted into the soil at the stem. For six weeks, the plants were kept in greenhouse conditions of 25°C with a 12-hour light and dark cycle and were watered when soil appeared dry. Plants were kept in a loosely sealed container constructed from wood and vinyl sheeting to minimize cross contamination with other plants grown in the same greenhouse. Plants were monitored weekly for charcoal rot symptoms. Symptoms that resembled charcoal rot, wilting and microsclerotia on the lower stem, were present on inoculated plants after about four weeks and symptoms were not present on the control plants. Isolates resembling M. phaseolina in culture were recovered from symptomatic plants; therefore, Koch's postulates were successfully fulfilled and the fungus was recovered from the inoculated plants. DNA was extracted from the pure cultures of both the initial isolate and the isolate obtained from Koch's postulates using GeneJet Plant Genomic DNA Purification Kit (Thermo Scientific, California, USA) and the internal transcribed spacer (ITS) region of ribosomal DNA including ITS1, 5.8S, and ITS4 regions were amplified using universal primers ITS1 and ITS4 (White et al. 1990). The ITS region was sequenced and compared to reference sequences in GenBank by BLAST analysis. Recovered isolates (GenBank accession no. OQ455934.1) showed closest sequence similarity (100%) to M. phaseolina accession number GU046909.1. Little is known about the life cycle, growth conditions, and possible inoculum buildup in the soil in hemp in Missouri. In addition, M. phaseolina is a known pathogen of corn and soybean and effective management strategies are challenging for these crops as well due to the broad host range of the pathogen. Cultural management practices, such as crop rotations to reduce inoculum in the soil and closely monitoring for symptoms, may help reduce the severity of this disease.

Effects of ILeVO Seed Treatment on Heterodera glycines Reproduction and Soybean Yield in Small-Plot and Strip-Trial Experiments in Iowa.

ILeVO (fluopyram) is a fungicide seed treatment for soybean sudden death syndrome (SDS) that also has nematicidal activity. ILeVO is sold with a base of insecticide Poncho (clothianidin), nematode-protectant VOTiVO (Bacillus firmus), and Acceleron fungicides (metalaxyl, fluxapyroxad, and pyraclostrobin). Yield and reproduction of the soybean cyst nematode (SCN) (Heterodera glycines) on soybean plants grown from seed treated with ILeVO plus the base were compared with those treated with only the base in 27 small-plot experiments and 12 strip-trial experiments across Iowa from 2015 to 2017. To increase the likelihood that yield results were related to effects on SCN, data were used only from 26 small-plot experiments and 12 strip trials in which symptoms of SDS were low or nonexistent. An SCN reproductive factor (RF) was calculated for each experimental unit by dividing the SCN population density at harvest by the population density at planting. ILeVO significantly reduced SCN RF by 50% in one strip-trial experiment and by 36 to 60% in four small-plot experiments but yields were not increased by ILeVO in any of those five experiments. Soybean yields were 2.8 to 3.7 bushels/acre (bu/ac) (188.3 to 248.8 kg/ha) greater with ILeVO in three small-plot experiments but SCN RF was not reduced in those experiments. Also, yield was 1.9 bu/ac (127.8 kg/ha) greater with ILeVO in one strip-trial experiment in 2016 but SCN samples were not collected at harvest from the study to assess the possible effects of ILeVO on SCN reproduction. When strip-trial data from 2015 and 2016 were combined, there was a small but significant 0.8 bu/ac (52.2 kg/ha) yield increase with ILeVO. Overall, the effects of ILeVO on SCN reproduction and soybean yield were variable in these field studies.

Effect of Wheat Cultivar on the Concentration of Fusarium Mycotoxins in Wheat Stems.

Effective control of Fusarium-mycotoxin accumulation in grain affected by Fusarium head blight (FHB) (caused by Fusarium graminearum) begins with selecting moderately resistant wheat cultivars; however, little is known about how this resistance affects mycotoxin levels in the stem. A study was conducted from 2011 to 2014 in a mist-irrigated FHB nursery in Urbana, IL to determine whether the FHB resistance class of a cultivar (very susceptible, susceptible, moderately susceptible, and moderately resistant) affects the concentration of Fusarium mycotoxins in the stem. FHB incidence, FHB severity, and Fusarium-damaged kernel ratings were collected and used to calculate FHB index; incidence, severity, and kernel damage (ISK) index; and deoxynivalenol (DON), incidence, severity, and kernel damage (DISK) index. Grain was assayed for levels of DON, and the bottom 25 cm of plant stems was collected from each plot and assayed for DON, 3-acetyl-deoxynivalenol (3ADON), and 15-acetyl-deoxynivalenol (15ADON). Significant differences in DON concentration in the grain were detected among cultivars (P = 0.0001) and for the concentration of all DON (P = 0.003), 3ADON (P = 0.03), and 15ADON (P < 0.0001) in the stem. Significant differences among resistance classes were observed for FHB index value (P < 0.0001), ISK index (P = 0.006), and DISK index (P = 0.004). In all years of this study, the concentration of DON in the grain and the concentrations of all mycotoxins in the stem were consistently lower in the moderately resistant cultivars. All three indices were poor indicators of mycotoxin concentrations in the stem. Overall, the selection of a moderately resistant cultivar provides effective control of DON accumulation in the grain and mycotoxin accumulation in the stem.

Effect of Fusarium Head Blight Management Practices on Mycotoxin Contamination of Wheat Straw.

Management of Fusarium graminearum-associated mycotoxins in wheat grain has been extensively evaluated, but little is known about management of mycotoxins in straw. Two research trials were conducted at four locations from 2011 to 2014. The objective of the first trial was to determine the efficacy of fungicides, and the objective of the second trial was to evaluate the use of integrated disease management strategies, for the control of Fusarium head blight (FHB) and reducing the concentration of the Fusarium mycotoxins deoxynivalenol, 3-acetyl-deoxynivalenol, and 15-acetyl-deoxynivalenol in straw. In the first trial, it was determined that demethylation inhibitor (DMI) fungicides did not offer significant (P ≤ 0.05) reductions of mycotoxin concentrations in the straw compared with a no-fungicide control treatment, but significant (P ≤ 0.05) reductions in mycotoxin concentration were observed in the control when compared with treatments with the application of quinone outside inhibitor (QoI)-containing fungicides. In the second trial, mycotoxin concentrations in the straw were significantly (P ≤ 0.05) reduced in the moderately resistant cultivar compared with the susceptible cultivar, but were not affected by the use of a fungicide. The practices typically used to manage Fusarium mycotoxins in wheat grain, especially the selection of resistant cultivars and not using a QoI fungicide, may be an effective means to reduce mycotoxin concentrations in the straw.