Frogeye leaf spot (FLS), caused by Cercospora sojina, is a common soybean disease across the U.S. Fungicides are a key management tool, particularly when susceptible cultivars are planted; however, widespread QoI resistance has raised concern about overreliance on the remaining effective fungicide classes. Protecting these chemical classes is essential for long-term sustainability, particularly under narrow profit margins. To develop an FLS prediction model that supports more efficient fungicide use, environmental and epidemiological data from multiple site-years were analyzed in 2024 using correlation analysis, logistic regression (LR), and machine-learning approaches. The most effective model combined a 30-day moving average (ma) of daily hours of relative humidity (RH) ≥ 80% and maximum temperature (°C) in a LR model. FLS risk peaked when the 30-d ma of daily hours of RH ≥ 80% was 15–20 h and maximum temperature was 24–36 °C. When daily hours of RH ≥ 80% averaged < 5 h, risk remained low regardless of temperature. Random forest and support vector machine models achieved greater accuracy and sensitivity than LR but showed poorer specificity. This research provides a strong epidemiological foundation for improving decision-making and advancing integrated disease management. The resulting prediction model is deployed in a public decision support system ( https://cropprotectionnetwork.org/crop-disease-forecasting ), enabling real-time FLS risk assessments and promoting stewardship-minded fungicide use.

Tar spot, caused by Phyllachora maydis, is a threatening disease to maize production across the Americas. This study aimed to test an inoculation protocol for P. maydis in maize, addressing variability across geographic regions. We tested a whorl inoculation method under semicontrolled environments in Ecuador and the United States, focusing on three objectives: (i) to develop a noninvasive inoculation protocol; (ii) to assess the effects of inoculum source, cultivar susceptibility, and leaf position on disease progression over time; and (iii) to evaluate the reproducibility of the protocol. Inoculum preparations from fresh or stored leaves were tested across multiple maize cultivars. Results indicated that the use of a fresh inoculum source led to faster disease onset (15 to 17 days after inoculation) and higher stromata counts compared with inoculum prepared from stored leaves. In Ecuador, fisheye lesions were prominent, whereas they were absent in the United States; however, the factors driving those differences remain unclear. Reproducibility was high in Ecuador and less consistent in the United States, likely because of environmental variability and inoculum quality. Future research should focus on optimizing artificial inoculation methods by exploring the interaction between environmental conditions, inoculum quality, and host susceptibility. Overall, the findings contribute to refining inoculation protocols for tar spot in maize.

The impact of plant diseases on soybean (Glycine max [L.] Merrill) yield was estimated across 29 states and Ontario, Canada from 2020 to 2024 by university and government plant pathologists. Losses from 29 pathogens or groups of pathogens were estimated at the end of each growing season through a survey and summarized across years and locations. Diseases reduced soybean yield by an estimated 1.2 billion bushels (32.8 million metric tons) valued at $14.6 billion USD for the survey period. Per acre, this estimated mean economic loss was equal to $32.93 USD ($81.37 USD per hectare) across all locations and years, excluding costs such as fungicide seed treatments and foliar applications. Soybean cyst nematode (SCN) (Heterodera glycines Ichinohe) reduced yield by 482.4 million bushels (13.1 million metric tons), a value nearly four times greater than the next greatest cause of yield loss, which was sudden death syndrome (SDS) (caused by Fusarium virguliforme O’Donnell & T. Aoki). Following SCN and SDS, the most significant yield losses were attributed to white mold (caused by Sclerotinia sclerotiorum [Lib.] de Bary), seedling diseases (caused by various pathogens), Phytophthora root and stem rot (caused by Phytophthora sojae Kaufm. & Gerd.), and root-knot nematodes (Meloidogyne spp.), in descending order. The most important diseases in the southern U.S. were generally different from those in the northern U.S. and Ontario. Data presented here will enable government agencies, scientists, educators, commodity groups, funding organizations, and plant breeders to enhance and prioritize policy, research, funding, and education regarding soybean disease management.

Soybean (Glycine max) production is severely impacted by Phytophthora sojae, the causal agent of Phytophthora root and stem rot, resulting in significant yield losses worldwide. Accurate detection of this pathogen is critical for effective disease management. In this study, we developed a novel loop-mediated isothermal amplification (LAMP) assay targeting the internal transcribed spacer (ITS) region of P. sojae DNA. Conventional PCR detected the pathogen in only 18.7% of infected soybean seedlings using a 1 μL DNA sample, while nested PCR detected 71.9%. In contrast, the ITS-LAMP assay achieved 100% detection with the same input volume, demonstrating superior sensitivity. Notably, nested PCR required a fivefold increase in template volume (5 μL) to reach comparable detection levels. The ITS-LAMP assay exhibited a detection limit of 1 pg μL-1, which is two orders of magnitude lower than that of the previously reported PS-PYpt1-LAMP assay. Visual detection was enabled through both colorimetric change and agarose gel electrophoresis. Primer specificity was confirmed through blind testing across a wide range of oomycete and fungal species that also infect soybean. With 100% specificity, sensitivity, and accuracy and robust resistance to inhibitors, the LAMP assay provides a valuable tool for rapid and accurate detection of P. sojae, enhancing Phytophthora root and stem rot management.

Corn stunt is one of the most significant corn diseases in the Neotropics, leading to severe plant stunting and substantial yield losses. Although four pathogens have been found either singly or in combination in infected plants in the Americas, corn stunt spiroplasma (Spiroplasma kunkelii) has been the most predominant pathogen associated with the disease in the United States, due to its widespread distribution in the Rio Grande Valley region and persistent occurrence in California and Florida. During the 2024 growing season, reports of chlorosis, leaf reddening, and stunting in corn fields in Southern, Great Plains, Central Corn Belt, and Northeastern states raised concern regarding the possibility of a more widespread distribution of corn stunt spiroplasma in the United States. Symptomatic corn leaf samples were collected in commercial and experimental field sites across the United States. Detection and identification of S. kunkelii were performed using a polymerase chain reaction assay targeting a section of the spiralin gene, followed by amplicon sequencing. This study provides the first report of the pathogen S. kunkelii associated with corn stunt symptoms distributed across 6 counties in Oklahoma, 14 counties in Kansas, 2 counties in Missouri and Arkansas, 4 counties in New York, and 1 county in each of Nebraska, South Dakota, Wisconsin, Minnesota, Indiana, and Alabama. All states with submitted samples had at least one confirmed case of S. kunkelii.

Diseases of soybean (Glycine max L.) caused by species of Diaporthe have resulted in estimated yield losses totaling $248.2 million in the United States over the past 10 years. To effectively manage species of Diaporthe, it is important to use an integrated approach. In this study, we evaluated the in vitro sensitivity of isolates of the soybean pathogens D. aspalathi, D. caulivora, and D. longicolla from 18 U.S. states to difenoconazole (a demethylation inhibitor fungicide) and fluopyram (a succinate dehydrogenase inhibitor fungicide). The fungicides were incorporated into 2% water agar (WA) in Petri dishes at various concentrations. A mycelial plug of each isolate (n = 59 for difenoconazole and n = 55 for fluopyram) obtained from a 7-day-old culture was placed at the center of the WA and incubated in the dark. After 5 days for D. caulivora and D. longicolla and 8 days for D. aspalathi, the colony diameter was measured, and the corresponding percent inhibition and effective concentration at which 50% mycelial growth was inhibited (EC50) were determined. Significant differences in EC50 values (P < 0.0001) were observed among the isolates of D. aspalathi (0.227 g/ml), D. caulivora (0.130 mu g/ml), and D. longicolla (1.860 mu g/ml) for difenoconazole. Similarly, for fluopyram, the EC50 values varied significantly (P < 0.001) among the D. aspalathi (2.233 g/ml), D. caulivora (1.610 mu g/ml), and D. longicolla (0.347 mu g/ml) isolates. This study established sensitivity profiles for difenoconazole and fluopyram fungicides for D. aspalathi, D. caulivora, and D. longicolla and provides valuable information that may help in the development of a Diaporthe disease management program.

Sudden death syndrome (SDS) of soybean, caused by Fusarium virguliforme, can significantly reduce soybean yield and is spreading in North America. Soybean cultivars with partial resistance are important for managing SDS, and seed treatments have been developed to supplement resistance and manage SDS where resistance is unavailable. Studies were conducted from 2020 to 2022 at two field locations in Minnesota to evaluate seed treatments containing fluopyram and pydiflumetofen and a biological seed treatment containing saponin extract from Chenopodium quinoa for managing SDS. Both locations had low soybean cyst nematode populations. The average SDS foliar disease index (FDX) severity was 50 and 19% in the untreated plots at the Rosemount and Waseca, Minnesota, field locations, respectively. Pydiflumetofen and fluopyram treatments significantly reduced FDX compared with untreated controls on susceptible and partially resistant soybean cultivars and at each field location. Pydiflumetofen significantly reduced FDX in 75% and fluopyram in 62% of the studies by cultivar. Yield was significantly greater with pydiflumetofen than with other treatments for both cultivars and significantly greater with fluopyram for one cultivar in Rosemount, where SDS severity was the greatest. None of the treatments provided a significant yield benefit in Waseca, where SDS levels were moderately low. Neither seed treatment nor cultivar significantly influenced plant population. In greenhouse experiments with the same treatments, fluopyram, pydiflumetofen, and saponin extract significantly reduced SDS development compared with controls. In summary, seed treatments can be effective for managing soybean SDS across a range of northern field and greenhouse environments with low soybean cyst nematode populations.

Bacterial leaf streak (BLS) of corn caused by Xanthomonas vasicola pv. vasculorum was first reported in the United States in 2017. The biology and management of BLS are poorly understood. The objective of this work was to determine the effects of hybrid, foliar treatments, and infection conditions (timing, temperature, inoculation site) on BLS of sweet corn. Field studies were conducted to determine if hybrid or foliar disease management treatments influenced BLS development and yield. Corn leaves were inoculated in plots with X. vasicola pv. vasculorum, and noninoculated plots were used for comparison. The leaf incidence and severity of BLS differed significantly among sweet corn hybrids, suggesting different levels of susceptibility to BLS. Grain yield was significantly reduced (14.7%) by BLS for one hybrid. The corn growth stage at time of infection influenced BLS, with incidence and severity significantly greater following inoculation at stage V6 than V9. Foliar application of Kocide®, LifeGard®, and Kocide®+LifeGard® significantly reduced leaf severity compared to nontreated controls in field studies. Kocide® significantly reduced leaf incidence, but no treatments significantly increased yield vs. controls. In comparisons of inoculation methods in a growth chamber, lesion length on leaves was significantly greater on stalk-inoculated than leaf-inoculated plants. Lesions developed on leaf-inoculated plants only at inoculation sites whereas lesions developed on stalk-inoculated plants on multiple leaves. In controlled environments, lesion length on leaves was significantly greater at 21°C than 27°C and 32°C. This study expands our understanding of factors that influence development and management of BLS of sweet corn.

Phyllachora maydis is a fungal plant pathogen that causes tar spot of corn ( Zea mays) in North and South America, causing devastating yield losses under favorable conditions. Although the causal agent is relatively easy to diagnose via macroscopic and microscopic observations, other diseases and conditions, such as insect frass, have been mistaken for tar spot of corn. Furthermore, conidia and ascospores in isolation can be difficult to visually distinguish from other fungi, and the development of signs and symptoms of the disease may not be observed until 12 to 20 days after infection. Therefore, we developed a TaqMan quantitative polymerase chain reaction (qPCR) assay for the detection and quantification of this pathogen to be used for diagnostics and airborne spore quantification. The assay was designed for the internal transcribed spacer region of P. maydis. The specificity of the assay was confirmed and tested against various nontarget Phyllachora species, corn pathogens, endophytes, and P. maydis samples from several states in the Midwest and from Mexico. The detection limit of this assay was determined to be 100 fg of genomic P. maydis DNA. To demonstrate the transferability of this technology, the assay was tested in different labs using various qPCR thermal cyclers. This assay can be used in downstream research involving latency period, disease prediction, and diagnostics. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

The genus Phyllachora contains numerous obligate fungal parasites that produce raised, melanized structures called stromata on their plant hosts. Most members of this genus are not of significant economic concern, with the exception of P. maydis, the causal agent of tar spot of maize (Zea mays). Tar spot of maize has emerged as a major threat to maize production throughout the Americas and continues to spread throughout North America. To date, species designations for Phyllachora have been based on host associations and morphology, and the origin and diversity of the pathogen that causes tar spot is unknown. We assessed the sequence diversity of 186 single stroma isolates collected from 16 hosts representing 15 countries by amplification of the ITS and LSU gene regions. Samples included both herbarium and contemporary strains that covered a temporal range from 1905-2019. These 186 isolates were grouped into 5 distinct species with strong bootstrap support. We found three closely related, but genetically distinct groups of Phyllachora are capable of infecting maize in the United States, we refer to these as the P. maydis species complex. Based on herbarium species, we hypothesize that these three groups in the P. maydis species complex originated from Central America, Mexico and the Caribbean. Although two of these groups were only found on maize, the third and largest group contained contemporary strains found on maize and other grass hosts, as well as herbarium specimens from maize and other grasses that include 10 species of Phyllachora. The herbarium specimens were identified based on morphology and host association, but our data indicates there may be significant synonymy in the Phyllachora genus and additional work on species delineation and host specificity should be considered.

Tar spot of corn (Zea mays L.) is a significant disease in the United States and Canada caused by Phyllachora maydis, an obligate biotroph fungus. However, field research critical for understanding and managing the disease has been hindered by a need for methods to inoculate corn with P. maydis in field environments. In this study, we developed and demonstrated the efficacy of a method to initiate tar spot in field settings using inoculations of corn leaves with P. maydis inoculum that had been stored at -20°C for 10 months. Stromata of P. maydis were observed 19 days after inoculations in two field experiments, and stromata resulting from secondary spread were initially observed 39 to 41 days after the initial inoculations. Tar spot was not present in the fields beyond the inoculated areas or localized spread area, signifying that the establishment of initial disease resulted solely from inoculations. This study enhances our understanding of inoculation and infection of corn with P. maydis and tar spot development in field environments. The results will aid new research into understanding the corn tar spot pathosystem and improving management strategies.

Sudden death syndrome (SDS), caused by Fusarium virguliforme, is an important yield-limiting disease of soybean (Glycine max). From 1996 to 2022, cumulative yield losses attributed to SDS in North America totaled over 25 million metric tons, which was valued at over US $7.8 billion. Seed treatments are widely used to manage SDS by reducing early season soybean root infection by F. virguliforme. Fluopyram (succinate dehydrogenase inhibitor [SDHI] - FRAC 7), a fungicide seed treatment for SDS management, has been registered for use on soybean in the United States since 2014. A baseline sensitivity study conducted in 2014 evaluated 130 F. virguliforme isolates collected from five states to fluopyram in a mycelial growth inhibition assay and reported a mean EC50 of 3.35 mg/liter. This baseline study provided the foundation for the objectives of this research: to detect any statistically significant change in fluopyram sensitivity over time and geographical regions within the United States and to investigate sensitivity to the fungicide pydiflumetofen. We repeated fluopyram sensitivity testing on a panel of 80 historical F. virguliforme isolates collected from 2006 to 2013 (76 of which were used in the baseline study) and conducted testing on 123 contemporary isolates collected from 2016 to 2022 from 11 states. This study estimated a mean absolute EC50 of 3.95 mg/liter in isolates collected from 2006 to 2013 and a mean absolute EC50 of 4.19 mg/liter in those collected in 2016 to 2022. There was no significant change in fluopyram sensitivity (P = 0.1) identified between the historical and contemporary isolates. A subset of 23 isolates, tested against pydiflumetofen under the same conditions, estimated an absolute mean EC50 of 0.11 mg/liter. Moderate correlation was detected between fluopyram and pydiflumetofen sensitivity estimates (R = 0.53; P < 0.001). These findings enable future fluopyram and pydiflumetofen resistance monitoring and inform current soybean SDS management strategies in a regional and national context.

Plant disease resistance genes are widely used in agriculture to reduce disease outbreaks and epidemics and ensure global food security. In soybean, Rps (Resistance to Phytophthora sojae) genes are used to manage Phytophthora sojae, a major oomycete pathogen that causes Phytophthora stem and root rot (PRR) worldwide. This study aims to identify temporal changes in P. sojae pathotype complexity, diversity, and Rps gene efficacy. Pathotype data was collected from 5121 isolates of P. sojae, derived from 29 surveys conducted between 1990 and 2019 across the United States, Argentina, Canada, and China. This systematic review shows a loss of efficacy of specific Rps genes utilized for disease management and a significant increase in the pathotype diversity of isolates over time. This study finds that the most widely deployed Rps genes used to manage PRR globally, Rps1a, Rps1c and Rps1k, are no longer effective for PRR management in the United States, Argentina, and Canada. This systematic review emphasizes the need to widely introduce new sources of resistance to P. sojae, such as Rps3a, Rps6, or Rps11, into commercial cultivars to effectively manage PRR going forward.

Background Tar spot of corn is a significant and spreading disease in the continental U.S. and Canada caused by the obligate biotrophic fungus Phyllachora maydis. As of 2023, tar spot had been reported in 18 U.S. states and one Canadian Province. The symptoms of tar spot include chlorotic flecking followed by the formation of black stromata where conidia and ascospores are produced. Advancements in research and management for tar spot have been limited by a need for a reliable method to inoculate plants to enable the study of the disease. The goal of this study was to develop a reliable method to induce tar spot in controlled conditions. Results We induced infection of corn by P. maydis in 100% of inoculated plants with a new inoculation method. This method includes the use of vacuum-collection tools to extract ascospores from field-infected corn leaves, application of spores to leaves, and induction of the disease in the dark at high humidity and moderate temperatures. Infection and disease development were consistently achieved in four independent experiments on different corn hybrids and under different environmental conditions in a greenhouse and growth chamber. Disease induction was impacted by the source and storage conditions of spores, as tar spot was not induced with ascospores from leaves stored dry at 25 ºC for 5 months but was induced using ascospores from infected leaves stored at -20 ºC for 5 months. The time from inoculation to stromata formation was 10 to 12 days and ascospores were present 19 days after inoculation throughout our experiments. In addition to providing techniques that enable in-vitro experimentation, our research also provides fundamental insights into the conditions that favor tar spot epidemics. Conclusions We developed a method to reliably inoculate corn with P. maydis . The method was validated by multiple independent experiments in which infection was induced in 100% of the plants, demonstrating its consistency in controlled conditions. This new method facilitates research on tar spot and provides opportunities to study the biology of P. maydis , the epidemiology of tar spot, and for identifying host resistance.