Robert Martin Harveson

Head rotting diseases of sunflower are particularly destructive due to their direct impact on the tissues of the head (capitulum, receptacle), which may result in loss of yield and reduction in seed or flower quality. This article focuses on Sclerotinia head rot, caused by Sclerotinia sclerotiorum, which is among the most yield-limiting diseases of sunflower globally. As S. sclerotiorum is among the most common plant pathogens of many crops, we place emphasis on diagnostic techniques most relevant to sunflower and signs and symptoms that distinguish the disease from other sunflower head rots. This article is the fifth in a series of Diagnostic Guides on sunflower diseases, produced in an effort to improve identification, diagnostics, and management of this globally important crop.

The ability of plant pathogens to infect multiple host species has substantial adverse effects on food security and ecosystem health. Rhizoctonia solani is a globally distributed, soil‐borne fungal pathogen capable of infecting a wide variety of economically important crops. Its broad distribution across geographic regions and host taxa is accompanied by high genetic diversity. Historically, the R. solani species complex has been classified into 14 anastomosis groups (AGs) based primarily on hyphal anastomosis compatibility. Despite being widely perceived as a generalist, R. solani 's host range has not been studied in a comprehensive cross‐AG framework to evaluate global patterns of pathogen diversity and host associations. We analyzed a global dataset of 4119 R. solani rDNA‐ITS sequences collected over the past 70 years, including newly sampled taxa from Nebraska, and used a smaller concatenated ITS and TEF‐1α dataset comprising 162 R. solani sequences to assess whether broad phylogenetic patterns were consistent across markers. Phylogenetic reconstruction suggests that most AGs formed well‐supported clades that largely corresponded to reported AG designations. Host family‐level transition inference suggests that generalist AGs may experience more transient host associations, while specialist lineages show stronger host‐associated genetic differentiation that contributes to the observed genetic structure. Our ancestral host‐state reconstruction analysis further suggests that the AG‐3 lineage likely originated from an ancestor with a broader host range and subsequently became predominantly associated with Solanaceae, with no inferred reversal in the sampled dataset. These findings suggest that host association and host‐transition patterns are important features of the generalism‐specialization continuum and contribute to population structure in a globally distributed fungal pathogen.

Phomopsis stem canker of sunflower (Helianthus annuus L.) is a major disease in the United States. This study quantified the effect of Phomopsis stem canker severity on sunflower yield and evaluated the efficacy of fungicides under field conditions across Minnesota, North Dakota, Nebraska, and South Dakota. To determine the yield-disease severity index (DSI) relationship, non-treated control (NTC) plots from 80 field trials (2009-2025) were analyzed, and yield was found to decrease with increasing DSI, described by the equation: Yield (kg ha⁻¹) = 2552.52 - 11.64 × DSI (R² = 0.14, P < 0.001). Seventeen trials conducted from 2022 to 2025 evaluated fungicides containing Quinone outside Inhibitors (QoI), Demethylation Inhibitors (DMI), and Succinate Dehydrogenase Inhibitors (SDHI) applied at V8 (eight true leaves), R1 (miniature floral head), and/or R6 (flowering complete and the ray flowers wilting) stages. Trials were categorized into low-, moderate-, and high-yield environments using ±1 SD residuals to determine relative differences in yield within the overall DSI-yield relationship. Yield decreased with increasing DSI in low (R²=0.91, P=6.13×10 ^⁻5 ) and moderate (R²=0.19, P=0.000359) yielding environments. In low yield environments, fluopyram + tebuconazole at R1 followed by pyraclostrobin at R6 increased yield by up to 55%. In moderate yield environments, fluopyram (SDHI) + tebuconazole (DMI) at V8 or R6, combined with pyraclostrobin (QoI) at R1, increased yield by up to 15% over NTC. These findings indicate that fungicide efficacy differed among yield environments, suggesting that treatment decisions should account for the production environment to maximize sunflower yield.

Although the high-altitude limit for microbial survival in the Earth–atmosphere system has remained a scientific curiosity and topic of study, the ecological significance of long-distance microbial dispersal in the atmosphere has been perceived to have marginal relevance. Here, we report the characterization of novel plant pathogenic species of Curtobacterium that were isolated from samples collected at altitudes ranging from 1.5 to 29 km above sea level. Whole genome-based phylogenies of three strains, paired with plant challenge assays, indicate that each is a previously unrecognized species and causes disease on beans comparable to Curtobacterium flaccumfaciens . Isolates from the stratosphere (strain L6-1) and agricultural millet (G77) were identified to be the same species and designated as Curtobacterium aetherium sp. nov. C. aetherium displays high levels of tolerance to desiccation and UV radiation, which are stresses that increase in intensity with altitude. Back trajectory air mass analysis implied that the phytopathogens may have had an intercontinental source, but regional origins in the continental US cannot be excluded. The environmentally robust phytopathogens we have documented in the upper atmosphere provide new perspective on the role that high-altitude transport may play in microbial dispersal, gene flow, and the epidemiology of aerially dispersed plant disease. IMPORTANCE: Enormous quantities and varieties of microorganisms are continually aerosolized and transported in the atmosphere, yet there is a limited understanding of the consequences to downwind ecosystems. While studying bacteria that survive extreme conditions at altitudes up to 29 km in the atmosphere, we discovered new species that have the capacity to cause disease in agriculturally relevant bean varieties. The hardiest isolate we characterized from the stratosphere is a member of the same species as an isolate from an agricultural source, which we have designated Curtobacterium aetherium . C. aetherium is a phytopathogen capable of enduring high-altitude and long-distance atmospheric transport while also possessing the potential to opportunistically infect crops in deposition locations.

Phomopsis stem canker, primarily caused by Diaporthe gulyae and D. helianthi, is a significant disease of sunflowers ( Helianthus annuus L.), leading to yield losses of up to 40%. Foliar fungicides containing pyraclostrobin (quinone outside inhibitors [QoI]) are recommended for disease management; however, their widespread use poses a significant risk of selecting for resistant fungal strains. In this study, a cytochrome b (cyt b) gene-based polymerase chain reaction assay was developed to identify QoI-resistant strains of D. gulyae and D. helianthi. Given that the cyt b gene sequence for D. gulyae and D. helianthi was not available in the National Center for Biotechnology Information (NCBI) database, forward and reverse primers for the assay were designed based on the cyt b sequence of D. longicolla, a fungus within the Diaporthe genus. A total of 182 isolates of D. gulyae and D. helianthi, collected from six U.S. states, were cultured in potato dextrose broth, and genomic DNA was extracted. The cyt b gene fragments from the isolates were amplified, sequenced, assembled, and analyzed for mutations. Our findings confirmed a point mutation in the cyt b gene at codon 143 (GGT→GCT, G143A) in one isolate of D. helianthi recovered from diseased sunflower stalks in Polk County, MN, U.S.A., in 2019. The detection of the G143A mutation in a D. helianthi isolate emphasizes the need for monitoring QoI-resistant strains to maintain the effectiveness of QoI fungicides in disease management. [ ] Copyright © 2025 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Sunflower is one of the most important oil crops globally and is impacted by numerous yield-limiting diseases. This publication provides information on signs and symptoms for identifying and distinguishing Rhizopus head rot from other head diseases, methods and techniques for working with the causal organism, and references to further resources.

Sunflower is affected by many difficult-to-distinguish diseases whose prevalence and economic importance varies by geography, time, and management practices. In this Diagnostic Guide, we present information on Botrytis head rot and bacterial rots. Botrytis head rot caused by Botrytis cinerea Pers. can be a common and significant disease in favorable environments, whereas bacterial rots usually occur opportunistically and sporadically, resulting in limited yield loss. In this paper, we concentrate on providing information to identify and distinguish these diseases and a comprehensive review of methods and techniques for working with the causal pathogens, and we emphasize the importance of disease cycles for identification and management.

Phomopsis stem canker reduces yield of sunflower (Helianthus annuus L.) up to or exceeding 40%; however, management recommendations have not been developed for U.S. farmers. Between 2009 and 2020, foliar fungicide trials were conducted in Minnesota, Nebraska, North Dakota, and South Dakota for a total of 49 location-years. Random effects meta-analyses were performed on the disease severity index (DSI) and yield data collected from the foliar fungicide trials to determine the overall and individual effectiveness of the tested fungicides. Effect sizes, Cohen’s f or Hedges’ g, were calculated as the difference in DSI or yield between the fungicide treatment and nontreated control (NTC) divided by the pooled SD. The pooled Cohen’s f for DSI and yield was 0.40 (95% CI = [0.29, 0.42]), indicating a large effect size and that fungicide treatments had a significant effect on DSI and yield (P < 0.0001). Among the fungicide groups, quinone outside inhibitor (QoI) (DSI [k = 45; g = −0.47] and yield [k = 46; g = 0.41]) is moderately effective and premixes of demethylation inhibitors (DMI), succinate dehydrogenase inhibitors (SDHI), and QoI (DMI + SDHI + QoI) (DSI [k = 3; g = −0.79] and yield [k = 3; g = 0.94]) are largely effective in comparison with NTC. Upon performing prediction analyses, the probability of not recovering the fungicide application cost (P_loss) associated with QoI (pyraclostrobin) was 5%).

Cowpea ( Vigna unguiculata) is grown on approximately 5,220 ha across California and several Midwestern and southern states. In August of 2020, a viral disease was observed infecting cowpea in commercial fields and research trials in Nebraska, United States. Based on RNA sequencing and serology, the virus species was identified as Alfalfa mosaic virus. To our knowledge, this is the first report of this virus on cowpea in the United States.