Foliar Fungicides Containing FRAC 11 Mitigate Phomopsis Stem Canker in Sunflower (Helianthus annuus).

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 (Ploss) associated with QoI (pyraclostrobin) was <0.35 for a range of sunflower grain prices suggesting a greater probability of return on investment from a single application of fungicide. Overall, our study suggests that the use of QoI fungicides is likely to be profitable in the presence of Phomopsis stem canker (DSI > 5%).

Clavibacter nebraskensis causing Goss's wilt of maize: Five decades of detaining the enemy in the New World.

Goss's bacterial wilt and leaf blight of maize (Zea mays) caused by the gram-positive coryneform bacterium Clavibacter nebraskensis is an economically important disease in North America. C. nebraskensis is included within the high-risk list of quarantine pathogens by several plant protection organizations (EPPO code: CORBMI), hence it is under strict quarantine control around the world. The causal agent was reported for the first time on maize in Nebraska (USA) in 1969. After an outbreak during the 1970s, prevalence of the disease decreased in the 1980s to the early 2000s, before the disease resurged causing a serious threat to maize production in North America. The re-emergence of Goss's wilt in the corn belt of the United States led to several novel achievements in understanding the pathogen biology and disease control. In this review, we provide an updated overview of the pathogen taxonomy, biology, and epidemiology as well as management strategies of Goss's wilt disease. First, a taxonomic history of the pathogen is provided followed by symptomology and host range, genetic diversity, and pathogenicity mechanisms of the bacterium. Then, utility of high-throughput molecular approaches in the precise detection and identification of the pathogen and the management strategies of the disease are explained. Finally, we highlight the role of integrated pest management strategies to combat the risk of Goss's wilt in the 21st century maize industry. DISEASE SYMPTOMS: Large (2-15 cm) tan to grey elongated oval lesions with wavy, irregular water-soaked margins on the leaves. The lesions often start at the leaf tip or are associated with wounding caused by hail or wind damage. Small (1 mm in diameter), dark, discontinuous water-soaked spots, known as "freckles", can be observed in the periphery of lesions. When backlit, the freckles appear translucent. Early infection (prior to growth stage V6) may become systemic and cause seedlings to wilt, wither, and die. Coalescence of lesions results in leaf blighting. HOST RANGE: Maize (Zea mays) is the only economic host of the pathogen. A number of Poaceae species are reported to act as secondary hosts for C. nebraskensis. TAXONOMIC STATUS OF THE PATHOGEN: Class: Actinobacteria; Order: Micrococcales; Family: Microbacteriaceae; Genus: Clavibacter; Species: Clavibacter nebraskensis. SYNONYMS: Corynebacterium nebraskense (Schuster, 1970) Vidaver & Mandel 1974; Corynebacterium michiganense pv. nebraskense (Vidaver & Mandel 1974) Dye & Kemp 1977; Corynebacterium michiganense subsp. nebraskense (Vidaver & Mandel 1974) Carlson & Vidaver 1982; Clavibacter michiganense subsp. nebraskense (Vidaver & Mandel 1974) Davis et al. 1984; Clavibacter michiganensis subsp. nebraskensis (Vidaver & Mandel 1974) Davis et al. 1984. TYPE MATERIALS: ATCC 27794T ; CFBP 2405T ; ICMP 3298T ; LMG 3700T ; NCPPB 2581T . MICROBIOLOGICAL PROPERTIES: Cells are gram-positive, orange-pigmented, pleomorphic club- or rod-shaped, nonspore-forming, nonmotile, and without flagella, approximately 0.5 × 1-2.0 µm. DISTRIBUTION: The pathogen is restricted to Canada and the United States. PHYTOSANITARY CATEGORIZATION: EPPO code CORBNE.

A longitudinal study on morpho-genetic diversity of pathogenic Rhizoctonia solani from sugar beet and dry beans of western Nebraska.

BACKGROUND: Root and stem rot caused by Rhizoctonia solani is a serious fungal disease of sugar beet and dry bean production in Nebraska. Rhizoctonia root rot and crown rot in sugar beet and dry bean have reduced the yield significantly and has also created problems in storage. The objective of this study was to analyze morpho-genetic diversity of 38 Rhizoctonia solani isolates from sugar beet and dry bean fields in western Nebraska collected over 10 years. Morphological features and ISSR-based DNA markers were used to study the morphogenetic diversity. RESULTS: Fungal colonies were morphologically diverse in shapes, aerial hyphae formation, colony, and sclerotia color. Marker analysis using 19 polymorphic ISSR markers showed polymorphic bands ranged from 15 to 28 with molecular weight of 100 bp to 3 kb. Polymorphic loci ranged from 43.26-92.88%. Nei genetic distance within the population ranged from 0.03-0.09 and Shannon diversity index varied from 0.24-0.28. AMOVA analysis based on ΦPT values showed 87% variation within and 13% among the population with statistical significance (p < 0.05). Majority of the isolates from sugar beet showed nearby association within the population. A significant number of isolates showed similarity with isolates of both the crops suggesting their broad pathogenicity. Isolates were grouped into three different clusters in UPGMA based cluster analysis using marker information. Interestingly, there was no geographical correlation among the isolates. Principal component analysis showed randomized distribution of isolates from the same geographical origin. Identities of the isolates were confirmed by both ITS-rDNA sequences and pathogenicity tests. CONCLUSION: Identification and categorization of the pathogen will be helpful in designing integrated disease management guidelines for sugar beet and dry beans of mid western America.

Bacterial wilt of dry beans caused by Curtobacterium flaccumfaciens pv. flaccumfaciens: A new threat from an old enemy.

Bacterial wilt and tan spot of dry beans (family Fabaceae), caused by Curtobacterium flaccumfaciens pv. flaccumfaciens, is an important emerging disease threatening the edible legume industry around the globe. The management of bacterial wilt has been a major problem since its original description in 1922. This is in part due to the seedborne nature of the pathogen allowing the bacterium to be transmitted long distances via infected seeds, as well as a lack of detailed molecular information concerning the pathogenicity repertoires and virulence determinates of the pathogen. Identification can also be difficult owing to the presence of five different colony colour variants (i.e., yellow, orange, pink, purple, and red) on culture media. In this review, we provide an overview of the aetiology, epidemiology, and management strategies of bacterial wilt disease. First, a comprehensive and comparative symptomology of the disease on different dry bean species is described. Then, the taxonomic history of the causal agent and utility of high-throughput sequencing-based approaches in the precise characterization of the pathogen is explained. Furthermore, we provide an updated outline on the global distribution of the pathogen, highlighting expansion of the causal agent into the areas with no history of the disease until the beginning of the current century. Finally, because there are limited options for use of conventional pesticides against the pathogen, we highlight the use of integrated pest management strategies, for example quarantine inspections, resistant cultivars, and crop sanitation, to combat the risk of bacterial wilt disease in the dry bean industry. DISEASE SYMPTOMS: Interveinal chlorosis on leaflets leading to necrotic areas and systemic wilt. Seed discolouration to yellow, orange, pink, or purple is seen in white-seeded cultivars. HOST RANGE: Causes bacterial wilt and tan spot disease on edible dry beans in the Fabaceae family, including common bean (Phaseolus vulgaris), cowpea (Vigna unguiculata), mungbean (Vigna radiata), soybean (Glycine max), as well as a number of weed species. TAXONOMIC STATUS OF THE PATHOGEN: Bacteria; phylum Actinobacteria; order Actinomycetales; suborder: Micrococcineae; family Microbacteriaceae; genus Curtobacterium; species Curtobacterium flaccumfaciens. SYNONYMS: Corynebacterium flaccumfaciens subsp. flaccumfaciens; Corynebacterium flaccumfaciens pv. flaccumfaciens, Corynebacterium flaccumfaciens, Phytomonas flaccumfaciens, Bacterium flaccumfaciens. MICROBIOLOGICAL PROPERTIES: Multicoloured (yellow, orange, pink, purple, and red), gram-positive, aerobic, curved rod, nonspore-forming, polar flagellated, motile cells. DISTRIBUTION: Widespread in America (Brazil, Canada, and the USA), Australia, and Iran. Restricted occurrence in Africa and Europe. PHYTOSANITARY CATEGORIZATION: EPPO A2 list no. 48, EU Annex II/B.

Quantitative PCR Assays Developed for Diaporthe helianthi and Diaporthe gulyae for Phomopsis Stem Canker Diagnosis and Germplasm Screening in Sunflower (Helianthus annuus).

Phomopsis stem canker of sunflower is caused by two fungal pathogens, Diaporthe helianthi and Diaporthe gulyae, in the United States. In this study, two quantitative PCR (qPCR) assays were developed to detect and quantify D. helianthi and D. gulyae in sunflower. The two assays differentiated the two fungi from each other, other species of the genus Diaporthe, and pathogens, and they have high efficiency (>90%). The qPCR assays detected the two pathogens on plant samples exhibiting Phomopsis stem canker symptoms sampled from commercial sunflower fields in Minnesota, Nebraska, North Dakota, and South Dakota. Furthermore, the assays were used to screen cultivated sunflower accessions for resistance to D. helianthi and D. gulyae. The disease severity index (DSI) of the accessions significantly correlated (P < 0.0001) with the amount of pathogen DNA from the qPCR assays. The qPCR assays identified PI664232 and PI561918 to be significantly less susceptible (P ≤ 0.05) to D. helianthi and D. gulyae, respectively, when compared with the susceptible check cultivar HA 288, and this was in agreement with the DSI. These results suggest that the qPCR assays for D. helianthi and D. gulyae can be used as a reliable tool to diagnose Phomopsis stem canker and screen sunflower germplasm for disease resistance.

Phenotypic and Molecular-Phylogenetic Analysis Provide Novel Insights into the Diversity of Curtobacterium flaccumfaciens.

A multiphasic approach was used to decipher the phenotypic features, genetic diversity, and phylogenetic position of 46 Curtobacterium spp. strains isolated from dry beans and other annual crops in Iran and Spain. Pathogenicity tests, resistance to arsenic compounds, plasmid profiling and BOX-PCR were performed on the strains. Multilocus sequence analysis (MLSA) was also performed on five housekeeping genes (i.e., atpD, gyrB, ppk, recA, and rpoB) of all the strains, as well as five pathotype strains of the species. Pathogenicity test showed that six out of 42 strains isolated in Iran were nonpathogenic on common bean. Despite no differences found between pathogenic and nonpathogenic strains in their plasmid profiling, the former were resistant to different concentrations of arsenic, while the latter were sensitive to the same concentrations. Strains pathogenic on common bean were polyphyletic with at least two evolutionary lineages (i.e., yellow-pigmented strains versus red/orange-pigmented strains). Nonpathogenic strains isolated from solanaceous vegetables were clustered within either the strains of C. flaccumfaciens pv. flaccumfaciens or different pathovars of the species. The results of MLSA and BOX-PCR analysis were similar to each other and both methods were able to discriminate the yellow-pigmented strains from the red/orange-pigmented strains. A comprehensive study of a worldwide collection representing all five pathovars as well as nonpathogenic strains of C. flaccumfaciens is warranted for a better understanding of the diversity within this phytopathogenic bacterium.

Phenotypic Diversity of Puccinia helianthi (Sunflower Rust) in the United States from 2011 and 2012.

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

Bacterial Wilt of Dry-Edible Beans in the Central High Plains of the U.S.: Past, Present, and Future.

Bacterial wilt, caused by Curtobacterium flaccumfaciens pv. flaccumfaciens, was first recognized and described as a new dry bean disease near Redfield, SD after the 1921 growing season on the farm of the Office of Forage Investigations. Between the late 1930s and the early 1950s it became one of the more problematic bacterial diseases of dry beans. It became an endemic problem in dry bean production throughout western Nebraska and other areas of the central high plains during the 1960s and early 1970s. By the early 1980s, the disease had virtually disappeared with the implementation of cultural practices. The disease was rediscovered in two fields in Nebraska late in the 2003 season. It was assumed to be an isolated incident. However, the next season the pathogen was widespread throughout western Nebraska production fields. Our research suggests that the return of bean wilt throughout the central high plains over the last decade is not due to a single factor but a combination of new changes in cultural practices, environmental stresses, and unfamiliarity with the pathogen and its past history.

Identification of Sources of Bacterial Wilt Resistance in Common Bean (Phaseolus vulgaris).

Over the last decade, bacterial wilt, caused by Curtobacterium flaccumfaciens pv. flaccumfaciens, has reemerged in the Central High Plains (Nebraska, Colorado, and Wyoming) and has been identified in almost 500 fields. Affected fields were planted with bean (Phaseolus vulgaris) from multiple market classes and seed sources, including yellow, great northern, pinto, kidney, cranberry, black, navy, pink, and small red, and incidence varied from trace levels to >90%. One wilt-resistant bean, 'Emerson', is available today but it is grown on a limited basis as a specialized cultivar for targeted markets in Europe and cannot be grown in all fields where the disease has recently been identified. Thus, we are faced with an emerging problem that must be addressed by utilizing newly developed resistant cultivars. This study was initiated to evaluate the Phaseolus National Plant Germplasm System (NPGS) bean collection for resistance to C. flaccumfaciens pv. flaccumfaciens in the ongoing effort to develop a new wilt-resistant cultivar adapted to this region. In total, 467 entries, including accessions from the NPGS, several commercial great northern and pinto cultivars, and University of Nebraska experimental lines, were screened with a highly virulent orange strain of C. flaccumfaciens pv. flaccumfaciens previously recovered from an infected great northern bean plant in Nebraska. Bacterial wilt severity ratings were 1.0 to 9.0 (0 to 90% incidence). Of the 427 accessions from the NPGS, only 1 showed resistance (0.23%), 19 showed intermediate resistances (4.45%), and the remainder were susceptible (95.34%). PI 325691 was identified as a source of bacterial wilt resistance. It was screened against six additional C. flaccumfaciens pv. flaccumfaciens strains and still produced resistant reactions. PI 325691 is a wild common bean (P. vulgaris) collected 8 miles South of Tzitzio, Michoacán, Mexico; however, it has a small seed size (5.3 g 100-1 seeds) that is commercially unacceptable. It will take several backcrosses to transfer this resistance to bacterial wilt and recover the seed size into a cultivated bean.