Dissemination of Goss's Wilt of Corn and Epiphytic Clavibacter michiganensis subsp. nebraskensis from Inoculum Point Sources

Herbicide safener isoxadifen-ethyl associated with increased Goss's wilt severity in corn (Zea mays)

BACKGROUND

Goss's bacterial wilt and leaf blight (Goss's wilt), caused by the bacterium Clavibacter nebraskensis, is a corn disease that has been a top ten yield-reducing disease in North America in the past 15 years. Isoxadifen-ethyl is an herbicide safener that effectively increases cytochrome P450 activity in corn which enhances a plant's metabolism of herbicide molecules. Recent research found a potential link between isoxadifen-ethyl and increased Goss's wilt severity.

RESULTS

The application of isoxadifen-ethyl increased (P = 0.014-0.046) area under disease progress curve (AUDPC) by 19%, 7%, and 9% at three environments, independent of accompanying herbicide or herbicide application timing. However, no significant differences in incidence of systemic wilt or corn grain yield occurred among treatments at any environment.

CONCLUSION

These data provide evidence for an association between isoxadifen-ethyl safener and Goss's wilt in corn. The reason for this association is unknown, but the safener may affect plant or pathogen physiological mechanisms. While the increased disease severity did not result in decreased grain yield in these experiments, an increase in pathogen inoculum due to higher disease severity could influence Goss' wilt epidemics in future years. © 2024 Society of Chemical Industry.

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 27794^T ; CFBP 2405^T ; ICMP 3298^T ; LMG 3700^T ; NCPPB 2581^T .

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.

Goss's Wilt Resistance in Corn Is Mediated via Salicylic Acid and Programmed Cell Death but Not Jasmonic Acid Pathways

A highly aggressive strain (CMN14-5-1) of Clavibacter nebraskensis bacteria, which causes Goss's wilt in corn, induced severe symptoms in a susceptible corn line (CO447), resulting in water-soaked lesions followed by necrosis within a few days. A tolerant line (CO450) inoculated with the same strain exhibited only mild symptoms such as chlorosis, freckling, and necrosis that did not progress after the first six days following infection. Both lesion length and disease severity were measured using the area under the disease progression curve (AUDPC), and significant differences were found between treatments. We analyzed the expression of key genes related to plant defense in both corn lines challenged with the CMN14-5-1 strain. Allene oxide synthase (ZmAOS), a gene responsible for the production of jasmonic acid (JA), was induced in the CO447 line in response to CMN14-5-1. Following inoculation with CMN14-5-1, the CO450 line demonstrated a higher expression of salicylic acid (SA)-related genes, ZmPAL and ZmPR-1, compared to the CO447 line. In the CO450 line, four genes related to programmed cell death (PCD) were upregulated: respiratory burst oxidase homolog protein D (ZmrbohD), polyphenol oxidase (ZmPPO1), ras-related protein 7 (ZmRab7), and peptidyl-prolyl cis-trans isomerase (ZmPPI). The differential gene expression in response to CMN14-5-1 between the two corn lines provided an indication that SA and PCD are involved in the regulation of corn defense responses against Goss's wilt disease, whereas JA may be contributing to disease susceptibility.

Colonization and Movement of Green Fluorescent Protein-Labeled Clavibacter nebraskensis in Maize

Clavibacter nebraskensis causes Goss's bacterial wilt and leaf blight, a major disease of maize. Infected crop residue is the primary inoculum source and infection can occur via wounds or natural openings, such as stomata or hydathodes. The use of resistant hybrids is the primary control method for Goss's wilt. In this study, colonization and movement patterns of C. nebraskensis during infection were examined using green fluorescent protein (GFP)-labeled bacterial strains. We successfully introduced a plasmid to C. nebraskensis via electroporation, which resulted in GFP accumulation. Fluorescence microscopy revealed that in the absence of wounding, bacteria colonize leaf tissue via entry through the hydathodes when guttation droplets are present. Stomatal penetration was not observed under natural conditions. Bacteria initially colonize the xylem and subsequently the mesophyll, which creates the freckles that are characteristic of the disease. Bacteria infiltrated into the mesophyll did not cause disease symptoms, could not enter the vasculature, and did not spread from the initial inoculation point. Bacteria were observed exuding through stomata onto the leaf surface, resulting in the characteristic sheen of diseased leaves. Resistant maize lines exhibited decreased bacterial spread in the vasculature and the mesophyll. These tools to examine C. nebraskensis movement offer opportunities and new insights into the pathogenesis process and can form the basis for improved Goss's wilt management through host resistance.

Infection, Survival, and Growth of Clavibacter nebraskensis on Crop, Weed, and Prairie Plant Species

Clavibacter nebraskensis is the causal agent of Goss's leaf blight and wilt, an important disease of maize in the United States and Canada. The epidemiology and ecology of this bacterial pathogen are poorly understood. Infested maize residue is often considered to be the primary source of inoculum for maize; however, the potential for many other plant species to be infected and serve as inoculum sources is unknown. The goal of this study was to determine if C. nebraskensis could infect, survive, and grow on common weed, crop, and grass species. Seedling leaves of 18 plant species that grow in maize production areas in the United States were inoculated with this pathogen in a controlled environment and in the field. Lesion development, bacterial streaming, and pathogen population size on leaves were then determined and used as criteria to evaluate host-pathogen interactions. Woolly cupgrass (Eriochloa villosa) and the native prairie grasses big bluestem (Andropogon gerardii) and little bluestem (Schizachyrium scoparium) developed lesions and bacterial streaming, identifying them as hosts and susceptible to infection. To our knowledge, this is the first report of these grass species being hosts of C. nebraskensis. Ten other grass species, including wheat and oats, were identified as potential sustaining hosts that maintained epiphytic or endophytic pathogen populations >10⁶ colony forming units per leaf sample but displayed no evidence of infection. Five broadleaf species tested were nonhosts based on the three criteria. This study suggests that multiple plant species support infection and growth of C. nebraskensis and further elucidates the ecology of this pathogen and the epidemiology of Goss's wilt.

Re-evaluation of Seed Transmission of Clavibacter michiganensis subsp. nebraskensis in Zea mays

The spread of Goss's bacterial wilt and leaf blight of corn (Zea mays), caused by Clavibacter michiganensis subsp. nebraskensis, to a wider geographic range in the early 2000s compared with the late 1960s has generated concern about the possible role of seed transmission in long-distance spread. The objectives of this research were: (1) to determine the percentage of seed infection found in seed harvested from inoculated and noninoculated plants of hybrids that varied in resistance to Goss's wilt; and (2) to estimate the seed transmission rate from these infected seed lots. The greatest percent seed infection was detected in seed from inoculated plants of the most susceptible hybrid and the least in seed from the most resistant hybrid. Seed lots with seed infection that ranged from 3.6 to 37.0% were planted in three field and three greenhouse trials. A total of 12 seed transmission events (Goss's wilt symptomatic seedlings) were identified among 241,850 plants examined, for a seed transmission rate of 0.005%. When the seed transmission rate was recalculated to consider only the infected seed portion of each seed lot, the rate increased to 0.040% (12 events from 30,088 potentially infected plants). Based on the low seed transmission rate observed and previous research on disease spread from a point source, it seems unlikely that seed transmission could introduce enough inoculum to create a serious disease outbreak in a single growing season. However, risk of seed transmission is relevant for phytosanitary restrictions and preventing the introduction of the pathogen to new areas. To date, Goss's wilt has not been detected outside North America, and while the risk of seed transmission is very low, the risk is not zero. Fortunately, the presence of C. michiganensis subsp. nebraskensis in corn seed is readily detectable by established seed health testing methods.

Specific and Sensitive Primers Developed by Comparative Genomics to Detect Bacterial Pathogens in Grains

Accurate and rapid detection of bacterial plant pathogen is the first step toward disease management and prevention of pathogen spread. Bacterial plant pathogens Clavibacter michiganensis subsp. nebraskensis (Cmn), Pantoea stewartii subsp. stewartii (Pss), and Rathayibacter tritici (Rt) cause Goss's bacterial wilt and blight of maize, Stewart's wilt of maize and spike blight of wheat and barley, respectively. The bacterial diseases are not globally distributed and not present in Korea. This study adopted comparative genomics approach and aimed to develop specific primer pairs to detect these three bacterial pathogens. Genome comparison among target pathogens and their closely related bacterial species generated 15-20 candidate primer pairs per bacterial pathogen. The primer pairs were assessed by a conventional PCR for specificity against 33 species of Clavibacter, Pantoea, Rathayibacter, Pectobacterium, Curtobacterium. The investigation for specificity and sensitivity of the primer pairs allowed final selection of one or two primer pairs per bacterial pathogens. In our assay condition, a detection limit of Pss and Cmn was 2 pg/μl of genomic DNA per PCR reaction, while the detection limit for Rt primers was higher. The selected primers could also detect bacterial cells up to 8.8 × 10³ cfu to 7.84 × 10⁴ cfu per gram of grain seeds artificially infected with corresponding bacterial pathogens. The primer pairs and PCR assay developed in this study provide an accurate and rapid detection method for three bacterial pathogens of grains, which can be used to investigate bacteria contamination in grain seeds and to ultimately prevent pathogen dissemination over countries.

Factors Affecting the Development and Severity of Goss's Bacterial Wilt and Leaf Blight of Corn, Caused by Clavibacter michiganensis subsp. nebraskensis

Goss's bacterial wilt and leaf blight, which is caused by Clavibacter michiganensis subsp. nebraskensis, is a disease of corn (Zea mays) that has been increasingly reported across the Midwest since its reemergence in western Nebraska, northeastern Colorado, and southeastern Wyoming during the 2006 growing season. The objective of this study was to identify environmental and agronomic factors contributing to the incidence of the disease across the Corn Belt through a multistate survey conducted during the 2011 growing season. Of the 2,400 surveys distributed throughout nine states, 486 were returned with corn leaf samples, of which 70% tested positive for C. michiganensis subsp. nebraskensis using an enzyme-linked immunosorbent assay. The agronomic data associated with each field were analyzed using classification and regression tree and random forest analyses to identify the factors that contributed most to Goss's bacterial wilt and leaf blight development. A χ² test of independence was also done to determine relationships between certain variables and disease incidence. The two best predictors of Goss's bacterial wilt and leaf blight were hybrid resistance to Goss's bacterial wilt and leaf blight, as indicated by the seed companies' score and a planting population density >67,500 plants ha^-1. Other important predictors included longitude, planting date, crop rotation, percent residue, yield history, tillage, and growth stage. Relationships between glyphosate applications, foliar fungicide applications, and corn rootworm beetle with samples testing positive for C. michiganensis subsp. nebraskensis were also detected. These data contribute to our understanding of factors that increase the risk of Goss's bacterial wilt and leaf blight, and should enable more effective management practices to be adopted or developed.