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Current Understanding of the History, Global Spread, Ecology, Evolution, and Management of the Corn Bacterial Leaf Streak Pathogen, Xanthomonas vasicola pv. vasculorum

    Affiliations
    Authors and Affiliations
    • Mary Ortiz-Castro1
    • Terra Hartman2 3
    • Teresa Coutinho4
    • Jillian M. Lang1
    • Kevin Korus5
    • Jan E. Leach1
    • Tamra Jackson-Ziems2
    • Kirk Broders1 6 †
    1. 1Agricultural Biology Department, Colorado State University, Fort Collins, CO, U.S.A.
    2. 2Department of Plant Pathology, University of Nebraska, Lincoln, NE, U.S.A.
    3. 3Bayer CropScience, Sabin, MN 56580, U.S.A.
    4. 4Department of Biochemistry, Genetics and Microbiology, Centre of Microbial Ecology and Genomics/Forestry and Agricultural Biotechnology Institute, University of Pretoria, Pretoria, South Africa
    5. 5Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL, U.S.A.
    6. 6Smithsonian Tropical Research Institute, Apartado 0843-03092, Balboa, Ancon, Republic of Panamá
    Published Online:4 May 2020https://doi.org/10.1094/PHYTO-01-20-0018-PER

    Abstract

    Bacterial leaf streak of corn, caused by Xanthomonas vasicola pv. vasculorum, has been present in South Africa for over 70 years, but is an emerging disease of corn in North and South America. The only scientific information pertaining to this disease on corn came from work done in South Africa, which primarily investigated host range on other African crops, such as sugarcane and banana. As a result, when the disease was first reported in the United States in 2016, there was very limited information on where this pathogen came from, how it infects its host, what plant tissue(s) it is capable of infecting, where initial inoculum comes from at the beginning of each crop season, how the bacterium spreads from plant to plant and long distance, what meteorological variables and agronomic practices favor disease development and spread, how many other plant species X. vasicola pv. vasculorum is capable of infecting or using as alternate hosts, and if the bacterium will be able to persist in all corn growing regions of the United States. There were also no rapid diagnostic assays available which initially hindered prompt identification prior to the development of molecular diagnostic tools. The goal of this synthesis is to review the history of X. vasicola pv. vasculorum and bacterial leaf streak in South Africa and its movement to North and South America, and highlight the recent research that has been done in response to the emergence of this bacterial disease.

    The emergence of Xanthomonas vasicola pv. vasculorum in North America likely has its origin in southeastern Nebraska and northeastern Colorado. The number and severity of reports by growers, crop consultants and extension agents in these parts of the country prompted researchers from Colorado State University and the University of Nebraska to meet near Grant, Nebraska in June 2016 to evaluate this widespread and almost ubiquitous bacterial disease that had escalated rapidly since the first symptoms were observed on grain corn and popcorn in the region in 2014 and possibly as early as 2010. The incidence of the disease was remarkable; in fields under center-pivot irrigation, field-level incidence was commonly greater than 90%. It was clear to all involved that day that a rapid response in both research and extension would be needed to inform both the grower community and the research community of this new disease in North America, bacterial leaf streak (BLS) of corn.

    While first reported in the United States in 2014, the disease became widespread in 2016, affecting grain corn, sweet corn, and popcorn (Korus et al. 2017). BLS of corn is now present in 10 states: Colorado, Kansas, Minnesota, South Dakota, Texas, Oklahoma, and the top three corn-producing states of Illinois, Iowa, and Nebraska (Damicone et al. 2018; Jamann et al. 2019; Korus et al. 2017; Lang et al. 2017; Smith et al. 2018). Both incidence and severity of the disease were most prominent in Colorado, Nebraska, and Kansas, likely due to corn being grown in continuous production under center-pivot irrigation (Hartman et al. 2020a). Based on previous performance of hybrids in the region, estimated yield loss in fields with the greatest disease severity ranged from 5 to 15%. In 2017, BLS was also reported on corn in Argentina (Plazas et al. 2018) and in 2018 in Brazil (Leite et al. 2018). Prior to 2016, the only reports of X. vasicola pv. vasculorum infecting corn came from South Africa in 1949 (Dyer 1949) and 1987 (Coutinho 1987). It was not clear how the pathogen was introduced to North and South America or if it was already present but undetected.

    The presence of X. vasicola pv. vasculorum in the Americas raised a number of questions from regulatory, disease management, and research perspectives. Hence, a network of regional and global collaborators was developed to fill the knowledge gaps on the basic biology, taxonomy, diagnostics, ecology and evolution of the pathogen as well as disease management, transmission, and epidemiology of the disease. The result of this collaborative work includes the articles published in this Pathogen Spotlight issue of Phytopathology (Arias et al. 2020; Hartman et al. 2020b; Perez-Quintero et al. 2020; Studholme et al. 2020; Stulberg et al. 2020) as well as papers published previously in Plant Disease (Jamann et al. 2019; Korus et al. 2017; Lang et al. 2017; Leite et al. 2018; Plazas et al. 2018).

    HISTORY OF BLS ON CORN, SUGARCANE, AND EUCALYPTUS IN SOUTH AFRICA

    BLS of corn was first reported in South Africa in 1948 when a culture of X. vasicola pv. vasculorum was deposited in NCPPB (strain 206). The disease at that time caused little damage and was regarded as economically unimportant (Coutinho and Wallis 1991). The next reported occurrence of the disease was in the mid-1980s in KwaZulu-Natal and the former Transvaal, now comprising four provinces, North West, Gauteng, Limpopo, and Mpumalanga. Today it occurs in the western, warmer South African provinces where corn is grown under irrigation. There are no completely resistant cultivars available in South Africa, although there are differences in the susceptibility of different genotypes to this pathogen, indicating resistance may be quantitative.

    In South Africa, X. vasicola pv. vasculorum has also been reported in sugarcane causing gummosis or gumming disease as early as 1977 (Gorter 1977). Cultures from South Africa deposited in NCPPB have all been identified as X. vasicola pv. vasculorum (Studholme et al. 2020). In South Africa, the most notable symptoms caused by X. vasicola pv. vasculorum on sugarcane are the deformation of the stalk, internal discoloration, and the formation of gum pockets containing bacteria (McFarlane and Coutinho 2010). In the early 2000s, an outbreak of bacterial blight and die-back caused by X. vasicola pv. vasculorum occurred on a single Eucalyptus grandis clone in KwaZulu-Natal. The plantation was surrounded by sugarcane fields and it was hypothesized that X. vasicola pv. vasculorum jumped from these fields to the newly established plantation (Coutinho et al. 2015). Subsequently, outbreaks of the disease occurred in Eucalyptus propagation nurseries located in Mpumalanga in 2007 and again in 2017.

    FIRST DETECTION AND EARLY SPREAD OF X. VASICOLA PV. VASCULORUM IN NORTH AND SOUTH AMERICA

    In the fall of 2014 and starting again in June 2015, corn leaf samples with interveinal yellow to brown streaks with irregular margins were submitted to the University of Nebraska-Lincoln Plant and Pest Diagnostic Clinic from Nebraska fields. Bacterial streaming was obvious from the cut lesions and yellow, mucoid bacteria were isolated on nutrient broth yeast extract agar. Completion of Koch’s postulates and subsequent testing of the pathogen by Korus et al. (2017) and Lang et al. (2017) confirmed the identity of the bacterium as X. vasicola pv. vasculorum. During the early months of the 2016 growing season, BLS was prevalent in many Nebraska, Kansas, and Colorado corn fields. By August 2016, the presence of X. vasicola pv. vasculorum causing BLS in corn had been confirmed via PCR in 50 Nebraska counties and six counties in Colorado using the diagnostic primers developed by Lang et al. (2017). Identity of the pathogen was confirmed by USDA-APHIS and a coordinated announcement was made identifying X. vasicola pv. vasculorum in the United States on 26 August 2016 (Bowman and Bissonnette 2016; Jackson-Ziems et al. 2016; Robertson 2016; USDA-APHIS 2016). Within a year of this announcement, the disease was also reported in Argentina (Plazas et al. 2018) and Brazil (Leite et al. 2018). According to Plazas et al. (2018), symptoms resembling BLS in Argentina were present since 2010 in the province of Córdoba.

    As mentioned above, the first reports of BLS in the United States were from eastern Colorado and western Nebraska and Kansas. Interestingly, these three states, and the regions of South Africa and Argentina where BLS is most prevalent, share the same type of cold semiarid climate, BSk classification (EarthData 2018) (Fig. 1). Therefore, it is possible this type of climate favored the initial establishment of the pathogen and since then spread to other regions with higher humidity such as those in the Corn Belt states (humid, continental climate with hot summers, Dfa classification) (Fig. 1). It is unknown whether other corn-producing countries sharing this same semiarid climate (parts of Mexico, China, and Australia) currently have the disease, but it is possible the disease will appear in these regions in the future.

    FIGURE 1.

    FIGURE 1. Current distribution of bacterial leaf streak of corn. Global map shows countries and states where Xanthomonas vasicola pv. vasculorum is confirmed (yellow); cold, semiarid regions (purple); humid continental climate with hot summers (blue); and countries at risk for X. vasicola pv. vasculorum (green) based on climate. Purple and blue regions correspond to the Koppen Climate Classification BSk and Dfa, respectively (EarthData 2018).

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    CURRENT UNDERSTANDING OF BLS DISEASE ECOLOGY

    Our ability to manage plant diseases depends on the fundamental knowledge of the ecology and biology of the pathogens causing the disease (Morris et al. 2017). By understanding how X. vasicola pv. vasculorum enters, exits, and interacts with different hosts and with other microbes, we can begin to develop an integrated management plan.

    Based on results from Ortiz-Castro (2019), X. vasicola pv. vasculorum colonizes the nonvascular apoplastic tissue, similar to other foliar bacterial pathogens such as Xanthomonas axonopodis pv. allii (causal agent of Xanthomonas leaf blight of onion) (Schwartz and Gent 2011) and X. translucens pv. undulosa (causal agent of BLS of barley) (Adhikari et al. 2012). This finding is consistent with reports from Hartman (2018) where the pathogen was only recovered from leaves present at time of inoculation. These results suggest that, unlike the strain of X. vasicola pv. vasculorum that infects sugarcane, corn-infecting X. vasicola pv. vasculorum remains localized to the site of infection, leading to a nonsystemic disease response on corn.

    The ability of a plant pathogen to enter and exit their host is a crucial first step in the development of a foliar infection and disease spread (Melotto et al. 2008). Based on observations in the field and spray inoculation in the greenhouse (Hartman et al. 2020b), it is likely that X. vasicola pv. vasculorum primarily infects through the stomata and does not require a wound to enter the leaf. The copious amounts of Xanthomonas-containing exudates observable as droplets on leaf surfaces are likely splashed to neighboring plants through rain and/or irrigation. These results are consistent with field observations that BLS symptoms travel from lower leaves to the upper canopy of plants under center-pivot irrigation (Fig. 2A to H).

    FIGURE 2.

    FIGURE 2. Symptoms of bacterial leaf streak of corn including A, early streak lesion development, B, small spot lesion development, C and D, symptom development starting at the base of the plant progressing up resulting in E, F, and G, coalescing severe lesions on upper leaves, and H, bacterial droplet signs of Xanthomonas vasicola pv. vasculorum from early lesion development.

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    Xanthomonas species have several mechanisms of survival. Several species, including X. phaseoli pv. fuscans (Darsonval et al. 2008), X. euvesicatoria (Jones et al. 1986), X. campestris pv. phaseoli (Gilbertson et al. 1990), and X. translucens pv. undulosa (Adhikari et al. 2012), survive associated with their respective hosts’ residue. Other bacteria, such as X. axonopodis pv. allii, may survive between susceptible crops in association with contaminated seed or infested crop residue, or epiphytically or pathogenically on volunteer onion, weeds, and leguminous plants (Gent et al. 2005). This would suggest that X. vasicola pv. vasculorum may survive from one production season to the next either in the soil, plant residue, seed, alternative hosts or a combination these.

    Preliminary data suggests that X. vasicola pv. vasculorum likely overwinters in infected corn residue left on the surface of the field from the previous growing season. Using qPCR, Ortiz-Castro (2019) showed that quantities of X. vasicola pv. vasculorum DNA recovered from corn residue on the soil surface were significantly higher than quantities of X. vasicola pv. vasculorum DNA in corn residue buried 10 cm below ground after a 6-month period. Although this study did not report live bacteria, it is suspected infected residue provides the primary inoculum for disease development since corn is often grown continuously in Colorado and western Nebraska and Kansas and in a corn-soybean-corn rotation in central and eastern Nebraska and Kansas. As seed is planted into residue and corn seedlings emerge, bacteria from the residue can be splashed onto new tissue through early-season rain and irrigation. After landing on the leaf surface, the bacteria may enter the plant through natural openings, such as stomata, or through wounds. Once inside, bacteria reproduce and cause characteristic interveinal streaks on corn leaves (Fig. 2A). Infections have been observed in the field as early as growth stage V4 (four leaves with collars visible), and in the greenhouse as early as VE (emergence).

    While infested residue represents one potential source of inoculum for initial infections within a field, the long-distance movement of X. vasicola pv. vasculorum within residue is less likely. Seed transmission of X. vasicola pv. vasculorum may be able to move the pathogen longer distances and introduce it into previously disease-free regions. In this issue, Arias et al. (2020) evaluate the occurrence of X. vasicola pv. vasculorum on seeds from diseased fields and its subsequent transmission to seedlings using both molecular- and culture-based methods. They found that X. vasicola pv. vasculorum was detected by TaqMan PCR in 22 of 41 corn seed lots harvested from naturally infected fields in Colorado, Nebraska, and Iowa. However, many of the PCR-positive seed samples did not yield viable cultures of X. vasicola pv. vasculorum colonies. In greenhouse experiments, putative seed transmission from naturally contaminated seed lots, estimated from PCR results, occurred at a frequency between 0.1 and 0.5% in 10-seedling pooled samples and at a frequency of 2.7% from individual plant assays. No seedling symptoms were observed in these assays and live X. vasicola pv. vasculorum colonies were not recovered from any of the PCR-positive seedlings. However, seed transmission was readily demonstrated from seed lots artificially contaminated with X. vasicola pv. vasculorum at a concentration of 106 colony-forming units (CFU) per milliliter, including leaf streak symptoms and recovery of live bacteria. In these assays, the lower bacterial populations on naturally contaminated seeds were not sufficient to result in diseased seedlings.

    Research on the persistence of X. vasicola pv. vasculorum has also focused on identifying potential alternative hosts for X. vasicola pv. vasculorum among plants commonly found in or near corn fields in the United States. By measuring the percentage of leaf tissue infected under greenhouse conditions, Hartman et al. (2020b) found that X. vasicola pv. vasculorum can colonize 15 new hosts, of which 12 were symptomatic and three were asymptomatic. Symptomatic hosts included crops such as oat (Avena sativa) and rice (Oryza sativa), as well as prairie grasses such as orchard grass (Dactylis glomerata), Indiangrass (Sorghastrum nutans), big bluestem (Andropogon gerardii), little bluestem (Schizachyrium scoparium), timothy (Phleum pratense), sand bluestem (Andropogon hallii), green foxtail (Setaria viridis), and bristly foxtail (Setaria verticillata). This study also confirmed infection of the weeds johnsongrass (Sorghum halepense) and yellow nutsedge (Cyperus esculentus). In addition to these hosts, Lang et al. (2017) demonstrated that X. vasicola pv. vasculorum from corn can infect sorghum (Sorghum bicolor), although this has not been observed in the field in the United States. Endophytic colonization by X. vasicola pv. vasculorum was found in three asymptomatic alternative hosts: downy brome (Bromus tectorum), tall fescue (Festuca arundinacea), and western wheatgrass (Pascopyrum smithii) (Hartman et al. 2020b). Field studies found that infection in big bluestem and bristly foxtail was possible, but with low incidence levels. These results suggest that infection of alternative hosts by X. vasicola pv. vasculorum can occur, but infection levels might be limited by environmental conditions (Hartman et al. 2020b).

    Agronomic practices also can have a significant impact on disease development and spread. This is highlighted in this issue by the investigation of agronomic factors contributing to incidence of BLS of corn by Hartman et al. (2020a). Using classification and regression tree (Breiman et al. 1984) and random forest (Breiman 2001) analyses of grower surveys (Kim et al. 2002; Langemeier et al. 2017; Paul and Munkvold 2004), they found that irrigation, crop rotation, growth stage, tillage, and planting date were the most important predictors of a sample testing positive for the presence of X. vasicola pv. vasculorum (Hartman et al. 2020a). Out of these factors, irrigation was the most relevant factor in creating a conducive environment for disease development. This is likely the result of frequent use of center-pivot irrigation in the area (Lichtenberg 1989; Turkington et al. 2004) leading to high humidity which favors the spread and colonization of X. vasicola pv. vasculorum in corn fields in Colorado and Nebraska where BLS has a high incidence.

    PROPOSED DISEASE CYCLE

    Understanding the major aspects of the BLS disease cycle is crucial to provide information enabling growers to make management decisions as well as preventing further spread of the disease to other corn-producing regions. No disease cycle had been described for BLS of corn previously. Based on observations over three growing seasons, comparisons with other Xanthomonas species causing foliar diseases, and previous research on survival, localization, entrance, and formation of secondary inoculum, we propose the following BLS disease cycle (Fig. 3).

    FIGURE 3

    FIGURE 3 Proposed disease cycle of bacterial leaf streak of corn including A, survival in crop residue; B, primary infection from rain or irrigation splash of Xanthomonas vasicola pv. vasculorum onto immature lower corn leaves; C, primary infection through natural opening or wounds; D, lesion development and production of bacterial ooze droplets on the leaf surface; E, secondary spread from lower leaves to upper leaves and plant to plant via overhead irrigation and rain splash as well as between field movement through wind-driven rain; and F, fully grown infected plant ready for harvest.

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    X. vasicola pv. vasculorum likely overwinters in infected crop residue from the previous growing season. Then, as corn seedlings emerge, bacteria from the residue are splashed onto new tissue through early-season rain and irrigation. The pathogen may enter the plant through natural openings or wounds, and once inside the plant, may reproduce and cause characteristic interveinal streaks on corn leaves. Symptoms are typically first observed on the lower leaves of the plant (Fig. 2C), and then begin to appear in the mid to upper canopy (Fig. 2D to G), usually after use of center-pivot irrigation or after wind-driven rain events as the season progresses (T. Jackson-Ziems and K. Broders, personal communication). These events provide the environmental conditions for secondary inoculum formation, and for plant-to-plant as well as field-to-field movement of X. vasicola pv. vasculorum. This is consistent with previous reports that wind-driven rain can disperse inoculum and aggravate diseases such as Xanthomonas leaf blight of onion (Schwartz et al. 2003). From mid- to late season, bacteria may continue to spread among fields until corn is harvested, with residue again left on the surface of the soil to provide the principal inoculum for the next growing season. Aspects related to potential alternative hosts and persistence of X. vasicola pv. vasculorum on seeds were not included in the proposed disease cycle because the frequency of these components, while important, are likely rare and sporadic.

    TAXONOMY AND EVOLUTION OF X. VASICOLA PV. VASCULORUM AND, MORE BROADLY, X. VASICOLA

    Studholme et al. (2020) provide an updated overview of the different evolutionary lineages of X. vasicola species based on whole-genome average nucleotide identity, dividing them into five groups: (i) X. vasicola pv. vasculorum infecting corn and sugarcane, (ii) X. vasicola pv. holcicola infecting sorghum, (iii) X. vasicola pv. musacearum infecting enset (Ensete ventricosum) and banana, (iv) strains isolated from Guatemalan Gamagrass (Tripsacum laxum), and (v) strains isolated from areca palm, Areca catechu (previously X. campestris pv. arecae). The taxonomic description of the first group, which contains X. vasicola pv. vasculorum, has been in a state of flux for over 30 years. Several changes have been made to the taxonomic nomenclature from X. campestris pv. zeae to X. vasicola pv. zeae to its current designation as X. vasicola pv. vasculorum (Bradbury 1986; Coutinho and Wallis 1991; Lang et al. 2017; Qhobela et al. 1990; Sanko et al. 2018). Studholme et al. (2020) stated that X. campestris pv. zeae fall within a clade of X. campestris pv. vasculorum (Cobb 1894) that belongs within the species X. vasicola pv. vasculorum (Vauterin et al. 1995).

    One distinction among X. vasicola pv. vasculorum is that the sugarcane-infecting strains can infect vascular tissue while corn-infecting strains are limited to foliar tissue (Karamura et al. 2015; Lang et al. 2017). This difference in colonization patterns resulted in the sugarcane pathogen being erroneously described as X. axonopodis pv. vasculorum (Lewis Ivey et al. 2010). More recent phylogenetic comparisons, through average nucleotide identity calculations and pathogenicity tests, solve this ambiguity demonstrating that sugarcane and corn X. vasicola pv. vasculorum strains are the same pathogen (Karamura et al. 2015; Lang et al. 2017; Wasukira et al. 2014).

    While X. vasicola pv. vasculorum on corn has been present in South Africa for over 70 years, it was unclear how and when X. vasicola pv. vasculorum was introduced into the United States and Argentina. Based on results from tip-dating inference, Perez-Quintero et al. (2020) determined there were two possible introduction events of corn X. vasicola pv. vasculorum strains, one around 1997 and one around 2010, the latter shortly before the detection of the epidemic (Korus et al. 2017; Plazas et al. 2018). The group corresponding to the 2010 introduction contains most of the sampled strains, and has Argentinian strains at the root of the clade, suggesting this introduction occurred first in South America. It is more difficult to determine whether this is also the case for the 1997 introduction since only one of the sampled U.S. strains falls within this group. Corn germplasm is moved between the United States and Argentina on an annual basis by international seed companies, and this may have facilitated genetic exchange between these distant populations.

    Perez-Quintero et al. (2020) also identified a genomic region, comprising a prophage, that is shared with X. vasicola pv. holcicola and North and South American X. vasicola pv. vasculorum isolates, but is absent in South African X. vasicola pv. vasculorum isolates and in X. vasicola pv. vasculorum from sugarcane. They hypothesize that this prophage region was transferred horizontally to an ancestor of the American X. vasicola pv. vasculorum populations, and that it may carry virulence genes or genes that provide a fitness advantage (Perez-Quintero et al. 2020). Prophages can have diverse effects on bacterial genomes such as increase of aggressiveness by transport of virulence factors, enhancement of fitness, and even decreasing virulence (Ahmad et al. 2014; Brüssow et al. 2004; Figueroa-Bossi et al. 2001).

    The study by Perez-Quintero et al. (2020) also identified another region in X. vasicola pv. vasculorum that contains eight genes found in the plant-pathogenic bacterium Pantoea ananatis. Intriguingly, Pantoea ananatis has frequently been isolated from corn leaves infected with X. vasicola pv. vasculorum (Lang et al. 2017) and has been reported as a pathogen causing corn white spot disease in Argentina in the same region where X. vasicola pv. vasculorum was first observed (Alippi and Lopez 2010). The function of Pantoea ananatis within the corn plant is still unclear; the bacterium has been shown to have antimicrobial activity (Coutinho and Venter 2009; Iimura and Hosono 1996), to induce systemic resistance (Kang et al. 2007), and to infect eucalyptus in South Africa (Coutinho and Venter 2009). Future research will need to focus on understanding how X. vasicola pv. vasculorum and Pantoea ananatis interact and compete for space inside the corn leaf, as well as understand if any genes X. vasicola pv. vasculorum has acquired from Pantoea ananatis impart any fitness advantage.

    MOLECULAR DIAGNOSTICS

    Accurate detection of a pathogen is critical to disease management and epidemiology in agricultural crops. Molecular diagnostic assays have become standard in monitoring plant diseases, particularly due to their speed and cost efficiency. Publicly available genomes representing the geographic and genetic diversity of X. vasicola pv. vasculorum were compared to develop PCR-based assays for rapid and accurate identification. The power of this approach is the identification of unique loci to X. vasicola pv. vasculorum strains infecting corn; assays based on these loci help to avoid misidentification. Conventional PCR with primers specific to corn-infecting X. vasicola pv. vasculorum was first established (Lang et al. 2017) using contemporary and historic strains, and remains to be the most cost-effective and commonly used method for detection of X. vasicola pv. vasculorum. This was followed by a sensitive TaqMan qPCR assay (Stulberg et al. 2020), which enabled the quantification of X. vasicola pv. vasculorum in residue and seed. A potentially mobile assay using loop mediated isothermal amplification that could aid in field detection was also developed (Stulberg et al. 2020) and may prove to be an effective tool for in-field diagnosis. A multilocus variable number of tandem repeat analysis scheme was recently developed for the banana Xanthomonas wilt pathogen, X. vasicola pv. musacearum, to enhance population surveillance and establish the genetic structure of this group, but the authors also identified a locus specific to X. vasicola that may prove useful in future work on X. vasicola pv. vasculorum (Nakato et al. 2019). Availability of reliable diagnostic tools for researchers and government agencies has improved our fundamental understanding of these organisms and the diseases they cause. Future assay development should be tailored to developing countries with significant corn production where equipment and resources may be limited.

    OPPORTUNITIES FOR DISEASE MANAGEMENT

    Standard management practices for residue-borne pathogens may mitigate disease incidence and severity caused by X. vasicola pv. vasculorum. These practices include tillage to promote the degradation of infested residue, thus reducing inoculum from previous years (Barak et al. 2001; Gent et al. 2005). Rotation to a nonhost crop is also advised, as it removes the bacterium’s primary host from the field, which in turn removes a source of survival for the pathogen. Preventing movement of residue between fields and harvesting heavily infected fields last are two strategies that may decrease disease incidence for the next growing season (Gent et al. 2005). Weed management is also an important consideration, as weeds, especially grasses, can potentially serve as alternative hosts.

    Chemical control via bactericides may inhibit disease development (Gent and Schwartz 2005); however, all available bactericides, which are mostly copper based, are contact products, meaning they remain on the surface of the plant without being absorbed systemically. Hence, they are washed off readily after rain or irrigation, requiring multiple applications to effectively control disease. Multiple applications of bactericidal products will likely be impractical in most corn growing situations, as the benefit will not justify the expense. Also, traditional application methods of bactericides predominantly to the upper leaf surfaces may not be effective; if coverage is not adequate on the lower leaf surface, the pathogen can still infect through stomata. Since chemical control is not readily available, sanitation of equipment is crucial to prevent further spread of the disease (Jackson et al. 2007).

    Based on observations in the field and greenhouse, all commercially available corn hybrids are susceptible to BLS, but there is wide variation in disease severity among varieties. This would indicate that resistance is a multigenic trait. Identification of pathogen effectors involved in virulence as well as quantitative trait loci (QTL) for disease resistance in corn will aid in the design of resistant hybrids which will likely be the most effective and practical means of disease control. In this sense, Qiu et al. (2020) conducted the first QTL mapping study for BLS resistance in three corn populations. They identified five significant QTL across five chromosomes in two populations. Out of those, one conferred a moderate effect, while the others conferred small effects confirming that resistance to BLS is a polygenic trait. Additionally, this study found a significant positive correlation of BLS resistance with northern corn leaf blight resistance in one of the populations. These findings constitute the basis for future breeding against BLS of corn.

    SUMMARY

    Agriculture in the United States will continue to face threats from emerging pathogens including both endemic and introduced species. The primary factors contributing to these threats are climate change and international trade including the movement of seeds and live plants, but also movement of pathogens and insects in pallet wood used to ship other items. In order to be prepared for the next emerging disease, it will be important to rapidly respond to each of these new threats. This will require collaboration among growers, extension professionals, industry, government and academic scientists, and regulatory agencies in order to rapidly identify these new disease threats and develop effective diagnostic tools and management strategies. This Pathogen Spotlight issue highlights such a collaborative effort by researchers across multiple countries and continents working together to determine (i) where this pathogen came from (Perez-Quintero et al. 2020); (ii) its relationship to other members of X. vasicola (Studholme et al. 2020); (iii) what meteorological variables and agronomic practices favor disease development and spread (Hartman et al. 2020a), how many other plant species X. vasicola pv. vasculorum is capable of infecting or using as alternate hosts (Hartman et al. 2020b); and (iv) how it infects its host, what plant tissue(s) it is capable of infecting, and where initial inoculum comes from at the beginning of each crop season (Arias et al. 2020; Ortiz-Castro 2019). There were also no rapid diagnostic assays available, which initially hindered prompt identification prior to the development of molecular diagnostic tools (Lang et al. 2017; Stulberg et al. 2020).

    In the years since X. vasicola pv. vasculorum was identified on corn in the United States, another pathogen, Phyllachora maydis, which causes tar spot, has become of increasing concern in corn-producing regions of Illinois, Indiana, Iowa, Michigan, and Wisconsin (McCoy et al. 2018; Ruhl et al. 2016). It will be important that federal agencies have the capacity to quickly provide funding to support such response efforts. The current system can take up to a year from the time a program is announced until funding is granted. That amounts to one or possibly two production years for a grower. The current research benefited significantly from early financial contributions from the Colorado Corn Administrative Committee and the Nebraska Corn Board as well as the Rapid Outcomes from Agricultural Research Foundation for Food and Agriculture Research program. Without these early funds, our ability to gather data on this disease that we then relay to growers prior to the next growing season would have been significantly reduced.

    The author(s) declare no conflict of interest.

    LITERATURE CITED

    The author(s) declare no conflict of interest.

    Funding: Support was provided by grants from the Colorado Corn Administrative Committee, the Nebraska Corn Board, USDA-Animal and Plant Health Inspection Service (grant 6.0533.01), and the Foundation for Food and Agriculture Research (grant 544722).