Xanthomonas vasicola pv. vasculorum
(bacterial leaf streak of corn)

Pictures

Picture	Title	Caption	Copyright
Title Symptoms Caption Xanthomonas vasicola pv. vasculorum (bacterial leaf streak of corn); foliar symptoms of bacterial leaf streak on corn (Zea mays). Clay Center, Nebraska, USA. August, 2018. Copyright ©Vinicius Garnica/Bugwood.org - CC BY-NC 3.0 US	Symptoms	Xanthomonas vasicola pv. vasculorum (bacterial leaf streak of corn); foliar symptoms of bacterial leaf streak on corn (Zea mays). Clay Center, Nebraska, USA. August, 2018.	©Vinicius Garnica/Bugwood.org - CC BY-NC 3.0 US
Title Symptoms Caption Xanthomonas vasicola pv. vasculorum (bacterial leaf streak of corn); foliar symptoms of bacterial leaf streak on corn (Zea mays). Clay Center, Nebraska, USA. August, 2018. Copyright ©Vinicius Garnica/Bugwood.org - CC BY-NC 3.0 US	Symptoms	Xanthomonas vasicola pv. vasculorum (bacterial leaf streak of corn); foliar symptoms of bacterial leaf streak on corn (Zea mays). Clay Center, Nebraska, USA. August, 2018.	©Vinicius Garnica/Bugwood.org - CC BY-NC 3.0 US
Title Symptoms Caption Xanthomonas vasicola pv. vasculorum (bacterial leaf streak of corn); early symptoms of bacterial leaf streak on corn (Zea mays). Clay Center, Nebraska, USA. August, 2018. Copyright ©Vinicius Garnica/Bugwood.org - CC BY-NC 3.0 US	Symptoms	Xanthomonas vasicola pv. vasculorum (bacterial leaf streak of corn); early symptoms of bacterial leaf streak on corn (Zea mays). Clay Center, Nebraska, USA. August, 2018.	©Vinicius Garnica/Bugwood.org - CC BY-NC 3.0 US
Title Symptoms Caption Xanthomonas vasicola pv. vasculorum (bacterial leaf streak of corn); early symptoms of bacterial leaf streak on corn (Zea mays). Clay Center, Nebraska, USA. August, 2018. Copyright ©Vinicius Garnica/Bugwood.org - CC BY-NC 3.0 US	Symptoms	Xanthomonas vasicola pv. vasculorum (bacterial leaf streak of corn); early symptoms of bacterial leaf streak on corn (Zea mays). Clay Center, Nebraska, USA. August, 2018.	©Vinicius Garnica/Bugwood.org - CC BY-NC 3.0 US
Title Symptoms Caption Xanthomonas vasicola pv. vasculorum (bacterial leaf streak of corn); early symptoms on lower canopy of an inbred line, showing foliar symptoms of bacterial leaf streak. Clay Center, Nebraska, USA. August, 2018. Copyright ©Vinicius Garnica/Bugwood.org - CC BY-NC 3.0 US	Symptoms	Xanthomonas vasicola pv. vasculorum (bacterial leaf streak of corn); early symptoms on lower canopy of an inbred line, showing foliar symptoms of bacterial leaf streak. Clay Center, Nebraska, USA. August, 2018.	©Vinicius Garnica/Bugwood.org - CC BY-NC 3.0 US

Identity

Preferred Scientific Name

• Xanthomonas vasicola pv. vasculorum (Cobb 1894)

Preferred Common Name

• bacterial leaf streak of corn

Other Scientific Names

• Xanthomonas campestris pv. vasculorum (Cobb 1894) Dye 1978
• Xanthomonas campestris pv. zeae
• Xanthomonas vasicola Vauterin et al. 1995
• Xanthomonas vasicola pv. zeae

International Common Names

• Spanish: bacteriosis de la raya del maiz
• Portuguese: estria bacteriana do milho

Local Common Names

• South Africa: eucalyptus leaf blight and dieback; gumming disease of sugarcane

Summary of Invasiveness

Xanthomonas vasicola pv. vasculorum (Xvv) is a bacterial pathogen that causes both bacterial leaf streak of maize and sugarcane gumming disease. After decades limited to South Africa, bacterial leaf streak of corn spread rapidly through maize-growing areas of Argentina, Brazil and the USA since 2014. The origin, method and biological underpinnings of this sudden spread are not well understood but are the subject of active research. Effective control methods remain elusive, but sanitation and control of crop debris may limit the disease. Yield impact data are not yet available, but lesions may become severe enough to limit plant productivity in some varieties. The pathogen is not currently considered a quarantine threat by the USDA, EPPO or IPPC.

Taxonomic Tree

• Domain: Bacteria
•     Phylum: Proteobacteria
•         Class: Gammaproteobacteria
•             Order: Xanthomonadales
•                 Family: Xanthomonadaceae
•                     Genus: Xanthomonas
•                         Species: Xanthomonas vasicola pv. vasculorum

Notes on Taxonomy and Nomenclature

The taxonomic nomenclature of the pathogen has undergone several changes and is still being resolved at the time of this report. The Xanthomonas clade causing bacterial leaf streak of maize was initially named X. campestris pv. vasculorum (Young et al., 1978), along with related strains of bacteria that cause a sugarcane gumming disease. Based on genetic typing, the maize pathogenic isolates were later assigned to X. campestris pv. zeae, and then in 1995 reassigned to the species X. vasicola, a species which also included strains causing diseases of several different hosts (reviewed in Lang et al., 2017). Some publications referred to the maize pathogenic strains as X. vasicola pv. zeae. Based on comprehensive genomic analyses, the species X. vasicola has been proposed to include four named pathovars, of which X. vasicola pv. vasculorum includes strains causing both bacterial leaf streak and sugarcane gumming (Lang et al., 2017; Studholme et al., 2020). The other pathovars holcicola, araceae and musacearum cause diseases of sorghum, areca nut and banana and enset, respectively, but do not threaten maize and sugarcane. Although the pathovar X. vasicola pv. vasculorum has not achieved formal standing in nomenclature as of January 2020, this pathovar designation has been widely adopted by the research community. A pathovar description has been formally proposed and is likely to be recognized (Studholme et al., 2020).  

It should be noted that a phenotypically similar sugarcane gumming disease is caused by X. axonopodis pv. vasculorum (Xav). Prior to the 1990s, many Xanthomonas strains were classified according to their host and not by biological relatedness; during this time sugarcane-associated Xav and Xvv strains were grouped into the same species and pathovar, X. campestris pv. vasculorum. However, genetic analysis revealed that X. axonopodis is a distinct species not closely related to X. vasicola (Harrison and Studholme, 2014).

Description

X. vasicola pv. vasculorum forms Gram-negative rods, 1.5-2 µm long and 0.4-0.5 µm in diameter, and is actively motile by means of a single polar flagellum. Bacteria produce yellow colonies on GYCA agar, eventually colouring the agar orange-yellow. It also produces abundant mucoid exopolysaccharides. Xvv is obligately aerobic, is asparagine-negative as the only source of carbon and nitrogen, oxidase-negative, and catalase-positive.

Xvv enters the leaves through stomata and colonizes the intercellular spaces (Ortiz-Castro, 2019). Despite its pathovar name, X. vasicola pv. vasculorum is not known to colonize the vasculature of maize, and has only been observed in nonvascular tissue. However, on sugarcane, the pathogen is reported to enter vascular or systemic phase in which it moves from the leaves to the stem vasculature, causing the symptom of gumming, or a vascular ooze emerging from the cut stem.

Distribution

Distribution within the USA expanded rapidly between 2016 and 2020. Reports in South Dakota, Texas, Wisconsin, Iowa and Minnesota have been documented through University extension bulletins, but not yet in the scientific literature.

History of Introduction and Spread

X. vasicola pv. vasculorum causing leaf streak of maize was observed in South Africa over 70 years ago (Dyer, 1949). There were no reports of Xvv on maize in other countries prior to 2010, when symptoms were first observed in Argentina. The pathogen may have spread regionally before being officially documented in 2018, the same year it was reported in Brazil (Plazas et al., 2018; Leite et al., 2019). The pathogen was first observed in the USA in 2014 and has now been confirmed throughout major US maize-producing states including Nebraska, Minnesota, Colorado, Kansas, Oklahoma, Iowa, Illinois and South Dakota (Korus et al., 2017; Damicone et al., 2018; Byamukama, 2019; Jamann et al., 2018). Isolates collected from the American epidemic tended to be more virulent than isolates from South African maize (Perez-Quintero et al., 2020). DNA sequence analysis has demonstrated that that the US epidemic isolates generally belong to a single group that is more closely related to strains from Argentina than those from South Africa, but these data are not sufficient to conclusively point to a single origin or time of introduction (Broders, 2017; Perez-Quintero et al., 2020).

Xvv was also collected on sugarcane in Zimbabwe and Madagascar in the 1960s (Wasukira et al., 2014). Interestingly, a Xanthomonas sugarcane disease called ‘false red stripe’ with leaf symptoms similar to Xvv was reported in Brazil in the 1990s. This Xanthomonas sp. was also capable of infecting maize and sorghum, but this pathogen has not been identified to species (Mantovani et al., 2006). Reports of Xvv disease on eucalyptus are thus far limited to South Africa.

Risk of Introduction

There is no evidence yet of yield or crop loss due to Xvv in maize, and it is not considered a quarantine pathogen by the USDA and EPPO. However, the spread of the disease within North and South America suggests a significant potential risk of introduction to new areas.

Hosts/Species Affected

Xvv most significantly affects maize and sugarcane. An outbreak in eucalyptus was reported in association with Pantoea ananatis, representing a host jump from monocot to dicot (Coutinho et al., 2015). In artificial inoculation studies in the field and greenhouse, isolates from North American maize caused symptoms on sorghum, Jerry oats, and several grass species, but did not infect wheat, barley, cereal rye or other grasses (Lang et al., 2017; Hartman, 2018). However, X. vasicola pv. vasculorum has not been detected on these hosts in a naturally-infected context. Infection attributed to Xvv has also been reported in naturally infected Tripsacum laxum, a wild species also called Guatemala grass (Broders, 2017), but genetic analysis shows that T. laxum strains are distinct from Xvv, and they are no longer considered part of this pathovar (Studholme et al., 2020).

Host Plants and Other Plants Affected

Growth Stages

Symptoms

Bacterial leaf streak symptoms on corn are identified by thin, long lesions that run parallel to the leaf vein and have wavy or jagged margins. The lesions can range from yellow and orange to brown, and when held up to the sun appear translucent and surrounded by a yellow halo. Early lesions may appear watersoaked and greasy. These lesions range from one to several inches, expanding over the time to cover a large area of the leaf. Ooze formed from xanthomonadins and extracellular polysaccharides is produced on leaves in humid environments.

Xvv also causes sugarcane gumming or gummosis disease. In sugarcane, Xvv similarly causes long, thin streak-like lesions on the leaves, initially appearing orange or yellow then turning brown or grey with age (Coutinho et al., 2015). Lesions can sometimes appear as white streaks, and bacterial ooze may appear (Peros and Lombard, 1992; Lang et al., 2017). The pathogen may enter a vascular wilt phase associated with gumming, seen as thick ooze emerging from a cut stem, and eventual death of the plant.

List of Symptoms/Signs

Biology and Ecology

Genetics

It is hypothesized that the rapid spread of the disease in the Americas may have been facilitated by the emergence of new genetic adaptations in the epidemic strains. The comparison of 41 X. vasicola genomes from the USA, Argentina, South Africa, and Zimbabwe confirmed that maize solates are genetically distinct from sugarcane isolates (Perez-Quintero et al., 2020). This suggests that while many maize isolates can cause symptoms when inoculated on sugarcane, and vice-versa (Lang et al., 2017), they probably segregate into specialized host niches in nature. Several genomic insertions were associated with the US isolates, including multiple regions acquired through genomic exchange with the sorghum pathogen Xanthomonas vasicola pv. holcicola. One cluster of genes was identified specifically in maize isolates from the USA and Argentina, but not found in older South African isolates (Perez-Quintero et al., 2020). This cluster contains several genes that could potentially contribute to virulence, but their role in Xvv pathogenesis has not yet been determined.

Genomic analysis also determined that most isolates from the USA epidemic are related to a single lineage of Argentine strains (Perez-Quintero et al., 2020). However, one US isolate was related to a separate group, while one South African isolate was closely related to the US group, so the historical global path of Xvv is still not fully resolved.     

Environmental Requirements

Moisture seems to be an important factor in Xvv infection, and irrigation is a top predictor of bacterial leaf streak infection in maize (Hartman, 2018). The temperature range and overwintering habits of Xvv are still not well understood. The presence of the disease over several seasons in Nebraska, USA, where temperatures routinely drop below -18°C for extended periods in the winter and exceed 37°C in the summer, suggests that the pathogen can survive in a broad range of temperatures. Crop debris is considered the most likely site for pathogen overwintering. Xvv survives in crop debris, but survival is greatly reduced in buried debris or inoculated soil (Ortiz-Castro, 2019).

Means of Movement and Dispersal

While the dispersal mechanisms of Xvv are not yet confirmed, other Xanthomonas foliar pathogens of rice and wheat can be dispersed by wind-blown water droplets, irrigation water, or agricultural machinery. The observance of new Xvv infections in non-irrigated fields through the season is consistent with wind and rain dispersal. Wind dispersal may be aided by drops of exopolysaccharide exudates, or ‘ooze’, that form on the leaf surface in humid conditions. It may also be possible for infected plant residue to spread between maize fields through baling, tillage or combines. The mechanisms of long-distance and international spread are still unclear. There are no known insect vectors for Xvv. 

Seedborne Aspects

Xvv was not detectable in a vast majority of field-collected seeds (Arias et al., 2018). However, the bacteria were detected on a few seeds from one site, and more research is needed to understand whether seeds carry a significant risk of disease transmission. 1% sodium hypochlorite (bleach) was effective in decontaminating artificially infested seeds (Arias et al., 2019).

Pathway Causes

Pathway Vectors

Plant Trade

Impact Summary

Economic Impact

Maize bacterial leaf streak infects field maize, seed corn, sweetcorn and popcorn varieties. Yield loss data are not yet available for maize bacterial leaf streak in the Americas, but lesions have been observed to cover 50% of the leaf surface in some varieties. The loss of leaf surface area to necrosis could potentially reduce yield by reducing photosynthetic capacity. No effects of bacterial leaf streak on grain quality have been reported.

In sugarcane, the disease is reported to enter a systemic phase associated with substantial yield loss (reviewed in Hartman, 2018). Yield loss specifically due to Xvv gummosis on sugarcane is not clear; quantitative data are scarce, and many reports do not distinguish this pathogen from the phenotypically similar X. axonopodis pv. vasculorum. 

Risk and Impact Factors

• Proved invasive outside its native range
• Highly adaptable to different environments
• Reproduces asexually

• Negatively impacts agriculture

Diagnosis

Preliminary diagnosis can be achieved by observation of streaming from lesions cut in water. The pathogen can be isolated from surface sterilized symptomatic leaves, forming mucoid, pale yellow colonies after 48h on GYCA. Molecular diagnostic tools have recently been developed for highly sensitive detection of Xvv in samples prior to symptom development. Lang et al. (2017) reported four primer sets for diagnosis of all strains of Xvv using conventional PCR, of which Xvv3 and Xvv5 were the most robust; these two tests have begun to be adopted for confirming Xvv in new regions of the USA (Jamann et al., 2018). Stulberg et al. developed two diagnostic tests that specifically detect the subgroup of Xvv associated with the corn epidemic in the Americas (2020), but not South African or sugarcane isolates. These tests include a LAMP PCR assay that is compatible with in-field detection, and a qPCR assay that could be used for quantitative detection. 

Detection and Inspection

The pathogen can be observed upon visual inspection of symptomatic plants by the appearance of elongated, yellow to reddish-brown lesions running parallel to the leaf veins of maize or sugarcane. Lesions have wavy or irregular margins and are surrounded by a yellow halo when held to the light. Advanced sugarcane disease can be confirmed by cutting the stem to look for ooze and gumming.

Similarities to Other Species/Conditions

Symptoms of leaf streak on maize can appear similar to common fungal diseases, such as northern corn leaf blight, southern corn leaf blight, and particularly grey leaf spot caused by Cercospora zeae-maydis. Bacterial leaf streak can be distinguished from grey leaf spot by its irregular lesion margins and, when backlit, the appearance of long, yellow halos surrounding the lesion. Grey leaf spot has rectangular lesions with smooth, straight margins and no halos. Bacterial leaf streak also tends to develop earlier in the season than grey leaf spot, and lesions produce visible bacterial streaming when cut in water.

Sugarcane gumming disease is also caused by Xanthomonas axonopodis pv. vasculorum, which is a distantly related species of Xanthomonas. The two species cause very similar diseases on sugarcane - in fact, there are no documented distinctions between the symptoms of sugarcane gumming caused by Xav and those caused by Xvv. However, they have distinct geographical distributions, and Xav has not been reported in South Africa, South America or North America, or naturally infecting maize. The two species may be distinguished by sequencing of the gyrB housekeeping gene. Because sugarcane-infecting strains of X. axonopodis and X. vasicola were both classified as X. campestris pv. vasculorum (1978-1995), they are often conflated in older publications, and sometimes by newer texts and bulletins citing those publications.

Prevention and Control

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

There are no completely effective prevention or control measures known for Xvv diseases. Current recommendations mainly focus on debris control and sanitary practices. More research is needed to evaluate optimal management and control strategies and identify genetic resistance.

Cultural Control and Sanitary Measures

Pathogen spread and disease incidence may be reduced by tillage or removal of crop residue at the end of the growing season and/or by rotation with non-host crops. In corn, late planting dates and irrigation are also associated with disease occurrence, so minimizing these factors may limit disease development.

Physical/Mechanical Control

Removing infested crop debris from equipment may prevent pathogen spread. Sanitation of equipment between fields may prevent the spreading of the pathogen to other fields. Harvesting severely infected fields last may help prevent or slow pathogen spread, although wind can still move infected water droplets and plant residues.

Biological Control

No biological control is available. However, colonization of maize plants with the endophytic bacterium Pantoea ananatis was associated with reduced symptoms after Xvv inoculation (Ortiz-Castro, 2019).

Chemical Control

Currently, there are no effective bactericides recommended for control of bacterial leaf streak or gummosis.

Host-Plant Resistance

Xvv infects a wide range of corn varieties, including field (dent) maize, sweetcorn, seed corn and popcorn, and there are no major (qualitative) resistance genes known. Some corn varieties appear to have partial resistance, and research is underway to identify the genetic basis of this resistance toward breeding resistant varieties. Five genomic regions associated with partial resistance have been mapped (Qiu et al., 2020).

Monitoring and Surveillance

University and government researchers have been actively scouting for bacterial leaf streak of maize in the USA, and communicating with growers to recognize and report the disease.

IPM

Weed management may play a role in disease mitigation, as it prevents host weeds from harbouring the bacteria when maize is not grown in the field.

Gaps in Knowledge/Research Needs

Although major early strides have been made toward understanding and managing Xvv since 2016, there remains much work to be done. Epidemiological and ecological studies are needed to understand the local and international mechanisms for disease spread in order to prevent further global expansion. Applied agricultural studies will be important to evaluate methods of cultural, chemical, and biological controls. Molecular and genetic work is necessary to understand whether the epidemic was enabled by specific mutations in the bacteria. Finally, the work of plant geneticists and breeders toward genetic resistance may be the best hope of controlling this disease.

References

Arias S, Block CC, Mayfield DA, Broders KD, Jackson-Ziems TA, Munkvold GP, 2018. Potential for seed transmission of Xanthomonas vasicola pv. vasculorum on maize collected from fields in the United States. Phytopathology, 108(10), 3340.

Arias S, Block CC, Mayfield DA, Munkvold GP, 2019. Effects of seed storage conditions and chemical disinfestation treatments on the survival of Xanthomonas vasicola pv. vasculorum on maize seed. In: Plant Health 2019 . Cleveland, Ohio, USA

Broders K, 2017. Status of bacterial leaf streak of corn in the United States. In: Proceedings of the Integrated Crop Management Conference . https://doi.org/10.31274/icm-180809-247

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Coutinho, T. A., Westhuizen, L. van der, Roux, J., McFarlane, S. A., Venter, S. N., 2015. Significant host jump of Xanthomonas vasicola from sugarcane to a Eucalyptus grandis clone in South Africa. Plant Pathology, 64(3), 576-581. doi: 10.1111/ppa.12298

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Dyer, R. A. , 1949. Botanical surveys and control of plant diseases. Farming in South Africa, 24(275), 119-121 pp.

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Jamann, TM, Plewa, D, Mideros, SX, Bissonnette, S, 2018. First Report of Bacterial Leaf Streak of Corn Caused by Xanthomonas vasicola pv. vasculorum in Illinois. Plant Disease, 103(5), 1018-1018. https://apsjournals.apsnet.org/doi/full/10.1094/PDIS-10-18-1895-PDN

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Principal Source

Draft datasheet under review

Contributors

07/02/20 Original text by:

Lindsay Triplett, Department of Plant Pathology and Ecology, The Connecticut Agricultural Experiment Station, USA

Ravikumar Patel, Department of Plant Pathology and Ecology, The Connecticut Agricultural Experiment Station, USA