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

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 maize 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 crop debris management 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.

Distribution within the USA expanded rapidly between 2016 and 2020.

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.

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).

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 as volunteer maize was also found with Xvv in Nebraska, USA. Xvv survives in crop debris, but survival is greatly reduced in buried debris or inoculated soil (Ortiz-Castro, 2019).

Xvv was not detectable in the 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).

Preliminary diagnosis can be achieved by observation of streaming from lesions cut in water under a stereo microscope. 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. (2020) developed two diagnostic tests that specifically detect the subgroup of Xvv associated with the maize epidemic in the Americas, 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. 

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 against the light. Advanced sugarcane disease can be confirmed by cutting the stem to look for ooze and gumming.

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.

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 management 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 maize, 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 maize varieties, including field (dent) maize, sweetcorn, seed maize 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.

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.

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