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

Effects of Hybrids, Foliar Treatments, and Infection Conditions on Bacterial Leaf Streak of Sweet Corn

Bacterial leaf streak (BLS) of corn caused by Xanthomonas vasicola pv. vasculorum was first reported in the United States in 2017. The biology and management of BLS are poorly understood. The objective of this work was to determine the effects of hybrids, foliar treatments, and infection conditions (timing, temperature, and inoculation site) on BLS of sweet corn. Field studies were conducted to determine if hybrid or foliar disease management treatments influenced BLS development and yield. Corn leaves were inoculated in plots with X. vasicola pv. vasculorum, and noninoculated plots were used for comparison. The leaf incidence and severity of BLS differed significantly among sweet corn hybrids, suggesting different levels of susceptibility to BLS. Grain yield was significantly reduced (14.7%) by BLS for one hybrid. The corn growth stage at the time of infection influenced BLS, with incidence and severity significantly greater after inoculation at stage V6 than at V9. Foliar application of Kocide, LifeGard, and Kocide and LifeGard significantly reduced leaf severity compared with nontreated controls in field studies. Kocide significantly reduced leaf incidence, but no treatments significantly increased yield versus controls. In comparisons of inoculation methods in a growth chamber, lesion length on leaves was significantly greater on stalk-inoculated than leaf-inoculated plants. Lesions developed on leaf-inoculated plants only at inoculation sites, whereas lesions developed on stalk-inoculated plants on multiple leaves. In controlled environments, lesion length on leaves was significantly greater at 21°C than at 27 and 32°C. This study expands our understanding of factors that influence the development and management of BLS of sweet corn.

Survival of Xanthomonas vasicola pv. vasculorum in Soil and in Corn Crop Residues under the Humid Subtropical Climate of Southern Brazil

Bacterial leaf streak caused by Xanthomonas vasicola pv. vasculorum (Xvv) is an emerging disease in several corn-producing regions around the world. In Brazil, there is a lack of information on the survival of this bacterium in soil and crop residues. Thus, the objective of this study was to determine the survival of Xvv in soil and also in infected corn crop residues under the humid subtropical climate of southern Brazil. The survival of Xvv in soil was initially investigated in sandy and clayey soils maintained at 20, 25 and 30 °C under controlled conditions. The survival of the bacterium under field conditions was studied in artificially infested clayey soil. The survival of Xvv in corn crop residues was investigated in infected residues maintained on the soil surface or buried in the soil at 20 cm deep. Under controlled conditions, regardless of the type of soil, the bacterium survived longer at 20 °C than at higher temperatures. The bacterium survived for 40 days in clayey soil kept at 20 °C and four days in sandy soil maintained at 30 °C. Under field conditions, the survival of Xvv in the soil was only for 48 h and in infected corn crop residues for up to 15 days in the samples maintained on the soil surface. In samples of infected corn residues buried in the soil, the bacterium was only detected at the time the experiment was set up. In general, the results obtained in this study revealed that Xvv survives for a short period of time in soil and in infected corn crop residues under humid subtropical conditions. Therefore, soil and corn residues may not be highly important sources of primary inoculum for the development of bacterial leaf streak on corn crops under these conditions.

The potential of bacteriocins and bacteriophages to control bacterial disease of crops with a focus on Xanthomonas spp

Crop production plays a crucial role in ensuring global food security and maintaining economic stability. The presence of bacterial phytopathogens, particularly Xanthomonas species (a key focus of this review), poses significant threats to crops, leading to substantial economic losses. Current control strategies, such as the use of chemicals and antibiotics, face challenges such as environmental impact and the development of antimicrobial resistance. This review discusses the potential of bacteriocins, bacterial-derived proteinaceous antimicrobials and bacteriophages, viruses that target bacteria as sustainable alternatives for effectively managing Xanthomonas diseases. We focus on the diversity of bacteriocins found within xanthomonads by identifying and predicting the structures of candidate bacteriocin genes from publicly available genome sequences using BAGEL4 and AlphaFold. Harnessing the power of bacteriocins and bacteriophages has great potential as an eco-friendly and sustainable approach for precision control of Xanthomonas diseases in agriculture. However, realising the full potential of these natural antimicrobials requires continued research, field trials and collaboration among scientists, regulators and farmers. This collective effort is crucial to establishing these alternatives as promising substitutes for traditional disease management methods.

Exploring the Relationship Between Pattern-Triggered Immunity and Quantitative Resistance to Xanthomonas vasicola pv. vasculorum in Maize

Bacterial leaf streak (BLS) of maize is an emerging foliar disease of maize in the Americas. It is caused by the gram-negative nonvascular bacterium Xanthomonas vasicola pv. vasculorum. There are no chemical controls available for BLS, and thus, host resistance is crucial for managing X. vasicola pv. vasculorum. The objective of this study was to examine the genetic determinants of resistance to X. vasicola pv. vasculorum in maize, as well as the relationship between other defense-related traits and BLS resistance. Specifically, we examined the correlations among BLS severity, severity for three fungal diseases, flg-22 response, hypersensitive response, and auricle color. We conducted quantitative trait locus (QTL) mapping for X. vasicola pv. vasculorum resistance using the maize recombinant inbred line population Z003 (B73 × CML228). We detected three QTLs for BLS resistance. In addition to the disease resistance QTL, we detected a single QTL for auricle color. We observed significant, yet weak, correlations among BLS severity, levels of pattern-triggered immunity response and leaf flecking. These results will be useful for understanding resistance to X. vasicola pv. vasculorum and mitigating the impact of BLS on maize yields.

Bacterial Leaf Streak Diseases of Plants: Symptom Convergence in Monocot Plants by Distant Pathogenic Xanthomonas Species

Bacterial leaf streak (BLS) is a disease of monocot plants caused by Xanthomonas translucens on small grains, X. vasicola on maize and sorghum, and X. oryzae on rice. These three pathogens cause remarkably similar symptomology in their host plants. Despite causing similar symptoms, BLS pathogens are dispersed throughout the larger Xanthomonas phylogeny. Each aforementioned species includes strain groups that do not cause BLS and instead cause vascular disease. In this commentary, we hypothesize that strains of X. translucens, X. vasicola, and X. oryzae convergently evolved to cause BLS due to shared evolutionary pressures. We examined the diversity of secreted effectors, which may be important virulence factors for BLS pathogens and their evolution. We discuss evidence that differences in gene regulation and abilities to manipulate plant hormones may also separate BLS pathogens from other Xanthomonas species or pathovars. BLS is becoming an increasing issue across the three pathosystems. Overall, we hope that a better understanding of conserved mechanisms used by BLS pathogens will enable researchers to translate findings across production systems and guide approaches to control this (re)emerging threat.

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.