The Importance of Xylella Fastidiosa in Maryland: Xylella fastidiosa is a Gram-negative bacterium known to infect a wide variety of plants. It affects the xylem, the tissue responsible for water transport, leading to leaf scorch, stunted growth, and even death of the plant. The pathogen is spread primarily by xylem-feeding insects like sharpshooters, and its effects can be devastating to both agricultural crops and ornamental trees in urban areas.

In Maryland, this bacterium’s impact is becoming more evident as it infects a range of plants, including those used for landscaping and in agricultural settings. The studies discussed here provide crucial evidence of the pathogen’s presence and its alarming potential to disrupt local ecosystems.

 

Study 1: Genome Sequence of Xylella Fastidiosa in Maryland: The first study, titled “Genome Sequence of a Xylella fastidiosa Strain Causing Mulberry Leaf Scorch Disease in Maryland,” was conducted by Wei Guan, Jonathan Shao, Tingchang Zhao, and Qi Huang. This study, published in Genome Announcements (March/April 2014), focuses on the genome sequencing of a specific Xylella fastidiosa strain found in mulberry trees in Beltsville, Maryland.

The research identified a unique strain, Mul-MD, responsible for causing bacterial leaf scorch (BLS) in mulberry trees. This discovery is critical, as it represents the first genomic data on Xf strains infecting landscape trees in the United States. Previously, genomic data on Xf had been limited to agricultural crops like grapes and citrus, but the Mul-MD strain highlights the pathogen’s broader reach into urban and suburban landscapes.

The methodology involved isolating the strain, culturing it in periwinkle wilt medium, and sequencing its genome using Roche 454 GS FLX Titanium pyrosequencing technology. The genome sequencing revealed over 850,000 reads, producing a total of 254 million base pairs, which were assembled into 188 contigs. The genome size was determined to be approximately 2.54 Mb, with a GC content of 51.65%, and a total of 2,286 predicted protein-coding genes.

 

Key Findings:

1.      Plasmid Discovery: The study also uncovered a significant ~25 kb plasmid within the Mul-MD genome. This plasmid is similar to others found in Xf strains from California and grapevine strains. This raises concerns about horizontal gene transfer and suggests that the pathogen may be adaptable, potentially allowing it to spread across regions and species.

2.      Importance for Maryland’s Urban Landscape: The identification of Xylella fastidiosa in mulberry trees in Maryland marks a critical point in understanding how this pathogen affects urban landscapes. The findings underscore the need for pathogen surveillance and management strategies, particularly for ornamental and native trees that can be impacted by this bacterium. The study stresses the importance of monitoring plant health in urban areas to prevent further spread.

 

Study 2: Natural Occurrence of Xylella Fastidiosa in a Commercial Nursery in Maryland: The second study, published in Canadian Journal of Plant Pathology by Qi Huang, investigates the presence of Xylella fastidiosa in a commercial nursery in Maryland. Conducted over a period of two years (2003-2004), this study used enzyme-linked immunosorbent assay (ELISA) and nested polymerase chain reaction (PCR) to detect the bacterium in a variety of plants within the nursery and its surrounding environment.

The study revealed that Xf was present in several plant species, some of which had never been reported as hosts of the bacterium. Notably, crape myrtle, Japanese stewartia, mimosa, sassafras, and Schipka laurel were identified as new hosts. While these plants did not exhibit visible symptoms of infection, their positive test results suggest they could harbor the bacterium asymptomatically.

 

Key Findings:

1.      Asymptomatic Hosts: This study provides groundbreaking information about asymptomatic hosts of Xylella fastidiosa. Plants like Japanese stewartia and mimosa can harbor the bacterium without showing the typical symptoms, such as leaf scorch. This finding complicates the management of the disease, as these plants could unknowingly contribute to the spread of the pathogen.

2.      Insect Vectors and Transmission: The study also highlights the role of insect vectors, such as sharpshooters and spittlebugs, in transmitting Xf. Plants with high bacterial populations, such as red oak and box-elder, were found to be more likely to spread the pathogen through insect activity. This finding emphasizes the need for targeted insect control in nursery and agricultural settings to limit pathogen transmission.

3.      Impact on Nursery Management: The study stresses the importance of controlling insect vectors and removing infected plants in the surrounding environment to prevent further spread of Xf. Nursery operators in Maryland are urged to test incoming plants for Xylella fastidiosa and monitor their plants for any signs of infection, even in the absence of visible symptoms.

 

Summary of Xylella Fastidiosa's Impact in Maryland:

Study	Location	Findings	Impact	Key Hosts
Genome Sequence of Xylella Fastidiosa in Maryland	Beltsville, MD	Discovery of Mul-MD strain in mulberry trees	Highlighted the need for pathogen surveillance in urban landscapes	Mulberry trees
Natural Occurrence of Xylella Fastidiosa in a Commercial Nursery	Maryland Nursery	Detection of Xf in new host plants	Emphasized the need for insect vector control in nurseries	Crape Myrtle, Mimosa, Sassafras, Box-elder, Red Oak

 

Conclusions and Implications for Maryland’s Plant Health: The findings from these two studies present a clear picture of Xylella fastidiosa’s growing presence in Maryland. The pathogen’s ability to infect a variety of plant species, including ornamental trees and nursery plants, poses a significant challenge to Maryland’s agriculture and urban landscapes.

Maryland's plant health professionals must prioritize pathogen surveillance and early detection systems to mitigate the spread of Xylella fastidiosa. The research also calls for the development of more effective management practices, including controlling insect vectors and removing infected plants from both urban and agricultural settings. Furthermore, the identification of asymptomatic hosts underlines the complexity of managing this pathogen, as it can silently infect plants without showing visible symptoms.

By continuing to monitor and study the presence of Xylella fastidiosa, researchers can better understand its ecological impact and develop more effective strategies for combating its spread.

 

References:

·         Guan, Wei, Jonathan Shao, Tingchang Zhao, and Qi Huang. "Genome Sequence of a Xylella fastidiosa Strain Causing Mulberry Leaf Scorch Disease in Maryland." Genome Announcements 2, no. 2 (March/April 2014): e00916-13. https://doi.org/10.1128/genomeA.00916-13.

·         Huang, Qi. "Natural Occurrence of Xylella fastidiosa in a Commercial Nursery in Maryland." Canadian Journal of Plant Pathology 29, no. 3 (2007): 299-303. https://doi.org/10.1080/07060660709507473.