Xylella Fastidiosa

Xylella fastidiosa, a bacterial plant pathogen, has become a growing concern for Delaware’s forests, landscape trees, and potentially, its agriculture. This post will walk you through what this bacterium is, the plant diseases it causes, and how it has already started affecting trees in Delaware. We’ll use solid research and clear examples from four major studies, all showing how this plant disease is more than just a theoretical threat, it's already here.

Xylella fastidiosa is a bacteria that lives in the xylem—the water-carrying tissue—of plants. It's not visible to the eye, but its effects are easy to spot once it infects a tree or shrub. It causes several diseases, including bacterial leaf scorch (BLS), phony peach disease, and Pierce’s disease of grapevines. It spreads through insect vectors, specifically xylem-feeding insects like leafhoppers and spittlebugs. These bugs feed on an infected plant, pick up the bacteria, and pass it along to the next plant they bite.

Plants and Trees at Risk

Over 30 plant families can be infected. In Delaware and nearby states, the most commonly affected trees include:

• Red oak

• Pin oak

• American elm

• Sycamore

• Red maple

• Sweetgum

Shrubs like wild grape, blackberry, Virginia creeper, and even weeds like dandelions have also been identified as hosts.

How the Disease Shows Up

Symptoms usually start in late summer and are often mistaken for drought stress:

• Leaf edges turn brown (scorch)

• Yellow bands form between healthy and dead parts of the leaf

• Leaves drop too early

• Branch dieback

• Tree decline over 5-7 years

Xylella fastidiosa in Delaware

Based on multiple peer-reviewed studies and official reports, Xylella fastidiosa has already been identified in Delaware, especially among urban oak trees. Here are the key findings from four important sources:

Study 1 – Sherald & Kostka (1992)

This early study identified oak leaf scorch in Delaware as a result of Xylella fastidiosa. Red, black, and pin oaks were specifically mentioned. It also noted that the symptoms began in late summer and that the disease persisted for years.

Study 2 – Gregory & Pollok (2025)

This report from the University of Delaware Cooperative Extension confirmed BLS in trees across Delaware since the early 1990s. Testing is available through their diagnostic clinic for a small fee, supporting local monitoring and diagnosis.

Study 3 – Sherald (2007)

This research highlighted that Delaware shares similar environmental and ecological risks as neighboring states like Pennsylvania and New Jersey, where BLS is well established. It emphasized that Delaware's oak, elm, and sycamore trees are highly likely to be affected.

Study 4 – Potter (2017), USDA Forest Service

The USDA report on Delaware's forests confirmed bacterial leaf scorch as a growing issue. Red oak had a 2.1% mortality rate due to the disease—double the state average. Sweetgum was the second most impacted.

Summary Table: Presence and Impact of Xylella fastidiosa in Delaware

Study	Author(s)	Year	Confirmed Presence in DE	Affected Trees	Notable Findings
Study 1	Sherald & Kostka	1992	Yes	Red oak, pin oak	Early signs of BLS in urban trees
Study 2	Gregory & Pollok	2025	Yes	Oaks, maples	University of Delaware offers testing
Study 3	Sherald	2007	Likely	Oaks, sycamore, elms	Shared risks with nearby states
Study 4	Potter (USDA)	2017	Yes	Red oak, sweetgum	Mortality rates due to BLS increasing

How it Spreads in Delaware

The primary method of transmission in Delaware is through local leafhoppers and spittlebugs. The report from Sherald (2007) mentions potential alternative hosts like wild grapevines and blackberry in riparian zones, areas near water, which are common in Delaware.

These insects and wild hosts make controlling the disease difficult. Trees in urban and suburban areas are at higher risk due to closer proximity and higher planting density.

Managing the Disease

Right now, there’s no cure for Xylella fastidiosa. But several practices can help manage or slow down its spread:

• Remove infected trees when symptoms worsen.

• Test trees regularly in late summer or early fall.

• Avoid planting monocultures. A mix of species is harder for the bacteria to take over.

• Use less-susceptible trees like white oak or willow oak in urban planning.

• Keep trees healthy through watering and mulching.

• Prune affected branches if symptoms are limited.

• Chemical options like oxytetracycline have been tried but aren't fully effective.

Why Delaware Needs to Take This Seriously

Forests in Delaware cover about 355,000 acres, with red oak making up a significant portion. These trees are not just beautiful, they provide shade, oxygen, and even help cool down cities. Losing them to BLS would be a big environmental and financial loss.

Annual mortality rates for red oak (2.1%) and sweetgum (1.2%) are already higher than the state average. The risk is especially high in urban areas where many of these trees are planted as part of city planning.

Final Thoughts

Xylella fastidiosa is not just a southern or California problem, it’s in Delaware. Oak trees in your neighborhood or city parks could already be infected. We need more monitoring, education, and early management to stop it from spreading further.

Local homeowners, city planners, and forest managers must stay alert. By identifying infected trees early and removing alternative hosts, we can help protect Delaware’s natural beauty and forest health.

References:

• Sherald, J. L., & Kostka, S. J. 1992. "Bacterial leaf scorch of landscape trees caused by Xylella fastidiosa." Journal of Arboriculture 18(2): 57–63.

• Gregory, N. F., and Pollok, J. 2025. "Bacterial Leaf Scorch." University of Delaware Cooperative Extension.

• Sherald, J.L. 2007. "Bacterial Leaf Scorch of Landscape Trees: What We Know and What We Do Not Know." Arboriculture & Urban Forestry 33(6): 376–385.

• Potter, Stephen. 2017. Forests of Delaware, 2016. Resource Update FS-143. U.S. Department of Agriculture, Forest Service, Northern Research Station. https://doi.org/10.2737/FS-RU-143

In recent years, the agricultural landscape in Oregon has faced a growing challenge: the emergence of Xylella fastidiosa, a bacterial pathogen known for causing a range of devastating plant diseases. Once thought to be a threat mostly confined to warmer climates like California or Italy, this pathogen has now appeared in Oregon, prompting serious concern from researchers, farmers, and state agencies.

This blog post provides a comprehensive overview of the presence of Xylella fastidiosa in Oregon, the plant diseases it can cause, and the impact it may have on the state's agriculture. Using verified sources and summaries from multiple studies, we will explore what Xylella fastidiosa means for Oregon's future and what is being done to contain its spread.

 

 

Xylella fastidiosa is a bacterium that lives in the xylem (water-conducting tissue) of plants and causes diseases by blocking water flow, which leads to leaf scorch, stunted growth, and eventual plant death. It can infect over 300 plant species including grapes, almonds, olives, citrus, pears, and ornamental plants.

It spreads mainly through xylem-feeding insects such as sharpshooters and spittlebugs, which makes it hard to control, especially in regions where these insects are active year-round.

 

Confirmed Presence of Xylella fastidiosa in Oregon (Study 1 & 2)

In 2015, Xylella fastidiosa was officially detected in Oregon for the first time, marking a major turning point. According to a report from Capital Press, symptoms of disease appeared at a pear nursery in Hood River County. Initially, the signs were difficult to identify as they resembled drought stress. After updated testing by the Oregon Department of Agriculture (ODA) and Washington State University, the presence of Xylella was confirmed (Perkowski, 2015).

The bacteria were also found at the National Clonal Germplasm Repository in Corvallis, and potentially spread through scion wood to 22 sites across the state. This prompted the ODA to issue a quarantine in nine counties: Hood River, Benton, Linn, Lane, Marion, Jackson, Yamhill, Multnomah, and Washington.

The ODA enforced fines of up to $10,000 for violations. Emergency steps included banning movement of host plants such as:

·         Pear, cherry, plum, and peach trees

·         Oaks and maples

·         Blueberries and caneberries

Infected plants were to be destroyed, and surveys conducted in surrounding areas.

 

Quarantine & Economic Impact

The quarantine wasn't just about plant health—it had economic implications too. Businesses like Fall Creek Farm & Nursery, which export plant materials to Europe, faced shipment delays. The ODA worked with the USDA's Animal and Plant Health Inspection Service (APHIS) to certify pest-free zones to meet international trade requirements.

Oregon's agriculture exports were at risk, as importing countries like those in Europe enforce strict plant health standards. Even a single outbreak can stall trade deals and impact farm income.

 

Risk to Oregon Crops (Study 2)

As Growing Produce highlighted, Xylella's presence in Oregon could hit local crops hard (Eddy, 2015). Key crops at risk include:

·         Wine grapes (similar to Pierce’s Disease outbreaks in California)

·         Pears and apples

·         Blueberries, almonds, and stone fruits

The disease is spread by insects like leafhoppers, blue-green sharpshooters, and spittlebugs, all of which are present in Oregon. With no cure, once a plant is infected, the only option is removal and destruction.

 

Insect Vectors & Climate Change Concerns (Study 3)

According to the Pacific Northwest Pest Management Handbooks (Pscheidt, 2025), while Pierce’s disease caused by Xylella fastidiosa hasn't taken hold in Oregon's grapevines yet, the blue-green sharpshooter is common in areas like:

·         Willamette Valley

·         Columbia Gorge

·         Medford

·         Milton-Freewater

Although the colder winter season has limited Xylella's spread in the past, climate change is warming Oregon's climate and could remove this natural barrier. If winters continue to get milder, these insects might thrive longer and spread the pathogen more widely.

Surveys from 2000 and 2017 didn’t detect Xylella in Oregon vineyards, but the presence of vectors and warming trends suggests the risk is growing.

 

Oregon Quarantines and Preventive Actions (Study 4)

Another section in the Pacific Northwest Pest Management Handbooks specifically discusses Oregon's plant quarantines (Pscheidt, 2023). The report confirms that:

·         Xylella fastidiosa is on Oregon's regulated pathogen list.

·         All imported grape plants must be tested for Xylella.

·         The glassy-winged sharpshooter, a dangerous vector, was found in 2000 near ornamental nurseries.

·         A statewide survey of 291 samples in 2000 found no evidence of Xylella.

Even though the bacteria hasn’t spread widely, these strict policies show the state's serious commitment to preventing an outbreak.

 

Summary Table: Research Findings on Xylella fastidiosa in Oregon

Study Source	Key Finding	Region Impacted	Confirmed Presence	Insect Vectors Mentioned
Capital Press (2015)	Confirmed Xylella in Hood River pear nursery and Corvallis repository	Hood River, Corvallis	Yes	Unknown initially
Growing Produce (2015)	Detailed risk to wine grapes and fruit crops	9 counties under quarantine	Yes	Leafhoppers, blue-green sharpshooters
Pacific NW Handbooks (2025)	Blue-green sharpshooter present; Xylella not yet spreading in grapes	Willamette Valley, Columbia Gorge, Medford, etc.	No	Blue-green sharpshooter
Pacific NW Handbooks (2023)	Xylella not detected in 291 samples; quarantine rules in place	Statewide	No	Glassy-winged sharpshooter

 

What Should Oregon Farmers and Gardeners Do?

Oregon isn’t in the clear yet. Even though Xylella fastidiosa hasn’t become widespread, the 2015 confirmation in pear trees is proof that it can happen.

Here’s what needs to happen moving forward:

·         Farmers and nurseries should report any suspicious symptoms like leaf scorch or unexpected plant decline.

·         Samples should be submitted with leaves and twigs attached.

·         Everyone in the agriculture sector should follow quarantine rules to stop the spread.

Ongoing monitoring of insect populations, especially in areas like the Willamette Valley and Columbia Gorge, will be key. Education and awareness are also crucial.

 

References

·         Eddy, David. "Bacterial Disease Discovered At Oregon Pear Nursery Causes Quarantine." Growing Produce, December 15, 2015. https://www.growingproduce.com/fruits/apples-pears/bacterial-disease-discovered-at-oregon-pear-nursery-causes-quarantine/.

·         Perkowski, Mateusz. "Bacteria Discovery Prompts Oregon Quarantine." Capital Press, December 9, 2015. https://capitalpress.com/author/capitalpressstaff/.

·         Pscheidt, Jay W. "Grape (Vitis spp.)-Pierce's Disease." Pacific Northwest Pest Management Handbooks, Oregon State University, March 2025. https://pnwhandbooks.org/plantdisease/host-disease/grape-vitis-spp-pierces-disease.

·         Pscheidt, Jay W. "Oregon Quarantines." Pacific Northwest Pest Management Handbooks, March 2023. https://pnwhandbooks.org/plantdisease/pathogen-articles/common/oregons-most-unwanted-invasive-species/oregon-quarantines.

 

Xylella fastidiosa (Xf) is a destructive bacterial pathogen responsible for causing several plant diseases across the United States. In Texas, its presence has raised significant concerns, particularly in the grapevine and orchard industries. As this bacterium spreads, it poses a serious threat to the state’s agriculture. This blog post delves into the various studies highlighting the presence of Xylella fastidiosa in Texas, the plant diseases it causes, and its impact on agriculture. Through these studies, we will explore the science behind the pathogen’s spread, the crops most affected, and how Texas is coping with its ever-increasing presence.

 

Xylella fastidiosa is a gram-negative bacterium that infects the xylem tissue of various plants, including grapes, almonds, citrus, and oleanders. The bacterium disrupts water transport within the plant, causing symptoms such as leaf scorch, chlorosis, and, in severe cases, plant death. Xf is primarily transmitted by insect vectors like sharpshooters, which feed on infected plants and spread the bacterium to healthy ones.

In Texas, Xylella fastidiosa has become a growing concern for farmers, particularly in grapevine production areas, where it causes Pierce’s Disease (PD), a devastating condition that can severely damage or even destroy vineyards.

 

The Presence of Xylella Fastidiosa in Texas: A Growing Threat

Study 1: An American Hybrid Vineyard in the Texas Gulf Coast

The study conducted on vineyards in the Texas Gulf Coast revealed significant findings regarding the prevalence of Xylella fastidiosa in the region. Grapevines, especially the Cynthiana variety, were found to have the highest bacterial load, which corresponded to reduced crop yields. This study confirmed that Xf is prevalent in Texas, particularly in areas where environmental conditions are conducive to the disease.

Key Findings:

• The Cynthiana grape variety showed severe symptoms of Pierce’s Disease, while other varieties, such as Blanc du Bois and Black Spanish, showed resistance.
• Insect vectors like glassy-winged sharpshooters were found to be significant transmitters of the disease.

Proof of Presence:

• The detection of Xylella fastidiosa was confirmed using advanced diagnostic techniques like PCR and ELISA testing, confirming the presence of the bacterium in various grapevine varieties.

Study 2: Oleander Leaf Scorch Caused by Xylella Fastidiosa

The first report of oleander leaf scorch caused by Xylella fastidiosa in Texas highlighted the bacterium’s ability to affect ornamental plants. Oleanders in Texas exhibited typical symptoms of bacterial leaf scorch, such as chlorosis, necrosis, and defoliation, making it the first recorded case of this disease in the state.

Key Findings:

• Symptoms were identified in multiple Texas cities, including Galveston and Austin.
• PCR and ELISA testing confirmed the presence of Xf in symptomatic plants, making this the first report of Xf affecting oleanders in Texas.

Proof of Presence:

• Positive results from ELISA testing and further confirmation through PCR testing validated the presence of Xylella fastidiosa in oleanders across Texas.

Study 3: Genetic Variation of Xylella Fastidiosa in Grapevines

This study analyzed the genetic variation of Xylella fastidiosa in grapevines from both California and Texas. The research focused on the genetic traits that influence how the pathogen behaves in Texas’ unique environment. It was found that Xf strains in Texas exhibit significant genetic variation, which may contribute to the pathogen’s persistence and spread.

Key Findings:

• Texas grapevines harbor multiple strains of Xf, suggesting that the pathogen has adapted to local conditions.
• The study emphasized the importance of genetic research for developing region-specific disease management strategies.

Proof of Presence:

• The genetic analysis identified specific Xf strains present in Texas, demonstrating the pathogen’s continued presence and adaptation in the state.

Study 4: Zot Gene in Xylella Fastidiosa

The study on the Zonula Occludens Toxin (Zot) gene in Xylella fastidiosa strains from Texas further advanced the understanding of how the pathogen affects plants. The Zot gene is believed to play a key role in the bacterium’s virulence. This study revealed significant genetic diversity among Texas strains, suggesting that the local strains may be more resilient to environmental stress.

Key Findings:

• The Texas strains of Xf displayed distinct genetic features, including variations in the Zot gene.
• The genetic differences among Xf strains in Texas could help develop better diagnostic methods and more effective control strategies.

Proof of Presence:

• The identification of the Zot gene in Xf strains from Texas provided further proof of the pathogen's widespread presence and diversity.

Study 5: Seasonal Increase of Xylella Fastidiosa in Hemiptera in Central Texas Vineyards

This study focused on the insect vectors responsible for spreading Xylella fastidiosa in Central Texas vineyards. It was found that the glassy-winged sharpshooter, a known vector, was particularly abundant during the growing season, correlating with increased Xf prevalence.

Key Findings:

• The prevalence of Xf in vector species, particularly Homalodisca vitripennis, increased as the growing season progressed.
• Vineyards severely impacted by Pierce’s Disease had higher rates of Xf in insect vectors.

Proof of Presence:

• Real-time PCR testing of collected insects confirmed the presence of Xylella fastidiosa, linking the pathogen’s spread to insect activity in Texas vineyards.

Study 6: Plum Leaf Scald Caused by Xylella Fastidiosa

The first identification of Xylella fastidiosa as the cause of plum leaf scald in Texas further confirmed the bacterium's geographic spread. This discovery adds plum trees to the list of plants affected by Xf in Texas, expanding the pathogen's impact.

Key Findings:

• The detection of Xf in plum trees in Central Texas marked the first case of plum leaf scald in the state.
• Both ELISA and PCR testing confirmed the presence of Xf in symptomatic plum trees.

Proof of Presence:

• Confirmation through both ELISA and PCR testing validated the presence of Xylella fastidiosa in Texas plum orchards.

Study 7: Potential Plant Reservoirs for Xylella Fastidiosa in South Texas

In South Texas, research identified potential plant species that could harbor Xylella fastidiosa, acting as reservoirs for the pathogen. These plants, often asymptomatic, contribute to the spread of the disease.

Key Findings:

• Several native and ornamental plant species tested positive for Xf, potentially serving as reservoirs.
• These findings highlight the complexity of managing Xf in Texas, as controlling the disease requires addressing the broader plant landscape, not just agricultural crops.

Proof of Presence:

• Diagnostic methods like ELISA and PCR confirmed the presence of Xf in a wide range of plant species across South Texas.

Study 8: Initial Genetic Analysis of Xylella Fastidiosa in Texas

This study focused on the genetic diversity of Xylella fastidiosa strains in Texas, providing important insights into how the pathogen adapts to local conditions. The findings underscored the importance of understanding the pathogen’s genetic makeup for disease management.

Key Findings:

• Two primary strains of Xf were identified in Texas: the grape strain (subsp. piercei) and the ragweed strain (subsp. multiplex).
• These strains exhibited genetic diversity, highlighting the need for tailored disease management strategies.

Proof of Presence:

• The genetic analysis confirmed the presence of Xf strains in Texas, further establishing the bacterium's widespread impact on local agriculture.

Study 9: Sequencing of Xylella Fastidiosa Genome from Texas

The sequencing of the Xylella fastidiosa GB514 strain from a Texas vineyard offered a deeper understanding of the pathogen’s genetic makeup. This research helped identify unique genetic features in Texas strains, potentially influencing how the pathogen spreads and infects plants.

Key Findings:

• The GB514 strain exhibited genetic differences from other Xf strains, including the presence of a plasmid with Type IV secretion systems, which could play a role in virulence.
• This discovery is crucial for understanding how the pathogen behaves in Texas and how it might evolve in response to control measures.

Proof of Presence:

• The sequencing of the GB514 strain confirmed that Xf is present in Texas vineyards, highlighting the need for continuous monitoring and management.

Conclusion: Combating the Spread of Xylella Fastidiosa in Texas

The presence of Xylella fastidiosa in Texas is undeniable, with the pathogen affecting a wide range of plant species across the state. From vineyards to ornamental plants, its impact is far-reaching, and its spread is facilitated by insect vectors and plant reservoirs. The studies discussed in this post highlight the importance of ongoing research, surveillance, and integrated pest management strategies to combat the spread of Xf. As Texas continues to grapple with the threat posed by Xylella fastidiosa, it is crucial for farmers, researchers, and policymakers to work together to protect the state’s agricultural industries from further damage.

 

Table: Summary of Xylella Fastidiosa Studies in Texas

Study	Location	Pathogen Detected	Plant Diseases	Diagnostic Method	Key Findings
Study 1	Texas Gulf Coast	Pierce's Disease	Grape Vines	PCR, ELISA	High bacterial load in Cynthiana grapevines
Study 2	Multiple Locations	Oleander Leaf Scorch	Oleander	PCR, ELISA	First report in Texas
Study 3	California & Texas	Pierce's Disease	Grape Vines	Genetic Analysis	Multiple strains in Texas
Study 4	Texas	Pierce's Disease	Grape Vines	Genetic Sequencing	Genetic diversity in Xf strains
Study 5	Central Texas	Pierce's Disease	Grape Vines	PCR, Real-Time PCR	Seasonal trends in vector populations
Study 6	Central Texas	Plum Leaf Scald	Plum Trees	PCR, ELISA	First report in Texas
Study 7	South Texas	Pierce's Disease	Various Plants	PCR, ELISA	Reservoir plants identified
Study 8	Texas	Pierce's Disease	Grape Vines	Genetic Analysis	Two strains of Xf identified
Study 9	Gillespie County, TX	Pierce's Disease	Grape Vines	Genome Sequencing	Genetic differences in Texas strain

References:

1.      Buzombo, Prince, Jose Jaimes, Vien Lam, Kimberly Cantrell, Melinda Harkness, Deanna McCullough, and Lisa Morano. "An American Hybrid Vineyard in the Texas Gulf Coast: Analysis within a Pierce’s Disease Hot Zone." American Journal of Enology and Viticulture 57, no. 3 (2006): 347-355. https://doi.org/10.5344/ajev.2006.57.3.347.

2.      Huang, Q., Brlansky, R. H., Barnes, L., Li, W., and Hartung, J. S. "First Report of Oleander Leaf Scorch Caused by Xylella fastidiosa in Texas." Plant Disease 88, no. 9 (2004): 1049. https://doi.org/10.1094/PDIS.2004.88.9.1049A.

3.      Lin, H., M.S. Islam, L. Morano, R. Groves, B. Bextine, E. Civerolo, and M.A. Walker. "Genetic Variation of Xylella fastidiosa Associated with Grapevines in Two Major Viticultural Regions in the United States: California and Texas." Journal of Plant Pathology 95, no. 2 (July 2013): 329-337. Accessed April 24, 2025. https://www.jstor.org/stable/23721523.

4.      Bextine, Blake, Lisa Morano, and Henry L. Schreiber IV. "Genetic Analysis of the Zonula Occludens Toxin (Zot) Gene in Texas Isolates of Xylella fastidiosa." University of Texas-Tyler, 2025.

5.      Mitchell, Forrest L., Jeff Brady, Blake Bextine, and Isabelle Lauzière. "Seasonal Increase of Xylella fastidiosa in Hemiptera Collected from Central Texas Vineyards." Journal of Economic Entomology 102, no. 5 (2009): 1743–1749. https://doi.org/10.1603/029.102.0503.

6.      Olawole, O. I., Uribe, P., Rodriguez, N. A., Gonzalez, C. F., and Ong, K. L. "First Report of Bacterial Leaf Scald of Plum Caused by Xylella fastidiosa in Texas." Plant Disease 106, no. 12 (2022): 3198. https://doi.org/10.1094/PDIS-03-22-0561-PDN.

7.      McGaha, Lee Anne, Brian Jackson, Blake Bextine, Deanna McCullough, and Lisa Morano. "Potential Plant Reservoirs for Xylella fastidiosa in South Texas." American Journal of Enology and Viticulture 58, no. 3 (2007): 398-401. https://doi.org/10.5344/ajev.2007.58.3.398.

8.      Morano, Lisa D., Blake R. Bextine, Dennis A. Garcia, Shermel V. Maddox, Stanley Gunawan, Natalie J. Vitovsky, and Mark C. Black. "Initial Genetic Analysis of Xylella fastidiosa in Texas." Current Microbiology 56, no. 4 (2008): 346-351. https://doi.org/10.1007/s00284-007-9088-2.

9.      Schreiber, H. L., M. Koirala, A. Lara, M. Ojeda, S. E. Dowd, B. Bextine, and L. Morano. "Unraveling the First Xylella fastidiosa Subsp. Fastidiosa Genome from Texas." Southwestern Entomologist 35, no. 3 (2010): 479-483. https://doi.org/10.3958/059.035.0336.