Xylella fastidiosa is a dangerous plant pathogen responsible for a wide array of plant diseases, including some of the most harmful to California’s agricultural landscape. From almond orchards to grapevine fields, this bacterium has embedded itself in the very fabric of the state's farming industry, impacting both local economies and crop health. Understanding the spread of Xylella fastidiosa, particularly in California, is crucial for protecting these valuable crops. This blog post takes a closer look at the presence of Xylella fastidiosa in California, the diseases it causes, and the ongoing research efforts to manage its spread.

 

Xylella fastidiosa (Xf) is a bacterial pathogen that thrives in the xylem vessels of plants. This bacterium disrupts the plant’s ability to transport water and nutrients, leading to severe symptoms such as leaf scorch, stunted growth, and in extreme cases, the death of the plant. Xylella fastidiosa is transmitted primarily through insect vectors, particularly sharpshooters like the glassy-winged sharpshooter (Homalodisca vitripennis) and other leafhoppers.

In California, Xf has caused significant damage to crops such as grapevines, almonds, and olives, as well as a range of ornamental plants. It is particularly notable for causing Pierce’s disease in grapevines and almond leaf scorch disease (ALSD) in almond trees, among other diseases.

 

The Spread of Xylella fastidiosa in California: A Growing Concern

The spread of Xylella fastidiosa in California has been well-documented across multiple studies. Its presence has been confirmed in a variety of crops, highlighting the vulnerability of California’s agricultural systems. Below, we explore the research findings and the areas affected by Xylella fastidiosa.

Study 1: Pecan Bacterial Leaf Scorch (PBLS)

In 2017, a significant study confirmed the presence of Xylella fastidiosa in pecan trees across Arizona, New Mexico, California, and Texas. The pathogen caused Pecan Bacterial Leaf Scorch (PBLS), which led to tan to light brown necrotic lesions on leaf margins, eventually causing leaf abscission. In California, 15 pecan cultivars, including popular varieties like 'Cape Fear' and 'Pawnee', were found to be infected. The discovery of Xylella fastidiosa in California’s pecan orchards underscores the pathogen’s expanding reach across the state.

Reference: Hilton, A.E., Jo, Y.K., Cervantes, K., et al. (2017). First Report of Pecan Bacterial Leaf Scorch Caused by Xylella fastidiosa in Pecan (Carya illinoinensis) in Arizona, New Mexico, California, and Texas. Plant Disease.

Study 2: Xylella fastidiosa in Olive Trees

Another pivotal study focused on California’s olive trees, specifically in Southern California, where symptoms of leaf scorch and branch dieback were observed. From 2008 to 2012, researchers tested 198 symptomatic olive trees and found that 17% tested positive for Xylella fastidiosa. However, while olive trees served as hosts for the pathogen, the study concluded that they did not contribute significantly to the disease’s spread, despite the presence of the bacterium in some trees.

Reference: Krugner, R., Sisterson, M.S., Chen, J., et al. (2014). Evaluation of Olive as a Host of Xylella fastidiosa and Associated Sharpshooter Vectors. Plant Disease.

Study 3: Mulberry Leaf Scorch in Southern California

The discovery of Mulberry Leaf Scorch (MLS) caused by Xylella fastidiosa in Southern California further expands the list of host plants affected by the bacterium. The study found that Xf was present in both white mulberry trees and ornamental species in urban areas, with strains genetically similar to those causing diseases in other regions. Although MLS was a new disease in California, it was clear that Xf’s impact on the state's plants was becoming more widespread.

Reference: Hernandez-Martinez, R., Cooksey, D.A., Wong, F.P. (2006). Discovery and Characterization of Xylella fastidiosa Strains in Southern California Causing Mulberry Leaf Scorch. Plant Disease.

 

Impact on California's Agriculture and Economy

The spread of Xylella fastidiosa has profound implications for California’s agricultural industry, which relies heavily on crops such as almonds, grapevines, and olives. The presence of Xf in these high-value crops threatens not only the health of these plants but also the livelihoods of farmers and the state's economy.

Pierce’s Disease in Grapevines

Perhaps the most well-known effect of Xylella fastidiosa in California is Pierce’s Disease in grapevines. This disease has led to significant yield losses in Napa Valley and other wine-growing regions. The glassy-winged sharpshooter, which feeds on the xylem sap of grapevines, spreads Xf, causing symptoms like leaf scorch and stunted growth. The economic impact of Pierce’s Disease is substantial, as vineyards face both loss of production and the costs of managing and replacing infected vines.

Almond Leaf Scorch Disease (ALSD)

Almond Leaf Scorch Disease (ALSD) is another major concern. The bacterium’s ability to infect almond trees has led to decreased yields and the weakening of tree health. As the almond industry in California is worth billions of dollars, the presence of Xylella fastidiosa poses a severe risk to this crop, further underscoring the need for effective management strategies.

 

Managing the Spread of Xylella fastidiosa

Research on Xylella fastidiosa in California has led to a better understanding of the pathogen’s transmission dynamics and spread. The role of sharpshooters as vectors of Xf is well-documented, and controlling these insect populations is a key component of disease management. Several strategies are being developed, including:

• Insecticide treatments to reduce vector populations.
• Removal of infected plants to prevent the spread of the bacterium to healthy crops.
• Development of resistant plant varieties to withstand Xf infections.
• Monitoring programs to detect the pathogen early and track its spread across the state.

Table: Summary of Xylella fastidiosa’s Presence in California

Study	Region	Disease	Key Findings	Impact on Agriculture
Study 1	Arizona, New Mexico, California, Texas	Pecan Bacterial Leaf Scorch (PBLS)	79 pecan trees infected across multiple states	Affects pecan trees, potential yield losses
Study 2	Southern California	Olive Tree Infection	17% of symptomatic trees tested positive for Xf	Olive trees act as secondary hosts
Study 3	Southern California	Mulberry Leaf Scorch (MLS)	Xf found in mulberry trees, new disease in California	Further spread of Xf to urban and agricultural areas
Study 4	Central Valley, California	Almond Leaf Scorch Disease (ALSD)	Detection of Xf in almond orchards	Risk to almond crop health and productivity
Study 5	Napa Valley, California	Pierce’s Disease in Grapevines	Xf spread by glassy-winged sharpshooters	Significant yield loss in grapevines

Conclusion: The Path Forward

As the presence of Xylella fastidiosa continues to expand across California, it is crucial for farmers, researchers, and policymakers to work together to mitigate the pathogen's impact on the state’s agricultural landscape. The research presented in this post emphasizes the need for proactive management strategies and ongoing surveillance to protect crops from this devastating disease. By staying informed and embracing new technological solutions, California’s agriculture can better navigate the challenges posed by Xylella fastidiosa and continue to thrive in the face of this growing threat.

 

References:

1. Hilton, A.E., Jo, Y.K., Cervantes, K., et al. 2017. "First Report of Pecan Bacterial Leaf Scorch Caused by Xylella fastidiosa in Pecan (Carya illinoinensis) in Arizona, New Mexico, California, and Texas." Plant Disease.
2. Krugner, R., Sisterson, M.S., Chen, J., et al. 2014. "Evaluation of Olive as a Host of Xylella fastidiosa and Associated Sharpshooter Vectors." Plant Disease.
3. Hernandez-Martinez, R., Cooksey, D.A., Wong, F.P. 2006. "Discovery and Characterization of Xylella fastidiosa Strains in Southern California Causing Mulberry Leaf Scorch." Plant Disease.
4. Krugner, R., Ledbetter, C.A., Chen, J., & Shrestha, A. 2012. "Phenology of Xylella fastidiosa and Its Vector Around California Almond Nurseries: An Assessment of Plant Vulnerability to Almond Leaf Scorch Disease." Plant Disease.
5. Beal, Dylan Joseph. 2021. "Assessing Vectors of Xylella fastidiosa in California: From Ecology to Transmission Dynamics." Doctoral dissertation, University of California, Berkeley.
6. Shapland, E.B., Daane, K.M., Yokota, G.Y., et al. 2006. "Ground Vegetation Survey for Xylella fastidiosa in California Almond Orchards." Plant Disease.
7. Raju, B.C., Goheen, A.C., & Frazier, N.W. 1983. "Occurrence of Pierce's Disease Bacteria in Plants and Vectors in California." Phytopathology.
8. Uyemoto, J.K., & Seoft, S. 1992. "Important Diseases of Prunus Caused by Viruses and Other Graft-Transmissible Pathogens in California and South Carolina." Plant Disease.
9. Kahn, A.K., Sicard, A., Cooper, M.L., et al. 2023. "Progression of Xylella fastidiosa Infection in Grapevines Under Field Conditions." Phytopathology.
10. Krugner, R., Sisterson, M.S., Chen, J., & Johnson, M.W. 2014. "Evaluation of Olive as a Host of Xylella fastidiosa and Associated Sharpshooter Vectors." Plant Disease.
11. Beal, Dylan Joseph. 2021. "Assessing Vectors of Xylella fastidiosa in California." University of California, Berkeley.
12. Krugner, R., Ledbetter, C.A., Shrestha, A., & Chen, J. 2012. "Evaluation of Almond Tree Susceptibility to Xylella fastidiosa." Plant Disease.