Xylella Fastidiosa in Arizona

Xylella fastidiosa (Xf) is a bacterial pathogen that affects plants by clogging their vascular system, leading to a variety of diseases. In Arizona, this pathogen has emerged as a significant concern for local agriculture, particularly in pecan orchards and vineyards. Over the years, Xf has been found to cause diseases like Pecan Bacterial Leaf Scorch (PBLS) and Pierce’s Disease (PD) in various crops. Recent studies reveal alarming insights into the spread of X. fastidiosa in Arizona, making it crucial for farmers and researchers to understand the pathogen's behavior and impact.

Infection due to Xylella fastidiosa leads to symptoms such as leaf scorch, branch dieback, and, in severe cases, plant death. X. fastidiosa is primarily spread by xylem-feeding insect vectors, such as sharpshooters and leafhoppers. While certain strains of X. fastidiosa are benign in native plants, others cause significant damage to crops and ornamental plants.

This pathogen has been responsible for devastating agricultural diseases, including:

·         Pierce’s Disease in grapevines

·         Citrus Variegated Chlorosis in citrus

·         Pecan Bacterial Leaf Scorch (PBLS) in pecans, as observed in Arizona and New Mexico .

The Spread of Xylella Fastidiosa in Arizona

Arizona has witnessed several outbreaks of Xylella fastidiosa, particularly in pecan trees, ornamental plants like Chitalpa tashkentensis, and grapevines. The pathogen's presence in these plants is concerning for Arizona’s agricultural industries.

Key Findings from Arizona Studies

1.      Pecan Bacterial Leaf Scorch (PBLS): PBLS, caused by X. fastidiosa, was first detected in Arizona in 2015. Symptoms such as leaf scorch, necrosis, and branch dieback have been observed in several pecan orchards across the state. The disease has severely affected the pecan industry, which contributes significantly to Arizona’s economy. The pathogen is primarily spread by insect vectors such as sharpshooters, with some studies indicating that grafting of infected plant material also contributes to its spread.

2.      Chitalpa and Other Ornamental Plants: X. fastidiosa has also been detected in Chitalpa trees, a popular ornamental species in Arizona. Chitalpa trees exhibit leaf scorch symptoms, which were initially thought to be due to environmental stress but were later linked to X. fastidiosa infection. This pathogen was also found in nursery stock, suggesting that it can spread through commercial plant trade .

3.      Genetic Analysis and Subspecies of Xylella Fastidiosa: Recent genetic studies have uncovered a novel subspecies of X. fastidiosa, known as X. fastidiosa subsp. tashke, which has been identified in Chitalpa trees in Arizona. This new subspecies is genetically distinct from other known strains and has been linked specifically to the southwestern U.S., including Arizona .

Impact on Arizona's Agriculture

The impact of Xylella fastidiosa in Arizona is far-reaching:

·         Economic Losses in Pecan Orchards: PBLS has led to declining yields and increased management costs in pecan orchards. The disease causes trees to suffer from poor water and nutrient transport, ultimately affecting tree health and productivity .

·         Threat to Vineyards: Pierce’s Disease, caused by X. fastidiosa, is a major threat to Arizona’s grape industry. The pathogen affects grapevines by blocking water transport, causing vine death, and reducing the quality of the grapes .

·         Spread through Nursery Stock: The discovery of X. fastidiosa in nursery stock emphasizes the risk of pathogen spread through the trade of plants. This can lead to the introduction of the disease into new areas, exacerbating the problem.

Table: Summary of Xylella Fastidiosa in Arizona

Region/Plant	Presence of X. fastidiosa	Disease Caused	Impact	Study Reference
Pecan Orchards	Positive for X. fastidiosa	Pecan Bacterial Leaf Scorch	Reduced tree growth, yield loss, economic impact	Study 2, Study 5
Chitalpa Trees	Positive for X. fastidiosa	Leaf scorch, dieback	Aesthetic and functional damage in landscapes	Study 3, Study 7
Vineyards	Positive for X. fastidiosa	Pierce’s Disease	Potential for grapevine death, loss of quality yield	Study 6

Prevention and Management Strategies

Managing Xylella fastidiosa in Arizona requires a multifaceted approach:

1.      Vector Control: Sharpshooters and other insect vectors play a key role in spreading X. fastidiosa. Using insecticides and implementing monitoring systems like sticky traps can help control vector populations .

2.      Improved Diagnostic Tools: Researchers are working on developing more reliable diagnostic methods, such as PCR-based tests, to detect X. fastidiosa early in infected plants .

3.      Tree Removal and Grafting Practices: Infected trees should be removed to limit the spread, and nurseries should ensure their stock is free from the pathogen. Additionally, using healthy scion wood for tree propagation can help prevent the introduction of X. fastidiosa .

The Pathogen’s Growing Threat in Arizona

Xylella fastidiosa presents a growing threat to Arizona's agriculture, particularly affecting pecan orchards and vineyards. The novel subspecies X. fastidiosa subsp. tashke, found in ornamental plants like Chitalpa, further complicates efforts to manage the pathogen. Ongoing research and proactive management strategies will be crucial in mitigating the impact of X. fastidiosa in Arizona and preventing its spread across the southwestern United States.

References

1.      Randall, J. J., Goldberg, N. P., Kemp, J. D., et al. (2009). "Genetic Analysis of a Novel Xylella fastidiosa Subspecies Found in the Southwestern United States." Applied and Environmental Microbiology, 75(17), 5631–5638.

2.      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, 101(6), 1027. https://doi.org/10.1094/PDIS-02-17-0298-PDN.

3.      "Prevalence and strain characterization of Xylella fastidiosa causing grapevine and pecan diseases in Arizona." University of Arizona.