Molecular Detection of the Laurel Wilt Fungus, Raffaelea lauricola

Area of Science:

• Plant Pathology
• Molecular Biology
• Forest Science

Background:

• Laurel wilt disease, caused by the fungus Raffaelea lauricola, is rapidly devastating redbay trees in the southeastern U.S. coastal plain.
• The pathogen poses a significant threat to Florida's vital avocado industry, necessitating rapid and accurate detection methods.
• Previous attempts to amplify the 5' region of the large ribosomal RNA (28S) gene from R. lauricola for diagnostic purposes have been challenging.

Purpose of the Study:

• To develop a reliable molecular tool for the detection of Raffaelea lauricola.
• To overcome amplification difficulties associated with the 28S gene of R. lauricola.
• To enable early and accurate diagnosis of laurel wilt disease in various tree species.

Main Methods:

• Optimized polymerase chain reaction (PCR) conditions using a high-fidelity polymerase and a modified dNTP mix (7-deaza-dGTP) to improve amplification of the 28S gene.
• Developed specialized sequencing protocols to address difficulties with GC-rich secondary structures in amplified products.
• Designed and validated a TaqMan real-time PCR assay based on species-specific 28S sequences for pathogen detection in wood tissues and cultures.

Main Results:

• A robust PCR protocol was established, enabling reliable amplification of 28S sequences from R. lauricola, overcoming previous limitations.
• Identification of GC-rich secondary structures (stem-loop, cruciform) that likely hindered amplification and sequencing efforts.
• The developed TaqMan real-time PCR assay successfully detected R. lauricola in symptomatic wood tissues of redbay, avocado, and sassafras trees in Florida, with results confirmed by Koch's postulates.

Conclusions:

• The optimized molecular protocol and TaqMan assay provide a sensitive and specific method for detecting Raffaelea lauricola.
• This diagnostic tool is crucial for managing laurel wilt disease and protecting the Florida avocado industry.
• Further characterization of internal transcribed spacers (ITS) sequences, despite secondary structure challenges, is now feasible.