Speaker: Dr. Fulya Baysal-Gurel, Associate Dean of Research and Associate Professor, Tennessee State University, Otis L. Floyd Nursery Research Center
Webinar Date: September 18, 2025
Moderator: Dr. Shimat Joseph, UGA Extension Entomologist
Duration: 50:24
CEU Categories: Category 24 (Ornamental and Turf Pest Control), Category 35 (Industrial, Institutional, Structural and Health Related Pest Control)
NARRATIVE SUMMARY
Dr. Fulya Baysal-Gurel, a leading woody ornamental plant pathologist at Tennessee State University, presented comprehensive research updates on vascular streak dieback (VSD), an emerging disease threatening the nursery industry across the United States. First detected on redbud (Cercis canadensis) in Tennessee in 2019, this devastating disease is caused by Ceratobasidium theobromae, a semi-obligate fungal pathogen previously known primarily from cacao production in South Asian countries. The presentation detailed six years of intensive research conducted at the Otis L. Floyd Nursery Research Center, providing critical insights into pathogen biology, disease management, and collaborative efforts to address this industry-wide challenge. Dr. Baysal-Gurel emphasized that VSD presents with distinctive symptoms including dieback, defoliation, water sprouts near grafting areas, leaf scorching, and characteristic vascular streaking in woody tissues showing marbled discoloration. Through molecular sequencing and phylogenetic analysis, her team confirmed that U.S. isolates are more closely related to Chinese honeysuckle isolates than to South Asian cacao strains, suggesting a different evolutionary pathway.
A major breakthrough from Dr. Baysal-Gurel’s program was the development and validation of new qPCR primers and protocols specifically designed to detect Ceratobasidium theobromae in U.S. nursery stock. The original primers developed for South Asian cacao cases produced false positive and false negative results with U.S. isolates, necessitating the creation of new diagnostic tools. Her team collaborated with multiple laboratories including Purdue University, Virginia Department of Agriculture, Virginia Tech, Oklahoma State University, Ball Horticultural Company, and Bartlett Tree Experts to validate these molecular protocols. Published in Plant Disease journal in May 2025, these primers are now available to all diagnostic labs under the National Plant Diagnostic Network (NPDN), providing critical tools for early detection and disease confirmation. The qPCR system has proven essential not only for tissue diagnosis but also for detecting basidiospores in air samples from spore traps, enabling researchers to track pathogen dispersal patterns and infection timing in commercial nurseries.
The presentation revealed critical findings about disease epidemiology and spore release patterns through multi-year field studies conducted in commercial nurseries. Using passive spore traps placed at various heights from ground level to two meters, Dr. Baysal-Gurel’s team documented that basidiospores are first detected at higher elevations in late June, appearing at ground level by late July. Disease severity remains low through May but increases dramatically from September through October. A pivotal discovery concerned the role of cultural practices in disease spread: growers who pruned plants and left residues on the ground inadvertently created inoculum reservoirs. Pruning activities occurred just before peak spore release, and plant debris left in fields continued releasing spores that initiated reinfection cycles. Similarly, defoliated leaves accumulating under plants in October served as additional spore sources. This research led to a critical management recommendation emphasizing thorough sanitation and prompt removal of all plant residues following pruning operations.
Dr. Baysal-Gurel presented extensive fungicide efficacy data from trials conducted between 2022 and 2024, testing an expanding array of products each year. Results demonstrated that several fungicides provided significant disease suppression compared to non-treated controls. Postiva emerged as a consistently promising product across multiple years when applied at middle rates. OHP’s Astun showed strong suppression in 2023 trials, as did an experimental Rainbow product applied as a single drench that performed comparably to Postiva and Astun. The 2024 trials expanded to include Orchestra, Morale, and rotation programs combining multiple products, all showing encouraging results for integration into disease management programs. Dr. Baysal-Gurel emphasized that fungicides should be applied immediately following any pruning or wounding events, and that rotation programs are essential to preserve long-term efficacy. She provided economic data from published literature showing that while sanitation labor costs range from $125-$293 per hectare per year, net profits after implementing sanitation practices increased from $83 to $650 per hectare annually, demonstrating clear economic benefits despite initial investment.
Redbud cultivar screening trials conducted in 2022 and 2023 revealed significant differences in disease susceptibility among varieties and species. Yellow-foliage cultivars consistently exhibited higher susceptibility to VSD compared to dark green and purple cultivars, which showed delayed symptom development and lower overall disease severity throughout the growing season. Eastern redbud (Cercis canadensis), commonly used as rootstock, displayed moderate susceptibility. Most significantly, Cercis chinensis (Chinese redbud) demonstrated notably higher tolerance to VSD. Dr. Baysal-Gurel emphasized using the term “tolerance” rather than “resistance,” as ongoing collaborative research with Dr. Ken Chen at North Carolina State University continues to characterize these differences. This work, supported by the North Carolina Department of Agriculture, is screening additional Cercis chinensis varieties at Tennessee State University to identify the most promising germplasm for future breeding programs and alternative rootstock development.
The current geographic distribution of VSD spans 13 confirmed states, with the disease documented not only in nursery production settings but also in landscape plantings, natural areas, and parks. The host range has expanded dramatically to over 50 different plant genera beyond the original redbud detections, including maple, dogwood, spicebush, and sweetgum. In Tennessee surveys coordinated with APHIS and the Tennessee Department of Agriculture, 44% of samples tested positive for Ceratobasidium theobromae, demonstrating widespread prevalence. Virginia Department of Agriculture surveys confirmed the pathogen’s presence in 37 different areas using the validated PCR protocols. Dr. Baysal-Gurel’s diagnostic laboratory has processed samples from multiple states and collaborates with Virginia Tech, making APHIS permits available to facilitate interstate sample submission. The pathogen’s ability to infect diverse woody ornamental hosts and establish in both cultivated and natural environments underscores the significant threat VSD poses to the broader nursery and landscape industry.
Management recommendations synthesized from six years of research emphasize integrated cultural and chemical approaches. Sanitation remains the highest priority: removing all pruning residues, defoliated leaves, and plant debris eliminates inoculum sources that perpetuate infection cycles. Growers should minimize unnecessary pruning and wounding, separating cosmetic pruning from diseased tissue removal operations. When pruning is essential, immediate fungicide application following wounding provides protection during vulnerable periods. Crop rotation is recommended though challenging given the extensive host range. Using only healthy, pathogen-free plants for propagation prevents introducing infected material into production systems. Proper cultural practices including appropriate irrigation management, correct planting depth, and avoiding herbicide injury reduce plant stress that predisposes plants to infection. Dr. Baysal-Gurel noted concerning interactions between VSD and root rot pathogens in samples submitted for diagnosis, suggesting that deep planting creates stress conditions favoring both disease complexes.
Dr. Baysal-Gurel highlighted extensive collaborative efforts advancing VSD research and management. In October 2024, USDA NIFA SCRI funding supported a working group meeting at the Nursery Research Center bringing together nearly 80 scientists, growers, and industry partners from multiple states and institutions. This productive gathering identified priority research objectives and led to development of a CAP (Coordinated Agricultural Project) proposal for expanded multi-state research efforts. The team has developed tech sheets published through Tennessee State University and Virginia Cooperative Extension, with information hosted on TSU diagnostic lab websites and AmericanHort platforms. Presentations to the Architect of the Capitol in Washington, D.C. addressed concerns about potential disease spread in federal landscapes. Publications in Plant Disease journal (March 2025 and May 2025) provide open-access resources detailing initial disease characterization and PCR protocol development. European plant health authorities have requested information and symptomatic plant images for alert systems, demonstrating international awareness. Media coverage in nursery industry publications has helped disseminate practical information to growers nationwide, supporting industry-wide efforts to manage this emerging threat.
YOUTUBE TIMESTAMPS
0:00 Introduction and Speaker Credentials
2:08 Webinar Overview and Acknowledgments
2:17 Vascular Streak Dieback: Overview of Emerging Disease
2:32 First Detection: Tennessee 2019 on Redbud
2:42 Characteristic Disease Symptoms
3:14 Symptom Recognition: Dieback, Leaf Scorching, Vascular Streaking
3:24 Diagnostic Laboratory Procedures
3:37 Fungal Isolation and Culturing
4:00 Molecular Identification: Ceratobasidium theobromae
4:20 Pathogen Biology and Semi-Obligate Nature
5:45 Historical Context: VSD on Cacao in South Asia
6:10 Koch’s Postulates and Pathogenicity Testing Challenges
7:00 VSD on Cassava: Recent Confirmations
7:17 Pathogenicity Testing on Eastern Redbud
8:06 Inoculation Studies and Symptom Development
9:05 Re-isolation Challenges and PCR Confirmation
9:47 Phylogenetic Analysis: Comparing U.S. and Asian Isolates
10:19 Genetic Relationships: China vs. South Asia
10:47 First Publication: Plant Disease Feature Article
11:15 Research Timeline: 2019 to Present
11:36 Development of qPCR Diagnostic Tools
12:21 Multi-Laboratory Validation Study
12:32 Collaborating Institutions
13:08 Protocol Validation Across Labs
13:18 Publication: PCR Primers and Protocols
13:31 NPDN Access to Diagnostic Tools
14:06 Virginia Survey Using PCR Protocols
14:18 Spore Detection Research
14:33 Understanding Pathogen Biology and Dispersal
15:08 Spore Trap Studies in Commercial Nurseries
15:38 Basidiospore Detection in Air
16:16 Passive Spore Trap Design and Placement
16:42 Field Trials: Observing Grower Practices
17:04 2022 Fungicide Efficacy Study Setup
17:21 Disease Severity Patterns: May Through September
17:32 Spore Detection at Multiple Heights
18:01 Critical Discovery: Pruning and Residue Management
18:14 Plant Debris as Inoculum Source
18:33 Sanitation Recommendations
19:00 Ground-Level Spore Detection in October
19:21 Defoliated Leaves as Infection Sources
19:40 Microbiome Community Studies
19:50 Challenges in Spore Detection Methods
20:03 New Trapping Methods: Burkard Spore Traps
20:48 Disease Biology and Seasonal Patterns
21:20 12-Month Production Cycle Overview
21:27 Basidiospore Release Timing: May Onwards
21:50 Grafting Operations and Disease Transmission
22:15 Rootstock-to-Scion Movement Questions
22:45 Overwintering Biology: Unanswered Questions
23:05 Seasonal Isolation Challenges: December-March
23:32 Latent Phase Hypothesis
24:00 Geographic Distribution: Current Status
24:38 Multi-State Detections: 13 States Confirmed
25:03 Native Plants and Park Detections
25:16 Tennessee Survey Results
25:43 Host Distribution: Redbud, Maple, Dogwood
26:02 Virginia Survey: 37 Confirmed Areas
26:21 Expanding Host Range
26:40 50+ Host Genera Confirmed
27:04 Fungicide Efficacy Research Program
27:28 2022 Commercial Nursery Trials
27:50 Postiva Fungicide Performance
28:29 Cultural Recommendations: Minimize Wounding
28:56 Post-Pruning Fungicide Applications
29:18 Economic Benefits of Sanitation
30:14 2023 Fungicide Trial Results
30:30 Expanded Product Testing
31:07 Astun, Postiva, and Experimental Products
31:38 Rainbow Product: Single Drench Application
31:47 2024 Fungicide Trials
32:05 Orchestra, Morale, and Rotation Programs
32:49 Redbud Cultivar Screening Studies
33:06 Cultivar Identification and Anonymity
33:36 2022 Cultivar Trial Results
34:19 2023 Cultivar Comparison
34:48 Weather Impact on Disease Severity
35:20 Molecular Detection in Asymptomatic Plants
36:00 Cultivar Tolerance Differences
37:00 Cercis chinensis Tolerance Research
38:00 Future Breeding and Rootstock Development
38:45 Industry Survey and Grower Feedback
39:30 Management Recommendations Summary
40:14 Diagnostic Services Available
40:35 Sanitation as Primary Management Tool
40:49 Crop Rotation Challenges
40:58 Cultural Practices: Irrigation and Planting Depth
41:24 VSD and Root Rot Pathogen Interactions
41:47 Herbicide Injury Avoidance
41:56 Chemical Control Recommendations
42:06 USDA NIFA Working Group Meeting
42:31 Multi-State Collaboration
42:44 Priority Research Topics
43:04 CAP Proposal Development
43:15 European Awareness and Alert Systems
43:34 Tech Sheets and Extension Publications
43:53 AmericanHort Information Hosting
44:15 Architect of the Capital Presentation
44:38 Media Coverage and Industry Outreach
45:03 Acknowledgments and Collaborative Partners
45:21 Question and Answer Session
46:01 Question: Cercis chinensis Resistance
46:23 Response: Tolerance vs. Resistance
47:02 Collaborative Research with NC State
47:35 Georgia Status Discussion
47:54 APHIS National Survey
48:17 Question: Rootstock Used for Grafting
48:34 Response: Cercis canadensis and C. chinensis
49:10 Research Publications Access
49:35 Closing Remarks
QUESTIONS & ANSWERS
Q: Is Cercis chinensis (Chinese redbud) resistant to vascular streak dieback?
A: Research shows that Cercis chinensis exhibits notably higher tolerance to VSD compared to other redbud species and cultivars, but Dr. Baysal-Gurel is hesitant to use the term “resistance.” Ongoing collaborative screening trials with Dr. Ken Chen at North Carolina State University, supported by the North Carolina Department of Agriculture, are evaluating multiple Cercis chinensis varieties at Tennessee State University. The tolerance level is definitely different and more favorable than many cultivars tested, making Chinese redbud promising for future breeding programs and potential rootstock alternatives.
Q: What rootstock do growers typically use when grafting redbud cultivars?
A: Most growers use Eastern redbud (Cercis canadensis) as rootstock for grafting ornamental cultivar scions. Some nurseries are beginning to experiment with Cercis chinensis as an alternative rootstock given its higher VSD tolerance. Research is currently exploring alternative rootstock options from different Cercis species, though changes must consider both grafting success rates and plant marketability. Collaboration with breeding specialists aims to identify rootstock-scion combinations that provide disease tolerance while maintaining desirable ornamental characteristics and commercial viability.
Q: Has vascular streak dieback been detected in Georgia?
A: As of September 2025, VSD has not been officially reported in Georgia, though the disease has been confirmed in 13 states including neighboring Tennessee and other southeastern locations. APHIS is funding a national survey, and Dr. Baysal-Gurel has introduced TSU’s diagnostic capabilities and PCR protocols to Georgia’s state plant pathologist. There is hope that Georgia Department of Agriculture may join the multi-state survey efforts in the near future, which would provide systematic monitoring to confirm the state’s current VSD-free status or detect any unreported infections.
Q: What fungicides have shown the best efficacy against VSD in research trials?
A: Multi-year trials from 2022-2024 identified several promising fungicides for VSD management. Postiva has demonstrated consistent disease suppression across three consecutive years of testing. OHP’s Astun showed strong performance in 2023-2024 trials. An experimental Rainbow product applied as a single drench application provided efficacy comparable to Postiva and Astun. Additional products including Orchestra and Morale showed promise in 2024 trials. Rotation programs combining multiple products are recommended to prevent resistance development. Fungicides should be applied immediately following any pruning or wounding events for maximum protection.
Q: How can growers access the PCR diagnostic protocols for VSD detection?
A: The validated qPCR primers and protocols are published in Plant Disease journal (May 30, 2025) as an open-access article, making them freely available to all diagnostic laboratories under the National Plant Diagnostic Network (NPDN). Growers should contact their state diagnostic lab, university extension specialists, or can submit samples directly to Dr. Baysal-Gurel’s laboratory at Tennessee State University. She is willing to share APHIS permits to facilitate interstate sample submission and has been processing samples from multiple states beyond Tennessee to support the national survey efforts.
Q: What is the most important management practice for controlling VSD in nurseries?
A: Sanitation practices are the single most critical management tool for VSD control. Growers must promptly remove all pruning residues and plant debris from production areas, as these materials serve as inoculum reservoirs that release basidiospores and perpetuate infection cycles. Research demonstrated that leaving pruned plant material on the ground directly coincided with spore detection on traps and disease spread. While sanitation labor costs range from $125-$293 per hectare per year, net profits after implementing sanitation increased from $83 to $650 per hectare annually, providing strong economic justification for this practice.
Q: Why is VSD difficult to work with in pathogenicity testing?
A: Ceratobasidium theobromae is classified as a semi-obligate pathogen, meaning it is dependent on living host tissue for survival and growth. While researchers successfully inoculated healthy Eastern redbud plants and observed characteristic vascular streaking symptoms developing 5-8 cm from inoculation points, they repeatedly failed to re-isolate the organism from inoculated plants using traditional culturing methods. The pathogen’s presence can only be confirmed using qPCR molecular techniques, which detect pathogen DNA even when the fungus cannot be recovered in pure culture. This same challenge was documented in South Asian cacao research over nearly 30 years, confirming the inherent biological characteristics of the pathogen.
Q: What is known about the overwintering biology of the VSD pathogen?
A: Overwintering biology remains one of the critical unanswered questions in VSD research. Between December and March, even when plants remain symptomatic from previous season infections, researchers are unable to successfully isolate Ceratobasidium theobromae from infected tissues. This suggests the pathogen may enter a latent phase during winter months, possibly tied to host dormancy and environmental conditions. Because many growers dig and store budded liners in December for spring sales, there are concerns about whether dormant infected plants can serve as overwintering inoculum sources that spread the pathogen to new locations when plants are shipped and transplanted in spring.
Q: What symptoms should growers look for when scouting for VSD?
A: The characteristic symptoms of vascular streak dieback include several distinctive features that appear progressively throughout the growing season. Watch for branch dieback and defoliation, particularly on newer growth. Water sprouts often develop near grafting areas on infected plants. Leaf scorching becomes prominent as the season progresses, typically intensifying from May through September with peak severity in late summer. The most diagnostic symptom is vascular streaking visible when cutting into symptomatic stems or branches—the vascular tissue shows marbled discoloration rather than healthy wood color. Symptoms may not appear immediately after infection, and molecular testing may detect the pathogen in asymptomatic plants.
Q: How does weather affect VSD disease development?
A: Comparative data from 2022 and 2023 cultivar trials demonstrated that environmental conditions significantly influence disease severity. The same redbud cultivars showed much lower disease severity in 2022 compared to 2023, with weather data including soil temperature, air temperature, humidity, and rainfall patterns all tracked to understand these differences. While specific threshold conditions are still being characterized, the research indicates that certain environmental conditions favor more aggressive disease development. This weather dependency has important implications for forecasting disease risk in different production regions and seasons.
Q: What is the current geographic distribution of vascular streak dieback?
A: As of September 2025, VSD has been confirmed in 13 states across the United States. The disease is present not only in nursery production settings but has also been detected in landscape plantings, natural forest areas, and public parks. Virginia Department of Agriculture surveys confirmed Ceratobasidium theobromae in 37 different areas using the validated PCR protocols. Tennessee surveys showed 44% of submitted samples testing positive. Native plants including spicebush in parks and sweetgum growing at woodland edges of nurseries have tested positive, demonstrating the pathogen’s ability to infect diverse hosts in both managed and natural environments. This broad distribution suggests VSD likely exists in additional states beyond those with confirmed detections.
Q: Beyond redbud, what other plants are susceptible to VSD?
A: The host range of vascular streak dieback has expanded dramatically from the initial redbud detections to encompass over 50 different plant genera. Confirmed hosts include important nursery crops such as maple, dogwood, spicebush, sweetgum, and honeysuckle. The pathogen’s ability to infect such a diverse array of woody ornamentals makes crop rotation extremely challenging as a management strategy. Virginia surveys using the validated PCR protocols detected Ceratobasidium theobromae on multiple host species beyond redbud, confirming field observations of expanding host range. This broad host spectrum significantly increases the economic threat VSD poses to the nursery industry and suggests many additional susceptible species may be identified as surveys continue.
ADDITIONAL RESOURCES
Dr. Fulya Baysal-Gurel Contact Information:
Tennessee State University
Otis L. Floyd Nursery Research Center
Email: Available through TSU Department of Agricultural and Environmental Sciences
Published Research:
- Baysal-Gurel et al. (2025). “Vascular Streak Dieback: First Report and Characterization.” Plant Disease (March 3, 2025) – Open Access Feature Article
- Baysal-Gurel et al. (2025). “Development and Validation of qPCR Primers for Detection of Ceratobasidium theobromae.” Plant Disease (May 30, 2025) – Open Access
- Plant Disease Management Reports (multiple years, 2022-2024) – Available through American Phytopathological Society
Extension Resources:
- Tennessee State University VSD Information: TSU Diagnostic Lab Website
- Virginia Cooperative Extension VSD Tech Sheet
- AmericanHort VSD Resource Page
- Workshop Proceedings (October 2024 Working Group Meeting)
Diagnostic Services:
- Contact Dr. Baysal-Gurel’s laboratory for sample submission (APHIS permits available for interstate samples)
- National Plant Diagnostic Network (NPDN) – Member labs can use validated PCR protocols
- State university extension diagnostic laboratories
Collaborating Institutions:
- Tennessee State University
- North Carolina State University (Dr. Ken Chen – Breeding Program)
- Virginia Tech University
- Virginia Department of Agriculture
- Purdue University
- Oklahoma State University
- Ball Horticultural Company
- Bartlett Tree Experts
- USDA APHIS (National Survey Program)
- USDA NIFA SCRI (Funding Support)
Industry Organizations:
- AmericanHort (HRI)
- American Phytopathological Society (APS)
This webinar was part of the Getting the Best of Pests (GTBOP) continuing education series, providing CEU credits for pest management professionals through the UGA Center for Urban Agriculture. For questions about woody ornamental diseases and VSD diagnostics, contact Dr. Fulya Baysal-Gurel at Tennessee State University.