Powdery mildew in hydrangea manifests as white, powdery spots on leaves, reducing plant vigor and aesthetic value. | Jane Cerza
Researchers at Clemson University are looking into new ways to control Asteraceae petal blight, a fungal disease that causes specks and streaks to appear on the petals of certain flowers. Their project seeks to minimize petal blight by finding the most common causal organisms, identifying them, and discovering the most effective strategies to combat them. Here’s a September 2024 update on their work and plans for next year from American Floral Endowment:
The project was created in response to growers in the cut flower industry, particularly those in the Bogotá, Colombia, area who face economic losses due to the occurrence of petal blight. Frequently, specks and streaks appear on ray florets of Asteraceae flowers (gerbera, sunflower, chrysanthemum), rendering them unsellable. Growers have reported that Itersonilia sp. has been the most common causal agent, but this diagnosis is based on symptoms that can be easily confused with other causal organisms. The goal of this research is to minimize petal blight by identifying the common causal organisms, facilitating their identification, and generating effective fungicide application strategies.
Specific objectives, based on questions raised by industry growers, that the team is currently addressing as part of the project are to:
- Determine if the Itersonilia that affects mums, gerbera daisies, and sunflowers is all the same species, or if a unique species or strain infects each host species.
- Differentiate symptoms from species of Botrytis, Stemphyllium, and Alternaria for identification by greenhouse staff.
The development of proper morphological and molecular identification of the microorganisms will be useful to avoid misdiagnosis and fine-tune disease management practices. These large growers have excellent research teams and facilities, with the capabilities to identify organisms with both microscopy and fundamental DNA sequencing methods. Accurate pathogen identification, knowledge of the disease cycle, and appropriate fungicide testing and application will improve control methods.
This past year, the researchers obtained a total of approximately 150 pounds of cut flowers from three growers in the Bogotá, Colombia, area. Samples included a variety of cultivars of gerbera, sunflower, and chrysanthemum, in addition to hydrangea. These were sent in response to growers scouting and noticing symptoms, and the team’s availability to process them. From these samples, photographs of the symptoms were taken, and cultivars were noted. Fungi were isolated using a brief rinse in 70% ethanol and water to remove potential saprotrophic organisms, and samples were plated on malt extract agar and corn meal agar containing antibiotics. Approximately 80 pure cultures are being maintained.
Control strategies graphic on Clemson research | American Floral Endowment (AFE)
The Clemson University researchers have an efficient system for processing flower samples to record symptoms and isolate associated fungi, and the team is ready to process the next season of samples. Generated data will be of use wherever environmental conditions are favorable for the pathogens. The goal is to characterize symptoms with specific pathogens. The researchers will continue to characterize symptoms and identify fungi involved until they can unite the two together.
Solutions for Managing Powdery Mildew on Bigleaf Hydrangeas
Powdery mildew in bigleaf hydrangea (Hydrangea macrophylla) is a destructive foliar disease. Now, a new study has identified effective chemical control methods for managing powdery mildew, a common fungal disease affecting bigleaf hydrangeas. The findings offer growers practical solutions to protect this popular ornamental plant and maintain its vibrant appearance in landscapes and nurseries.
Powdery mildew manifests as white, powdery spots on leaves, reducing plant vigor and aesthetic value. This study evaluated various chemical treatments to determine the most effective options for suppressing the disease while minimizing environmental impact.
Researchers tested multiple fungicides and application schedules, revealing several products and strategies that significantly reduced powdery mildew incidence and severity. These treatments not only improved plant health but also maintained the hydrangea’s ornamental quality, ensuring its appeal for gardeners and commercial markets.
This research was conducted by Dr. Baysal-Gurel and her team at Tennessee State University to determine the efficacy of the fungicides on powdery mildew of bigleaf hydrangea at different application rates and intervals. Plant height and width increase, defoliation percentage, defoliation progress, disease severity, and disease progress over the duration of the study were evaluated across several different experiments in both shade house and greenhouse conditions between 2022-2023.
The study underscores the importance of integrated disease management, where chemical control complements other practices like proper spacing, air circulation, and resistant cultivars. By adopting these findings, growers can better protect their crops from powdery mildew and enhance the sustainability of hydrangea production.
