Knoxville, TN - Researchers have reported new findings on the combined use of plant growth regulators and plant defense activators to manage both tree growth and disease in young apple orchards. The work evaluates how the compounds prohexadione calcium and acibenzolar-S-methyl interact to influence vegetative growth and fire blight management in apple production systems.
Fire blight, caused by the bacterium Erwinia amylovora, is one of the most destructive diseases affecting apple orchards in many growing regions. At the same time, excessive vegetative growth in young orchards can complicate canopy management, reduce light penetration, and create conditions favorable for disease development. The paired studies examined whether a coordinated management approach could address both challenges.
The research was conducted in a young commercial apple orchard in North Carolina. Prohexadione calcium, a plant growth regulator widely used to limit shoot growth, was evaluated alongside acibenzolar-S-methyl, a compound that activates the plant's natural defense responses. Treatments were applied alone and in combination to determine their effects on tree growth and fire blight incidence.
Results from the first study showed that prohexadione calcium significantly reduced vegetative shoot growth, helping maintain a more compact canopy structure during the early establishment years of the orchard. Acibenzolar-S-methyl did not substantially affect vegetative growth when used alone, but the combined treatments maintained the growth-reducing benefits of prohexadione calcium without negatively affecting overall tree development.
The second study focused on fire blight suppression under the same treatment programs. Applications involving acibenzolar-S-methyl were associated with reduced fire blight symptoms, supporting its role as a tool for activating plant defenses against the disease. When used alongside prohexadione calcium, disease management benefits were maintained while vegetative growth remained controlled.
Together, the findings suggest that integrating these two products may provide orchard managers with a practical strategy to manage canopy growth while also reducing the risk of fire blight during the critical early years of orchard establishment.
The studies highlight the potential value of combining physiological growth management with induced disease resistance as part of an integrated orchard management program. As growers continue to seek effective strategies to balance productivity, tree health, and disease control, coordinated approaches such as this may help improve the sustainability and resilience of modern apple production systems.
Dr. Annie Vogel is Assistant Professor and Fruit Extension Specialist at the University of Tennessee - Knoxville, Dr. Tom Kon is Associate Professor and Southeastern Apple Research Specialist at NC State, and Dr. Sara Villani is Assistant Professor and Extension Specialist-Fruit and Ornamental Pathology at NC State.
Dr. Vogel stated that her work in commercial fruit production focuses on the improvement of commercial standards through modified cultural management practices and implementation of integrated pest management strategies. The use of plant growth regulators, vegetative growth management, and orchard sanitation have been major topics in self-funded research and future grant efforts. Her current projects include testing formulations of chemically similar products from multiple manufacturers, using biopesticides to aid in crop load management, and developing alternative winter pruning strategies in young high-density orchards. Future areas will address improving canopy microclimate to mitigate disease pressure, testing varieties for disease resistance, and inducing lateral branch development and promoting yield in fruit crops
The full articles can be found on the ASHS HortScience electronic journal website at: https://doi.org/10.21273/HORTSCI18807-25 and https://doi.org/10.21273/HORTSCI18806-25
Established in 1903, the American Society for Horticultural Science is recognized around the world as one of the most respected and influential professional societies for horticultural scientists. ASHS is committed to promoting and encouraging national and international interest in scientific research and education in all branches of horticulture.
Comprised of thousands of members worldwide, ASHS represents a broad cross-section of the horticultural community - scientists, educators, students, landscape and turf managers, government, extension agents and industry professionals. ASHS members focus on practices and problems in horticulture: breeding, propagation, production and management, harvesting, handling and storage, processing, marketing and use of horticultural plants and products. To learn more, visit ashs.org.
