Improved understanding of the threat posed by Phellinus heart-rot in California prune orchards

Prunes have been commercially grown in California for more than a century and prune production and processing are important to local economies, especially in the Sacramento Valley. In recent decades, wood decay has emerged as a significant threat to the industry by reducing orchard longevity. However, little information is available about the incidence and severity of wood decay. Consequently, effective management and control strategies are lacking. Over the course of three years, we conducted systematic surveys of wood-decay fungi in prune orchards throughout the Sacramento Valley. We determined disease severity in individual trees and the incidence of infection within orchard blocks across multiple age classes and created a model of disease progress. We found the near ubiquitous presence of Phellinus tuberculosus heart-rot throughout Sacramento Valley prune orchards, resulting in substantial limb breakage. Infections were associated with pruning wounds and began to appear in orchards as young as 7-years, with 50% or more trees infected by age 15. This study shows that control and management strategies must focus on prevention rather than eradication.