Category: Citrus Industry

Recently, extensive wood decay-related limb breakages have been reported in commercial prune orchards in the Sacramento Valley, adversely affecting fruit production and limiting the value of salable firewood upon orchard removal. Several fungal genera such as Phellinus, Rhodoformes, Schizophyllum, Sterum, Trametes, Ceriporia, and Perenoporia have been recognized for wood-decaying activity on California prunes.  Phellinus pomaceus has been the primary organism associated with prune decay symptoms in the Sacramento Valley; however, it is yet unknown whether P. pomaceus is present in southern San Joaquin Valley prune orchards.

Phellinus pomaceus, specific to prunes and other plums, is one of the commonly reported wood-decaying fungi that attack the heartwood (non-functional xylem) of mature trees (Figure 1). It tends to attack the trunk and large-diameter branches, often resulting in broken limbs and the loss of fruit-bearing scaffolds. In fact, older trees are more likely to contain infection by the fungus, and frequent pruning of large branches may increase the probability of infection due to exposure of the internal heartwood. The fungus can be identified based on its fruiting bodies that emerge as conks or shelf-like brackets that are usually hard, woody, and hoof-shaped (Figure 2). Under the right conditions, these fruiting bodies are often perennial and may exhibit a darkened upper surface after several years of development.

UCCE Tulare County assisted UC Davis researchers from the Department of Plant Pathology in surveying Tulare County prune orchards for presence of P. pomaceus. Laurel Hoffman, a PhD student working under Dave Rizzo, Professor of Plant Pathology, visited our local UCCE Tulare County office, coordinating with Elizabeth Fichtner, UCCE farm advisor, to visit and survey prune orchards for the pathogen. With the assistance of Walter Martinez, Tulare County Ag Technician, and Santosh Bhandari, Assistant Specialist, six local ‘French’ prune orchards were surveyed with data collected on tree canopy status and presence or absence of fruiting bodies associated with decay fungi. Surveyed orchards were all over 15 years old and were in varying states of overall productivity. Samples from putative decay fungi were collected and brought back to UC Davis for genetic sequencing to identify the specimens. A preliminary observation based on the initial survey suggests that the prevalence of putative decay fungi in prune orchards is lower in the southern San Joaquin Valley than in the Sacramento Valley. The presence of P. pomaceus in the southern San Joaquin Valley has not yet been confirmed.

To date, there are no control measures for management of P. pomaceus. Chemical control strategies are not available for management of this disease. Removal of fruiting bodies may limit sporulation, thus having the potential to slow disease transmission. However, the value of this technique is limited by the ability to remove conks prior to sporulation and conks may be difficult to see, particularly after leaf out. Additionally, if the pathogen is present at a high level in (or near) affected orchards, the removal of conks may not significantly influence the total load of spores at a site. Fruiting body removal would not affect the health of infected trees because they are already colonized by the fungus.

Most of California’s prunes are sold in the dried fruit market; however, a few orchards are reserved for fresh prune production. After the economic lifespan of prune orchards, trees are removed, generating wood that can either be sold as firewood, or reintroduced to the soil through whole orchard recycling.  Infection with decay fungi such as P. pomaceous may adversely affect fruit production, limit the lifespan of infected trees, and reduce the economic longevity of orchards. — By Santosh Bhandari, Laurel Hoffman & Elizabeth Fichtner, UC Cooperative Extension