The ARR Problem

Summary and Concerns

Armillaria root rot (ARR) is a major threat to the long-term productivity of stone-fruit and nut crops across the major production areas in the U.S. There are no efficient methods of eradicating the long-lived inoculum buried in the soil before replanting nor are there therapeutic methods to offset reduced productivity and tree death. Industry stakeholders, local economies and communities are in critical need of long-term durable solutions to mitigate this threat. Funding from state commodity boards and a multistate block grant established the foundation for members of this project to initiate coordinated national efforts to address this problem. A national transdisciplinary team with active participation of stakeholders (growers and nurseries) was put together for this proposal to advance current knowledge. With comprehensive, coordinated projects across labs and Prunus crops, in cooperation with industry partners at all stages of the research, we propose to deliver contemporary short and long-term strategies to effectively address ARR. The long-term goal of this project is to develop phenotyping and genotyping tools to enable breeding for ARR resistance and therefore improve profitability and sustainability of Prunus crops.

The Problem

Thcausal soil fungi of Armillaria Root Rot (ARR) represent several Armillaria species that are geographically isolated across the nation, including A. mellea in California, Desarmillaria tabescens in the southeastern U.S. and A. ostoyae in MichiganAs a facultative necrotroph, these fungi colonize and kill the root system, effectively killing the tree prior to its maximum productivity. ARR disease is easy to recognize and diagnose when the bark of infected trunks or roots is peeled back as the characteristic cream-colored fungal mycelia is visible.

The first above ground symptoms are typical of water stress as the pathogen mycelium reduces the flow of water and nutrients to the plant. Infected trees will then collapse, but the effects are long-lasting. Orchard land is rendered unsuitable for further peach or cherry production as the inoculum (vegetative fungal mycelium) can survive in the soil on infected root pieces for years to come. Current ARR management practices such as chemical applications are unable to eradicate the pathogen and have a temporary effect. Thus, longer-term solutions are required to overcome the ARR threat.

The four main project goals to address the ARR problem and promote sustainable peach and cherry crop production were accomplished under the Multistate Specialty Crop Block Grant from the United States Department of Agriculture (USDA) Agriculture Marketing Service Specialty Crop Multistate Project, “Short and long-term solutions for Armillaria root rot in Prunus” (USDA-AMS-SCMP-2015-10.170).

Objective 1. Identify, Characterize, and Utilize Sources of Resistance to Armillaria/Desarmillaria

Objective 1a. Identify sources of resistance in the germplasm

Objective 1b. Characterize the anatomical and biochemical responses of root tissue to infection

Objective 2. Discover Genomics To Enable Breeding Solutions for ARR Resistance/Tolerance

Objective 2a. Genome characterization of ARR resistant germplasm

Objective 2b. Characterize the transcriptomic response to infection

Objective 2c. Characterize the metabolic response to infection

Objective 3. Implement Short- and Long-Term Approaches to Manage ARR

Objective 3a. Identify resistant germplasm that also has optimal horticultural characteristics

Objective 3b. Evaluate binary sectoral rootstocks as a strategy to increase longevity on ARR sites

Objective 3c. Evaluate the efficacy of root-collar excavation (RCE) for ARR on peach and cherry

Objective 4. Determine the Economic Effect of ARR and Cost-Benefit of Cultural Practice Adoption
Objective 5. ​Conduct Outreach and Extension Activities To Facilitate the Adoption of Results