Location: Horticultural Crops Disease and Pest Management Research Unit
2022 Annual Report
Objectives
Objective 1: Improve and expand knowledge of the prevalence of genotypic and phenotypic diversity in existing and emerging plant pathogens of small fruits and nursery crops.
Sub-objective 1.A: Evaluate boxwood blight in Oregon nurseries.
Sub-objective 1.B: Develop molecular diagnostics and tools to facilitate research on a nematode/virus disease complex.
Objective 2: Understand how pathogen biology and diversity interacts with environmental factors to cause disease of small fruits and nursery crops.
Sub-objective 2.A: Influence of plant spacing and irrigation frequency on the spread of boxwood blight from infected plants to healthy plants.
Sub-objective 2.B: Cellular level response of Meloidogyne spp. to nematicides.
Objective 3: Develop chemical and host resistance disease management strategies for nursery crops and small fruits.
Sub-objective 3.A: Evaluate newer fungicide chemistries for control of Phytophthora root rot of rhododendron by sensitivity assays.
Sub-objective 3.B: Novel nematicide discovery: Solanum sisymbriifolium as a source of nematicidal compounds.
Approach
The long-term goal of this project is to develop sustainable disease management strategies that are based on a knowledge of the identity and biology of the causal agent(s) and on knowledge of pathogen co-infections and interactions with the environment. This will be accomplished by: (1) determining the prevalence of key pathogens and nematodes constraining production of nursery and small fruit crops; (2) understanding how environment and management practices influence disease; and (3) identifying new pesticides for the management of nursery and small fruit diseases.
Knowledge about the prevalence of pathogens and nematodes in agricultural systems is key for establishing effective disease control methods. Surveys will be conducted to assess the incidence of fungal pathogens in nurseries and nematodes in small fruit production fields. Molecular diagnostic tools will further be developed to evaluate the ability of nematodes to vector plant viruses. Pathogen and nematode prevalence is influenced by their response to multiple environmental factors. Therefore, studies will be established to assess the influence of irrigation and plant spacing on the spread of fungal plant pathogens in outdoor container trials, and on the effect of nematicides on nematode fitness in laboratory trials. New pesticide chemistries are also needed because multiple oomycete pathogens have developed resistance to fungicides and many traditional nematicides have been phased out because of harmful environmental and human health effects. New pesticide chemistries will be evaluated for their efficacy against oomycete plant pathogens and nematodes in laboratory, greenhouse and field experiments.
Together, results from this research will identify chemical and nonchemical practices to reduce plant disease, and that can be deployed in horticultural systems in the future.
Progress Report
This report documents progress for project 2072-22000-046-000D, “Disease Management in Small Fruit and Nursery Crops Based on Knowledge of Pathogen Diversity, Biology, and Environmental Effects,” which started May 2022 and continues research from project 2072-22000-043-000D, “Development of Knowledge-based Approaches for Disease Management in Small Fruit and Nursery Crops.”
Objectives and sub-objectives for virology are in the process of being added to this project through the Office of Scientific Quality Review (OSQR) process. Progress towards those proposed objectives and sub-objectives for virology are included, pending OSQR review and final approval.
Progress towards Sub-objective 1A has been made by sampling for boxwood blight at two boxwood nurseries in Oregon. Single-spore cultures of the boxwood blight pathogen are being prepared to share with collaborators by December 2022.
For Sub-objective 1B, plans to collect diverse tomato ringspot virus isolates from different geographical regions and hosts are in place.
For proposed Sub-objective 1C, “Identify factors that cause blueberry shock virus recurrence in commercial fields and blueberry breeding lines,” about 1,400 samples were collected from commercial fields and blueberry breeding lines. Samples were prepared for RNA extractions and enzyme-linked immunosorbent assay (ELISA) testing.
For Sub-objective 2A, data were collected at each nursery from Sub-objective 1A on plant spacing and irrigation timing. Preliminary tests are being conducted to confirm that the pathogen isolates cause disease in preparation for inoculations.
For Sub-objective 2B, samples were collected from small fruit fields in the Pacific Northwest (PNW). Nematodes were extracted from soil and quantified. Plant material has been archived to allow for virus determination in the future.
For Sub-objective 3A, Phytophthora species inoculum is being prepared and fungicides have been acquired. Preliminary tests are being conducted to determine rates that will be used to evaluate the sensitivity of the Phytophthora isolates.
For Sub-objective 3B, greenhouse studies were initiated to determine the host status of Solanum sisymbrifolium to root-knot and root lesion nematodes, both important plant-parasitic nematodes in the PNW. Preliminary results suggest that S. sisymbrifolium is not a host for these nematodes.
For proposed Sub-objective 3C, “Screening selected Rubus idaeus selections for resistance to raspberry bushy dwarf virus,” a plan was devised to graft about 50 breeding lines with raspberry bushy dwarf positive material to evaluate potential resistance.
For proposed Objective 4, “Develop robust and reliable diagnostic assays for plant virus detection in small fruits,” and proposed Sub-objective 4A, “Compare graft indexing, RT-PCR, and high-throughput sequencing methods for virus detection in strawberry,” strawberry standard indicators were obtained from the University of California to be used in graft experiments.
For proposed Sub-Objective 4B, “Compare graft indexing, RT-PCR, and high-throughput sequencing methods for virus detection in Rubus,” Rubus occidentalis ‘Munger’ plants were obtained to be used in graft experiments. Seventy-five donor plants known to be infected with viruses were obtained from the ARS Corvallis, Oregon, repository and are being maintained for grafting and further testing.
Accomplishments
1. Cool temperatures favor boxwood blight in Oregon. Oregon nurseries produce most of the boxwood plants in the United States, but production is threatened by the introduced disease, boxwood blight, which renders infected plants unsalable. Oregon boxwood growers have noticed that the boxwood blight disease forecasting model is not accurate and that supposedly resistant boxwood cultivars often develop severe blight. ARS researchers at Corvallis, Oregon, showed that the pathogen grows faster and causes more severe disease at cooler temperatures (15 degrees C) than have been previously reported (25 degrees C). In addition, a resistant cultivar that exhibited few disease symptoms at warm temperatures (25 degrees C) developed severe disease symptoms at cool temperatures (15 degrees C). Disease forecasting models and boxwood resistance screening programs therefore need to incorporate cool temperatures to better predict disease outbreaks and to select for boxwood cultivars that are also resistant to blight at cool temperatures.
