The following projects at the National Ornamentals Research Site at Dominican University of California (NORS-DUC) were funded in Fiscal Year 2010-2011 with farm bill monies.
TITLE: The risk of asymptomatic Phytophthora ramorum infection on fungicide treated rhododendrons
RESEARCHERS: Gary Chastagner and Marianne Elliott, Washington State University
A number of systemic and contact fungicides have been shown to be effective in controlling Phytophthora ramorum development on several nursery crops. One of the concerns about using fungicides to manage this pathogen on nursery stock relates to the possibility that fungicides are masking symptom development, thus making it more difficult to detect infected plants during routine visual inspections and increasing the risk of spreading this pathogen on asymptomatic infected plants. Results from earlier artificial inoculation studies have indicated that Subdue MAXX and Insignia fungicides may pose a high risk of masking symptom development on rhododendron foliage. This research project will determine the risk that fungicide applications will mask symptom development on rhododendrons under commercial production practices and determine how long suppression of symptom development lasts following the cessation of fungicide treatments.
TITLE: Use of Trichoderma to remediate Phytophthora ramorum-infested soil
RESEARCHERS: Tim Widmer and Nina Shishkoff, USDA Agricultural Research Service
Phytophthora ramorum has been repeatedly detected in nurseries even after the removal of infected plants and sanitation of the growing area. Although methods, such as chemical fumigation, oxidation, and heat treatment, exist to sterilize soil they are often costly, impractical, and raise health and environmental concerns. It is the purpose of this study to examine methods to remediate P. ramorum-infested soil that are environmentally friendly, safe and effective. The field site will be infested with a known amount of P. ramorum propagules and then partitioned off using fiberglass, circular microplots. A specific treatment will then be applied to the microplots in replication. These treatments will be: 1) a non-treated control; 2) a commercially-registered chemical treatment; 3) a commercially-available biological control agent; 4) a different commercially-available biological control agent; and 5) an experimental fungal isolate of Trichoderma that was demonstrated to reduce P. ramorum populations to non-detectable limits in the laboratory. Over the course of the experiment, soil samples will be taken within each microplot and the populations of P. ramorum and the biological control agents will be monitored. It is the hope that the biological control agents will be effective in reducing or eliminating the populations of P. ramorum.
TITLE: Episodic abiotic stress and ramorum blight in nursery ornamentals: impacts on symptom expression and chemical management of Phytophthora ramorum in Rhododendron
RESEARCHERS: Richard M. Bostock and Tatiana Roubtsova, University of California-Davis
APHIS/NORS-DUC AMOUNT: $13,624
USDA FOREST SERVICE AMOUNT: $68,000
Episodes of abiotic stress, such as soil salinity, waterlogging, and chilling can affect the physiology of plants to increase their vulnerability to diseases. Previous work demonstrated that a brief episode of salt stress predisposes roots of Rhododendron sp. and Viburnum tinus to infection by Phytophthora ramorum to significantly increase disease severity. Root infections may play a role in the disease cycle of ramorum blight in some hosts, and it is likely that such infections can remain cryptic, or asymptomatic, for a time. This project will provide a better understanding of specific factors that contribute to disease development from soilborne infections, with the intent to inform and guide management decisions. These factors could have a large effect on inoculum thresholds necessary for disease, the extent and significance of root infections in various hosts, the consistency and reliability of assessment tests for host resistance, and the efficacy of chemical treatments to manage disease. We will examine how stresses that are encountered in nurseries, such as inappropriate nitrogen fertilization, waterlogging, water deficit, and chilling, can serve as potential triggers for disease development arising from low inoculum levels or cryptic root infections. We will assess the importance of these stresses as they may contribute to rapid development of ramorum blight in seemingly healthy Rhododendron plants, such as might occur following shipment and planting. We will also examine the impact of mild episodic stress on the efficacy of selected chemicals for managing ramorum blight. This research will illustrate interactions that can increase the disease proneness of plants to P. ramorum, and suggest measures to complement or refine disease management practices.
TITLE: Effect of fungicides and biocontrol agents on inoculum production and persistence of Phytophthora ramorum on nursery hosts
RESEARCHERS: Steve Tjosvold, University of California Cooperative Extension, Santa Cruz; Gary Chastagner and Marianne Elliott, Washington State University
Phytophthora ramorum is the causal agent of Sudden Oak Death (SOD) and can infect many commonly grown nursery crops. Once P. ramorum is introduced into a nursery on a host, its local spread and establishment is primarily dependent on sporangia and zoospores production and spread, and pathogen persistence. Nursery operators commonly use fungicides to prevent the establishment of Phytophthora diseases, although current research only supports the use of fungicides for preventing infection. It is still unknown, however, what effect fungicide treatments have on sporulation, spread, and persistence of the pathogen on established infections. With this knowledge, fungicide treatments could more effectively be used to prevent the spread and establishment of the pathogen in nursery operations. This research will evaluate activity of foliar applied fungicides and biocontrol agents to inhibit sporulation and reduce pathogen viability in ornamental hosts.