Phytophthora ramorum Ornamental Nurseries
What should a commercial ornamental nursery do to prevent the introduction of Phytophthora ramorum (the causal agent of Sudden Oak Death)?
For most nurseries, this can be, in part, accomplished by careful inspection of new incoming host propagative material and stock. First, make sure incoming stock from infested counties are inspected by agricultural inspectors. Symptoms are not always readily apparent to inspectors on stock initially. So, a weekly, systematic, monitoring of stock by a trained scout in the nursery helps insure that the pathogen has not been introduced. Other practices that should be helpful include:
• All nursery personnel need to be aware of the issues and disease symptoms regarding this pathogen and should be ready to alert the nursery scout or other authority if characteristic symptoms are seen.
• Infected leaves often drop from plants. For high-risk incoming shipments, off- load the nursery stock in an area that can be cleaned of leafy debris. Sweep debris from the receiving area and delivery truck and bag for disposal. Loading and delivery areas should be as far from production areas as possible
• Maintain good shipping and receiving records to facilitate trace-backs and trace-forwards if contaminated stock is detected.
What about nurseries that are surrounded by native host trees and shrubs and in an immediate area where Phytophthora ramorum is found ?
• Periodically inspect nearby native hosts for disease symptoms. Infected California bay laurel trees near the perimeter of nurseries may produce inoculum that can spread and cause infection of nearby host plants, so removal of these trees may be warranted.
• Rain runoff coming down slope from areas containing infected hosts may contain P. ramorum. Consider building berms to prevent water and soil movement into production areas from hillsides surrounding the nursery.
• Irrigation water pumped from streams and ponds in areas of infected native hosts may be contaminated with P. ramorum. Consider having this water periodically tested to detect P. ramorum. If it is found to be present, consider alternative irrigation sources, such as well water, or disinfection treatments.
What about fungicides? Wouldn’t fungicides control diseases of Phytophthora ramorum on ornamentals?
• If applications of fungicides are made to nursery stock they should be made as preventative treatments. Currently, even the most active fungicides do not stop the development of P. ramorum once foliar lesions are present. They need to be applied before environmental conditions favor pathogen infection, for example, before a period of rainy weather that would allow water to linger on leaf surfaces for many hours.
• The regular and blanket use of fungicides will drive the mechanism that develops resistant pathogen strains. Fungicides with specific modes of action-as many Phytophthora-active fungicides are- will be especially vulnerable. Minimizing fungicide use, in any way, is the first priority to prevent resistant strains from developing. When fungicides are used, use different chemical classes- either in rotation or combining products in tank mixes. Fungicides active on P. ramorum may already be used in the nursery to control other foliar or soil-inhabiting Phytophthora species or related pathogens (such as downy mildews), and their use should be considered in planning the overall fungicide treatment strategy.
• Fungicides active on Phytophthora should not be applied to high-risk nursery stock or cuttings that will be monitored for P. ramorum infection because detection of symptoms may be delayed or masked.
How do I know what the diseases look like on ornamentals? What else can I do?
Arm yourself with information and images of the disease symptoms. A draft document “Nursery Guide for Diseases of Phytophthora ramorum on Ornamentals: Diagnosis and Management” by S.A. Tjosvold, K.R. Buermeyer, C. Bloomquist, and S. Frankel is available for download. “Nursery Guide for Diseases of Phytophthora ramorum on Ornamentals: Diagnosis and Management”