OREGON STATE UNIVERSITY EXTENSION SERVICE Phytophthora Ramorum: A Guide for Oregon Nurseries Hazel A. Daniels, Jennifer Parke, Jay W. Pscheidt and Chris Benemann Summary ¾ Phytophthora ramorum became a federally quarantined pathogen in 2005. ¾ This pathogen causes the diseases known as ramorum leaf and twig blight, and sudden oak death. ¾ In the United States, the regulated host list includes more than 150 plant species in 37 genera. ¾ Repeated shipments of P. ramorum-infested plants by nurseries have resulted in spread of the disease. In 2019, for example, nationwide trace investigations were triggered due to the detection of P. ramorum positive plants in commerce. The infected plants originated from two nurseries in Washington state and Canada and were shipped to 18 states. Photo: Jennifer Parke, © Oregon State University ¾ Nurseries should be vigilant in monitoring plant Figure 1: Ramorum shoot dieback and leaf blight on Viburnum x material brought in from external sources. bodnantense ‘Dawn’. ¾ Management efforts in Pacific Northwest nurseries (Quercus spp.) in the San Francisco Bay Area were dying are focused on eradicating the pathogen when it is from a new disease, sudden oak death. The cause of both found and preventing new infections. sudden oak death and the nursery diseases was found to be Phytophthora ramorum, which was formally described ¾ If P. ramorum is found in your nursery, and you ship in 2001. material outside of Oregon, you will be required to sign a cooperative agreement with the United By 2001, P. ramorum had spread to 10 counties in States Department of Agriculture and Oregon coastal California. The pathogen was also detected Department of Agriculture. in 2001 in forest sites near Brookings, in southwest Oregon, on tanoak, Pacific rhododendron and evergreen ¾ To be released from the program, P. ramorum huckleberry. A 9-square-mile area was quarantined, must not be detected in your nursery for three and infected plant material was cut, piled and burned in consecutive years. Background Hazel A. Daniels, Ph.D. candidate, Department of Botany and Plant Pathology; Jennifer Parke, professor (senior research), A fungus-like organism was discovered in 1993 Department of Botany and Plant Pathology, and Department causing leaf blights, stem cankers and tip dieback of Crop and Soil Science; Jay W. Pscheidt, Extension plant on nursery-grown rhododendrons and viburnums in pathology specialist, and professor, Department of Botany and Germany and the Netherlands. At about the same time, Plant Pathology, all of Oregon State University; Chris Benemann, many tanoaks (Notholithocarpus densiflorus) and oaks Oregon Department of Agriculture. EM 8840 Revised March 2021 Table 1. Nursery hosts reported in Oregon, plant part infected and known symptoms of Phytophthora ramorum infection.1, 2 Plant species Common name Plant part infected or cultivar name and symptoms Rhododendron hybrids rhododendron Leaf lesions, shoot dieback; death of mature plants Viburnum x bodnantense Bodnant viburnum Stem lesions Viburnum plicatum var. tomentosum doublefile viburnum Leaf lesions, shoot dieback; death of mature plants Viburnum davidii David viburnum Stem lesions, shoot dieback Pieris japonica Japanese pieris Leaf lesions, shoot dieback Pieris floribunda x japonica Pieris x ‘Brouwer’s Beauty’ Leaf lesions, shoot dieback Pieris formosa x japonica Pieris x ‘Forest Flame’ Leaf lesions, shoot dieback Pieris formosa x japonica Pieris x ‘Fire Mountain’ Leaf lesions, shoot dieback Camellia japonica Japanese camellia Leaf lesions, defoliation Camellia sasanqua Sasanqua camellia Leaf lesions, defoliation Kalmia latifolia mountain laurel Leaf lesions 1Adapted with permission from Davidson et al., 2003. 2Additional hosts provided by Oregon Department of Agriculture, personal communication. an attempt to eradicate the pathogen. The eradication program failed to fully eliminate P. ramorum from Oregon forests, and new infections have since been detected in Curry County, Oregon, each year. In response to the continued spread of P. ramorum, the quarantine area expanded to its current size of 515 square miles (1,333 km2) in 2015. A 58-square-mile (150 km2) Generally Infested Area was established in 2012 where eradication treatments are no longer required; however, the Bureau of Land Management continues to treat new infestations on its lands within the GIA. The GIA was later expanded to 123 square miles (319 km2). The disease was also discovered in a botanical garden landscape on the north end of Bainbridge Island, Washington, in 2015. Several plants were Photo: Jennifer Parke, © Oregon State University infected, including Camellia, Gaultheria, Mahonia, Pieris, Figure 2: Initial symptoms of ramorum leaf blight on Viburnum Rhododendron, Vaccinium, Viburnum and Vinca. Soil at plicatum var. tomentosum ‘Mariesii’, showing necrotic leaf spots the location was also infested, and was steam-treated and shoot dieback. to eradicate P. ramorum. The pathogen persisted on the sent to wholesale and retail nurseries in the U.S., P. site, however, requiring additional containment efforts. ramorum became a federally quarantined pathogen in Phytophthora ramorum has been shipped regionally 2005. Nurseries shipping host plants out of state were and internationally on nursery stock. This stock is the required to participate in a state certification program. likely source of most wildland, garden and nursery In 2014 regulations were relaxed for most nurseries infestations. In 2003, a series of P. ramorum infections shipping regulated host material out of state. Only were discovered in several wholesale and retail nurseries those nurseries within the regulated areas of California, in Oregon. Rhododendron, Viburnum, Pieris and Camellia Oregon and Washington, where P. ramorum had been were the most heavily infected plants. In response to detected, on or after March 31, 2011, were required a large shipment of infected nursery stock that was to be inspected and certified by regulatory officials to 2 Table 2. Most common woodland plant hosts in California and Oregon, plant part infected and known impact of Phytophthora ramorum. Notholithocarpus densiflorus is of highest importance to P. ramorum spread and infection in Oregon woodlands. Plant species Common name Location Plant part infected and impact Notholithocarpus densiflorus tanoak California, Bole cankers, leaf and twig lesions; Oregon death of large trees Rhododendron Pacific rhododendron California, Leaf lesions, shoot dieback; death of macrophyllum Oregon mature plants Umbellularia californica California bay laurel, Oregon California, Leaf lesions (tip necrosis or speck- myrtlewood Oregon ling) Vaccinium ovatum evergreen huckleberry California, Leaf lesions, shoot dieback Oregon Quercus agrifolia coast live oak California Bole cankers; death of large trees Quercus chrysolepis canyon live oak California Bole cankers; death of saplings, pos- sible death of large trees Quercus kelloggii California black oak California Bole cankers; death of large trees Quercus parvula var. shrevei Shreve oak California Bole cankers; death of large trees Arctostaphylos manzanita manzanita California Leaf lesions, dieback, stem and branch cankers Arbutus menziesii madrone California Leaf lesions, branch cankers Adapted with permission from Davidson et al., 2003 ship out of state. The pathogen continues to be found in West Coast nurseries (California, Oregon, Washington and British Colombia) and a few nurseries in the southeastern U.S. In 2019, rhododendron plants infected with the pathogen and originating in the Pacific Northwest were detected at retail chains in 18 states. State and federal authorities implemented additional regulatory actions to halt the movement of infested nursery stock both in and out of Oregon. For more information, visit https://www.aphis.usda.gov/aphis/ ourfocus/planthealth/plant-pest-and-disease-programs/ pests-and-diseases/phytophthora-ramorum. Hosts Pacific Northwest nursery growers need to be alert for inadvertent introductions of this pathogen, especially when importing from areas with known infestations. Growers should be mindful as to where they purchase host material. In the United States, the regulated host list includes more than 150 plant species in 37 genera. The most common hosts found infected in Oregon nurseries in order of importance are Rhododendron, Viburnum, Pieris, Camellia, and Kalmia (Table 1). All species and hybrids of Photo: Jean Williams-Woodward, University of Georgia these genera are included in the regulated host list. Figure 3: Symptoms of leaf blight and dieback on Viburnum caused by Phytophthora ramorum. 3 Photo: Oregon Department of Agriculture Photo: Alameda County, California, Department of Photo: Alan Kanaskie, Oregon Department of Figure 4: Rhododendron plants with Agriculture Forestry ramorum leaf blight and shoot dieback Figure 5: Leaf lesions on Rhododendron Figure 6: Ramorum shoot dieback and at advanced (foreground) and early ‘Trilby’. leaf lesions on wild Rhododendron. (background) stages. The most common infected host in Oregon forests is tanoak (Notholithocarpus densiflorus). In California Symptoms forests, Quercus, Umbellularia, Rhododendron and Phytophthora ramorum causes different Notholithocarpus are most common (Table 2, page symptoms on different hosts. On Rhododendron, 3). Several more plant species are associated with P. Pieris, Viburnum, Camellia and other woody ramorum but are not regulated, pending experimental ornamental hosts, the disease is characterized by