Beat the blight! Phytophthora ramorum Grower Tips
Welcome to a new feature in the Member Update: P. ramorum Grower Tips. Plant pathologists from OSU, WSU, Ascent Agricultural Services and USDA-ARS will be offering tips to help you stay free of Phytophthora ramorum, the pathogen that causes Sudden Oak Death. Watch for more each week.
Tip #1. Pre-notification requirement loosened, but be extra vigilant
As of Dec. 10, 2012, most Oregon growers are no longer required to notify receiving states when shipments include any of the five high-risk genera for Phytophthora ramorum. Only 14 nurseries in Oregon – including 11 nurseries that tested positive for P. ramorum in 2012, and 3 nurseries that tested positive since 2010, need to pre-notify. These 14 nurseries will need to pre-notify until they test clean for three years. This spring, growers should be extra vigilant to ensure that P. ramorum does not get sent out of state. This week's tip is provided by OAN, Gary McAninch of the Oregon Department of Agriculture, and Jennifer Parke of Oregon State University.
Tip #2. Those rhody roots may look healthy, but...
The sudden oak death pathogen Phytophthora ramorum is able to infect roots of Rhododendron, Camellia and Lithocarpus without showing visible signs. The pathogen can persist in roots by production of long-lived resistant chlamydospores, without producing symptoms. This discovery has several important ramifications. First, growers have been focusing on above-ground symptoms to check incoming stock. These inspections will not detect latent root infections and would potentially allow the pathogen to move onto and throughout the production facility. Second, state and federal nursery inspectors also focus on above-ground symptoms for their annual certification testing and for delimitation sampling at confirmed-positive nurseries and thus, may allow the pathogen to go undetected. To reduce the risk of introducing P. ramorum on asymptomatic root tissue, growers should: (1) propagate with cuttings from symptom-free plants, and (2) purchase only host material tested by baiting or ELISA root testing prior to acquisition.
Tip #3. Rhododendron cultivars commonly infected by P. ramorum
Rhododendrons account for most of the infected plants associated with Phytophthora ramorum positive nurseries in the Pacific Northwest. Rhododendron species and cultivars vary in their susceptibility to foliar infection by P. ramorum and thus pose different risk in spreading this disease. In a screening study, very susceptible species and cultivars were mostly in the subgenus Hymenanthes. Resistant species and cultivars included R. keiskei, R. arboreum, R. mallotum, R. 'Ken Janeck' and R. 'Purple Gem'. A list of cultivars may be found here. For more information about rhododendrons and P. ramorum, click here.
Tip #4. Watch out for Phytophthora species in your water
Phytophthora species are water molds, aquatic organisms that live in rivers and ponds, and are abundant in recirculated water systems. They produce motile spores that spread in water. You should assume that your water is contaminated with pathogenic species of Phytophthora unless it is from a well or municipal source. Several water treatment methods including UV, chlorination, copper ionization and slow sand filtration are effective in killing this pathogen. To learn more about testing water and water treatment options, see: Safe Production Manual, OSU Phytophthora Online Course, and the Water Education Alliance for Horticulture.