A soilborne plant pathogen not previously seen in the United States — Phytophthora austrocedri — has been detected at two Oregon nurseries and has infested an unknown number of plants.
The detections have prompted an aggressive quarantine of some of the plant material on both operations as growers and authorities strive to identify infested areas and contain the pathogen’s spread. The Oregon Association of Nurseries is involved as well.
“[We are] monitoring this issue with great care and working closely with agencies, scientists and affected nurseries,” OAN Executive Director Jeff Stone said. “This pathogen still has many unknowns, but we’re bringing every appropriate resource to bear.”
P. austrocedri is part of the Phytophthora genus of water molds. More than 200 species of Phytophthora have been described, according to USDA. Hundreds of others may exist unidentified in the wild. One of the Phytophthora species that has been of greatest concern to nurseries is P. ramorum, which causes the disease known as sudden oak death.
Another Phytophthora species, P. infestans, is historically significant for causing the Irish potato famine from 1845–1852, leading to mass emigration.
P. austrocedri attacks conifers of the Cupressaceae family, including Austrocedrus, Juniperus, Chamaecyparis andCupressus and potentially others. Like its siblings in the Phytophthora genus, P. austrocedri loves soggy conditions and standing water.
According to the United Kingdom Forest Research agency, the pathogen attacks the roots of its host plants, working its way up to the main stem, forming lesions along the way. The lesions eventually girdle the main stem, cutting off the flow of nutrients and killing the plant.
The UK Forestry Commission issued a guide (PDF) to the pathogen that includes the following chief symptoms:
- Discolored foliage. The first sign of decline in juniper is that the foliage will turn a dull green. As the disease progresses, the crown of infested trees will turn bronze/brown. The pathogen may attack the stem or individual branches, causing patches of bronze-colored foliage amongst healthy green foliage.
- Discolored inner bark. When the outer bark of the tree is cut away at the infested area, it will reveal discolored inner bark, or phloem. The diseased tissue is usually a cinnamon brown color with yellow lesion edges. Resin islands may be present. Healthy tissue, on the other hand, will be white.
The handout notes that other pathogens, such as P. cinnamomi, can produce similar symptoms. Heavy snow or drought can also cause them, but there would be no associated lesions.
P. austrocedri was first isolated by researchers in 2007 as the cause of cedar sickness, aka mal del ciprés, which had plagued Cordillerian cypress trees (Austrocedrus chilensis), native to the mountains of Argentina and Chile, for decades. In the U.K., Lawson cypress (Chamaecyparis lawsoniana), Nootka cypress (Chamaecyparis nootkatensis), and the common juniper (Juniperus communis) were then confirmed as hosts in 2011. The latter put a crimp on the production of gin, made from juniper berries.
The pathogen also has been detected in Iran.
It’s not known how the pathogen arrived in the United States or in Oregon. Inspectors with the USDA Animal and Plant Health Inspection Service (APHIS) are set to begin inspecting the two affected Oregon nurseries to determine the extent of the pathogen’s spread.
In the meantime, millions of dollars in inventory at the two nurseries must remain isolated and untouched. The growers are allowed to irrigate the material, but not prune it or touch it in any way. They’ve been forced to cancel contracted sales of the material while the plants are in limbo.
A systems approach
One of the best ways to prevent the spread of water-borne pathogens is to utilize a systems approach to pest and disease management. This approach prevents establishment at critical control points and thus inhibits the spread of known pests and pathogens, as well as unknown ones also. The goal is to stop introduction into the nursery and spread of pathogens from unhealthy to healthy material.
The Oregon Association of Nurseries published the Safe Procurement and Production Manual in 2011 with a revised version in 2012 (https://www.oan.org/page/861). It was written by researchers with Oregon State University, Washington State University and the U.S. Department of Agriculture.
Another article by OSU and USDA scientists features system approaches for managing pathogens in nurseries.
These remain valuable resources for methods that prevent the spread and the shipment of infested plant material. Its methods focus on identifying and monitoring critical control points before pathogens and pests emerge and implementing best management practices to reduce risk
