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PORT-ORFORD-CEDAR ROOT DISEASE Pathogen: Phytophthora lateralis Tucker and Milbrath Identification: Trees infected by P. lateralis exhibit evidence of rapid decline. Crowns fade from yellow to red to brown in a matter of weeks in the case of small trees and in a year to 4 years when large trees are infected. A cinnamon-brown stain abutting against creamy white unstained wood in the root collars or lower stems of symptomatic but not yet-dead hosts is diagnostic. Hosts in Southwest Oregon: Port-Orford-cedar; occasionally Pacific yew. Effects: Port-Orford-cedar grows in a narrow coastal strip in Oregon and northern California. It grows on a wide variety of sites including streambanks, bogs, coastal sand dunes, deep productive soils, and dry sites. Although its domestic market is limited, for decades the export market for Port-Orford-cedar has been substantial, making it one of the most valuable timber species in North America.
Port-Orford-cedar root disease is very severe on suitable sites within its host's limited range. P. lateralis is an introduced pathogen first reported on ornamentals in Seattle, Washington in 1923 and not found within the native range of Port-Orford-cedar at Coos Bay, Oregon until 1952. Since then, spread into mountainous regions has been slow but progressive. P. lateralis is now present throughout much of the range of Port-Orford-cedar. Infected and healthy Port-Orford-cedar often occur intermingled.
Ecological Role: P. lateralis is an exotic pathogen that has not coevolved with its hosts. Port-Orford-cedar is a very important species in riparian ecosystems. It provides important stream-side shade, and its snags and logs are long-lasting components of terrestrial and aquatic wildlife habitat. Therefore, the extensive killing of large Port-Orford-cedar in riparian zones by this disease is likely to have severe ecological consequences. Port-Orford-cedar also often provides the only large tree components of forest stands on ultramafic soils. Loss of Port-Orford-cedar in these environments due to P. lateralis has been devastating in some locations.
Life History: Port-Orford-cedar root disease is caused by an Oomycete, or "water mold" and is spread by water-borne spores (zoospores) or resistant spores (chlamydospores) in contaminated soil. The pathogen is spread over long distances mainly by people moving infected seedlings and infested soil into disease-free sites. Livestock and wildlife may move the pathogen in soil on their feet but that is believed to be a minor factor in most situations. A much greater threat is the movement of soil on tires and machinery from an infested area to a noninfested area during road building, travel, or logging activities. Zoospores of the fungus, which can swim short distances, move in water and are attracted to the uninjured growing tips of cedar roots. They are totally dependent on free water for infection and spread and cannot survive in dry soil. Natural spread by zoospores is virtually all downslope or downstream in water moved by gravity. After the zoospores germinate, mycelia penetrate the roots, kill the phloem, and spread internally to the base of the tree and to other roots. If the roots of adjacent trees are grafted, the mycelia can grow from root to root, with estimated movement of about 15 cm (6 in) per year. However, this mode of spread is not considered to be very significant relative to zoospore spread in water. Chlamydospores, the durable resting spores are produced within infected phloem. As a dead root deteriorates these spores are released to the soil where they may remain viable for at least 6 years under favorable conditions and be available for passive spread in soil. Conducive Habitats: The disease is favored in areas of slow-moving or Management: Port-Orford-cedar's importance within its range makes it critical that this disease be managed aggressively, regardless of resource objectives. Mitigation strategies involve prevention of the movement of infested water or soil from an infested site to the root zone of healthy trees. They include closing roads and restricting operations to reduce movement of infested soil, cleaning vehicles and equipment before entering or leaving specified areas to remove soil that may contain spores, and berming roadsides to reduce splash and runoff. Photos:
Links: Port-Orford-cedar root disease on the Siskiyou NF Abstracts from Port-Orford-cedar conference Breeding resistance to Port-Orford-cedar root disease References: Hadfield, J. S., Goheen, D.J., Filip, G.M., Schmitt, C.L., and Harvey, R.D. 1986. Root diseases in Oregon and Washington conifers. R6-FPM-250-86. USDA For. Serv., PNW Region, 27 p. Roth, L. F., Harvey, R.D., and Kliejunas, J.T. 1987. Port-Orford-cedar root disease. R6-FPM-PR-010-91. USDA For. Serv., PNW Region. 11 p. | |||||||||||
standing water and on poorly drained microsites, especially when such sites occur below roads where new introductions are likely.
Port-Orford-cedar on microsites not favorable to the pathogen should be favored for retention during stand entries.
Preventive strategies also include removing Port-Orford-cedar from roadside strips to prevent infestation of adjacent disease-free stands and, where the pathogen is already present, to allow inoculum to die by removing all hosts. Some resistance to P. lateralis has been identified in the Port-Orford-cedar population. These trees are currently considered tolerant or "slow to die" but not immune. An aggressive search is underway to identify other resistant individuals throughout the range of the species and to breed for enhanced resistance.