Port-Orford-cedar, Chamaecyparis lawsoniana (POC), is a valuable forest tree that has a limited distribution along the Pacific coast from southern Oregon to northern California. The species has highly aromatic wood and is widely used as a landscape plant. Port-Orford-cedar populations have been heavily damaged by a root disease caused by the exotic algal fungus Phytophthora lateralis. The disease was first reported in 1923 near a Seattle nursery (Hunt in Zobel et al., 1985), which is outside Port-Orford-cedar’s natural range. Roth et al. (1987) reported that in 1938, the disease was again discovered in Oregon’s Willamette Valley, where it virtually halted nursery production of ornamental Chamaecyparis. The disease was discovered in the naturally occurring Port-Orford-cedars in 1952 (Roth et al., 1987) and has since spread throughout the host’s range. Mortality can reach more than 90 % on riparian sites – causing devastating impacts effects on stream ecology (Jung et al. 2018). P. lateralis also occasionally infects a second tree species in these forests, Taxus brevifolia (Jung et al. 2018).
How Phytophthora lateralis entered the country or even the exact country of origin is uncertain. Scientists long considered an eastern Asian origin likely. In 2009, P. lateralis was isolated at several sites in natural, high-altitude cloud forests of Taiwan from soil in wet seeps and from necrotic foliage of mature C. obtusa var. formosana with generally healthy crowns and non-damaged fine root system. However, the exact origin of the lineage established in Oregon and California is not yet known (Jung et al. 2018).
The fungus invades fine roots and subsequently colonizes the entire root system. Multiple root infections will spread to the root collar, and the tree will die from girdling. The fungus will attack seedlings as well as mature trees. Seedling mortality can occur within a few days, while mature trees may take two to four years to die. Aerial infections of branches that come into contact with infected litter or soil can occur during wet weather. The infection will eventually migrate to the trunk, and girdling will occur. Fungal spores are spread by surface water or rain splash, usually in the wet spring. The pathogen can be transferred in spore-contaminated soil by machinery and animals but does not occur independently in the soil (Ostrofsky et al., 1977). Movement of infected Port-Orford-cedar nursery stock, including other Chamaecyparis species, also can spread the disease. Zobel et al. (1985) believe that the disease “probably never would have emerged in epidemic form without the widespread planting of ornamental Chamaecyparis in northwestern Oregon and Washington.”
The USDA Forest Service at the Oregon Dorena Tree Improvement Center, in conjunction with the USDI Bureau of Land Management and Oregon State University, have been working many years on the detection of resistance to this disease. Over 9,000 trees have been screened for resistance, and controlled pollinations are now being made to develop resistant genotypes and to understand the mechanisms of resistance. Meanwhile, surviving trees and stands are being protected by management practices, including restricted movement of people and machinery during wet periods where spores are abundant. Several of the POC families resistant to the PNW lineage of P. lateralis are also resistant to the British and Taiwanese lineages (see below) (Jung et al. 2018).
P. lateralis has also been introduced to Europe, where it is causing severe decline and mortality of plantings of C. lawsoniana trees. Initial detections were in nurseries in France (in the 1990s) and the Netherlands (2004 or 2005). These outbreaks were eradicated (https://www.hortweek.com/fera-plant-pest-factsheet-phytophthora-lateralis/arboriculture/article/1095532 )
In 2010, France announced further outbreaks of P. lateralis on hedges or windbreaks of C. lawsoniana planted at several sites in Brittany in north-west France. Signs of decline and mortality were noticed in 2005 but the cause was only confirmed in 2009. The hedges were planted in the 1970s – it is not known where they were infected at the time of planting or became infected later. At about the same time, P. lateralis was detected on declining POC and C. pisifera (Sawara cypress) trees at numerous sites in forests, parks and shelterbelts in England, Scotland and Northern Ireland. Finally, that same year, P. lateralis was detected on POC nursery stock in the Netherlands (https://www.hortweek.com/fera-plant-pest-factsheet-phytophthora-lateralis/arboriculture/article/1095532 ).
Genetic testing has shown that the pathogen on trees in France, the Netherlands, and the majority of the British outbreaks are identical to isolates from the California and Oregon, indicating that these outbreaks were introduced from the United States on infested nursery stock. Several isolates from Scotland constituted a separate British lineage which might have resulted from a hybridization between the Pacific Northwest and Taiwanese lineages. On Taiwan, there are two distinct evolutionary lineages, called TWJ and TWK. As noted above, the exact origin of the PNW lineage in Asia has not yet been determined (Jung et al. 2018).
Jung, T., A. Pérez-Sierra, A. Durán, M. Horta Jung, Y. Balci, B. Scanu. 2018. Canker and decline diseases caused by soil- and airborne Phytophthora species in forests and woodlands. Persoonia 40, 2018: 182–220
Ostrofsky, W. D., R. G. Pratt, and L. F. Roth. 1977. Detection of Phytophthora lateralis in soil organic matter and factors that affect its survival. Phytopathology 67: 79-84.
Roth, L. F., R. D. Harvey, Jr., and J. T. Kliejunas. 1987. Port-Orford-Cedar Root Disease, United States Department of Agriculture, Forest Service, R6 FPM-PR-294-87, 1987.
Zobel, D. B., L. F. Roth, and G. M. Hawk, 1985. Ecology, Pathology, and Management of Port-Orford-Cedar (Chamaecyparis lawsoniana). United States Department of Agriculture Forest Service. Pacific Northwest, General Technical Report, PNW-184.