Cause The fungus-like microorganisms Phytophthora lateralis and P. cinnamomi kill Port Orford cedar seedlings and trees. The disease was found in nursery stock in 1923 and has since spread throughout the Pacific Northwest, in the tree's native range of southwest Oregon, and in landscapes with extensive hedgerows. Spread into the southwest Oregon mountains has been slow but progressive. Dead trees can be found along streams and in drainage ditches on the down-slope sides of roads.
Among the hosts to P. lateralis, Port Orford cedar is highly susceptible and the worst impacted. Other members of the Cupressaceae (cypress family), including other Chamaecyparis species such as Alaskan cedar (C. nootkatensis), show variable degrees of tolerance. Additional genera from which P. lateralis has been recovered include Juniperus, Microbiota, Taxus, and Thuja. All are considered susceptible to P. cinnamomi, which has an extensive host range.
Phytophthora austrocedri was detected for the first time in the USA on juniper and arborvitae at two different Oregon nurseries in 2024. This pathogen can also cause root rot of Port Orford cedar and other members of the Cupressaceae, including popular ornamental species such as Leyland cypress (Cupressus x leylandii) and Chinese juniper (Juniperus chinensis).
These fungus-like microorganisms survive as oospores or chlamydospores in plant debris such as rotted roots or foliage. Spores may be moved long distances in contaminated soil through human activities (for example, by logging equipment), by cattle, or through moving water. Spores germinate and produce sporangia and a swimming spore stage called zoospores, which attack fine roots and begin to rot the root system. Tree-to-tree spread can occur through root grafts. Spores splashed on leaves or stems can initiate a foliage blight. Infection also can occur through wounds, but wounds are not required for pathogen establishment.
Symptoms The organism infects small roots first, then larger and larger roots until the tree is girdled near the collar. The fine roots are at first water soaked, then darken and quickly disintegrate. Infected inner bark turns cinnamon-brown in contrast to the normal cream color of healthy tissue. There generally is a sharp line between infected and healthy tissues. Infected trees' foliage is a slightly lighter color and may wilt on warm days. All foliage soon turns chlorotic, then bronze, and finally brown. Seedlings may die a few weeks after infection; large trees may take 2 to 4 years to die. In hedgerow landscape plantings, trees typically die one by one over several years as the organism moves between adjacent trees. Dead and dying trees may be attacked by bark beetles and are more likely to be blown down by strong winds.
Foliar symptoms are rare in the Pacific Northwest. Infections that begin on leaves or stems start locally but can spread down the stem or to other branches. Stem lesions are sunken and kill all foliage distal to the canker.
Cultural control
- Plant healthy seedlings in soil known to be free of the organism.
- Do not plant in soil with poor drainage or in areas that receive drainage from roads or other Cupressaceae plantings.
- Prevent wounding at the base of trees or to roots from nearby construction or harvesting operations.
- Avoid extensive gardening (mulching and planting susceptible flowering plants) under cedar plantings because it may introduce the disease and/or wound tree roots.
- Promptly remove and destroy dead and dying trees to help protect other trees in the area. In landscape plantings, also remove healthy-looking trees next to diseased ones. Cedar bark beetle activity in trees has been associated with trees with root rot and may be good indicators of which healthy appearing trees need to be removed. Avoid moving infested soil while removing trees and/or stumps. Consider the ground to be infested, even after the removal of affected plants.
- Plant tolerant or resistant species in contaminated ground. If P. lateralis is the only problem (which is very likely) avoid susceptible plants such as C. lawsoniana (unless using resistant root stock), Microbiota, Thuja sp., or Juniperus communis; in general members of the Cupressaceae (cypress family) should be avoided, but tolerant species include Chamaecyparis formosensis, C. thyoides, C. pisifera, and Cupressocyparis leylandii, which may be planted. If P. cinnamomi is the problem (which is possible), there are fewer options for planting. P. cinnamomi has a much wider host range, although a few crabapple and rhododendron species are tolerant. Correct any drainage problems before replanting to minimize disease.
- Resistant Chamaecyparis lawsoniana are now available including susceptible scions grafted onto resistant rootstocks.
- In sensitive forested areas of southwest Oregon, unpaved roads are closed during the rainy season. Also, consider removal of cedars along the road side to prevent infections in those areas. This pathogen moves in infested soil, so clean boots and equipment after leaving infested area.
- Avoid reusing pots from a previous crop for propagation. If pots must be reused then wash off all debris and soak in a sanitizing solution or treat with aerated steam for 30 min.
Chemical control These chemicals may help prevent infection or allow slightly infected trees to survive the disease given annual spring and/or fall applications. Severely infected trees will not benefit and will die. Rotate fungicides from different groups that have a different mode of action for resistance management.
- Aliette at 2.5 to 5 lb/100 gal water as a foliar spray to nursery plants. Do not reapply within 30 days. Do not use with adjuvants. Group P7 fungicide. 24-hr reentry.
- ArborFos (from Mauget) can be injected into trees at a rate of 5 ml/inch diameter at breast height. Group P7 fungicide. See label for details. Unknown efficacy in the Pacific Northwest.
- Banrot 40 WP at 6 to 12 oz/100 gal water for container- or bed-grown plants. Group 1 + 14 fungicide. 12-hr reentry.
- Fosphite at 1 to 2 quarts/100 gal water. Do not use copper products within 20 days of treatment and do not use spray adjuvants. Group P7 fungicide. 4-hr reentry.
- Mefenoxam 2 AQ at 0.98 to 1.96 oz/100 gal water as a soil drench or at 1.23 to 2.45 oz/1,000 sq ft followed by at least 0.5 inch rain or irrigation. Group 4 fungicide. No restrictions on reentry when used as a soil drench or media incorporation.
- Monterey Garden Phos at 16 fl oz in 16 fl oz water plus 1 oz Pentra-Bark as a basal trunk spray. Also labeled for injection, see label for details. Can be used in landscape sites. Group P7 fungicide. H
- Phospho-Jet at 1 to 2 quarts/100 gal water as a soil drench. Group P7 fungicide. 4-hr reentry.
- Phostrol at 51 fl oz/74 fl oz water plus an organosilicone surfactant used as a basal bark spray. Also labeled for injection, see label for details. Group P7 fungicide. 4-hr reentry.
- Subdue MAXX as a soil surface spray at 1.25 to 2.5 fl oz/1,000 sq ft; irrigate with 0.5 inch water within 24 hours of application. Group 4 fungicide. 48-hr reentry.
- Terrazole 35 WP at 3.5 to 10 oz/100 gal water as a soil drench. Use only in commercial nurseries and greenhouses. Group 14 fungicide.12-hr reentry.
References Hansen, E.M., Goheen, D.J., Jules, E.S., and Ullian, B. 2000. Managing Port-Orford-cedar and the introduced pathogen Phytophthora lateralis. Plant Disease 84:4-14.
Peterson, E. K., Rupp, F., Eberhart, J., and Parke, J. L. 2020. Root rot of Juniperus and Microbiota by Phytophthora lateralis in Oregon horticultural nurseries. Plant Disease 104:1500-1506.
Robin, C., Brasier, C., Reeser, P.W., Sutton, W., Vannini, A., Vettraino, A.M., and Hansen, E. 2015. Pathogenicity of Phytophthora lateralis lineages on different selections of Chamaecyparis lawsoniana. Plant Disease 99:1133-1139.